Article

Jet Properties from Dihadron Correlations in p+p Collisions at s\sqrt{s}=200 GeV

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

The properties of jets produced in p+p collisions at √s=200 GeV are measured using the method of two-particle correlations. The trigger particle is a leading particle from a large transverse momentum jet while the associated particle comes from either the same jet or the away-side jet. Analysis of the angular width of the near-side peak in the correlation function determines the jet-fragmentation transverse momentum jT. The extracted value, √⟨jT2⟩=585±6(stat)±15(sys) MeV/c, is constant with respect to the trigger particle transverse momentum, and comparable to the previous lower √s measurements. The width of the away-side peak is shown to be a convolution of jT with the fragmentation variable, z, and the partonic transverse momentum, kT. The ⟨z⟩ is determined through a combined analysis of the measured π0 inclusive and associated spectra using jet-fragmentation functions measured in e+e- collisions. The final extracted values of kT are then determined to also be independent of the trigger particle transverse momentum, over the range measured, with value of √⟨kT2⟩=2.68±0.07(stat)±0.34(sys) GeV/c.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... In the following, the near and away-side jet fragmentation yields are calculated to verify the template fit method by comparing the jet fragmentation yields to the PYTHIA model. The away-side jet fragmentation yields in the PYTHIA model are obtained using the standard ∆φ analysis [106], while in the data, the away-side jet fragmentation yields are extracted using the template fit method because of the flow modulations in the data. The comparison between the data and the PYTHIA model provides a validation of the template fit method. ...
... While PYTHIA overestimates both near-side and away-side yields measured by ALICE, the corresponding ratio is consistent with the ALICE data in the all considered V0M multiplicity intervals. The value of this ratio can be explained by the pair acceptance effect caused by the limited ALICE η acceptance [106]. implies that the enhanced jet fragmentation yields in the away-side in high-multiplicity events with respect to low-multiplicity events [81,82] are taken into account by the low-multiplicity template method. ...
... In summary, the difference between the near-side and away-side jet fragmentation yields in PYTHIA is solely caused by the jet acceptance effects which affect the two-particle correlation functions. The corresponding ratio in data, where the away-side jet fragmentation yields are measured with the low-multiplicity template, agrees well with PYTHIA as well as with the expectation in ref. [106]. The flow coefficients, v n , of the trigger particles, can be extracted from the template fit with the use of the observed factorization of v n,n coefficients to single harmonics [10,19] by using v n (p T,trig ) = v n,n (p T,trig , p T,assoc )/ v n,n (p T,assoc , p T,assoc ), (3.6) ...
Article
Full-text available
A bstract Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s \sqrt{s} s = 13 TeV and p–Pb collisions at sNN \sqrt{s_{\textrm{NN}}} s NN = 5 . 02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆ φ and pseudorapidity separation ∆ η for pairs of primary charged particles within the pseudorapidity interval | η | < 0 . 9 and the transverse-momentum interval 1 < p T < 4 GeV/ c . Flow coefficients are extracted for the long-range correlations (1 . 6 < |∆ η | < 1 . 8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
... The transverse fragmentation of partons is often studied using the jet fragmentation transverse momentum, j T , that describes the momentum component of particles produced in the fragmentation perpendicular to the momentum vector of the hard parton initiating the fragmentation. Previously, j T has been studied using two-particle correlations by the CCOR collaboration at ISR with pp collisions at centerof-mass energy √ s = 31, 45 and 63 GeV [5] and the PHENIX collaboration at RHIC with pp collisions at √ s = 200 GeV [6] and d-Au collisions at center-of-mass energy per nucleon pair √ s NN = 200 GeV [7]. Jet measurements to study j T have been done by the CDF collaboration at the Tevatron with pp collisions at √ s = 1.96 ...
... In the highest x bin, a small underestimation of the data is visible for all the simulations at mid-p Tt and for Herwig also in the lowest p Tt bins. The narrow component j 2 T results from three x bins are compared to the earlier results from CCOR [5] and PHENIX [6] in figure 8. These experiments use different methods to extract j T from the data. ...
... This is connected to the current analysis given that especially at the lower energies the high-p T trigger particles are likely to have a high z t . PHENIX reported in [6] that this value is z t ∼ 0.6. Since ISR had lower collision energy than RHIC, z t can not be lower in the CCOR experiment. ...
Article
Full-text available
A bstract The transverse structure of jets was studied via jet fragmentation transverse momentum ( j T ) distributions, obtained using two-particle correlations in proton-proton and proton-lead collisions, measured with the ALICE experiment at the LHC. The highest transverse momentum particle in each event is used as the trigger particle and the region 3 < p Tt < 15GeV/ c is explored in this study. The measured distributions show a clear narrow Gaussian component and a wide non-Gaussian one. Based on Pythia simulations, the narrow component can be related to non-perturbative hadronization and the wide component to quantum chromodynamical splitting. The width of the narrow component shows a weak dependence on the transverse momentum of the trigger particle, in agreement with the expectation of universality of the hadronization process. On the other hand, the width of the wide component shows a rising trend suggesting increased branching for higher transverse momentum. The results obtained in pp collisions at s=7 \sqrt{s}=7 s = 7 TeV and in p–Pb collisions at sNN=5.02 \sqrt{s_{\mathrm{NN}}}=5.02 s N N = 5.02 TeV are compatible within uncertainties and hence no significant cold nuclear matter effects are observed. The results are compared to previous measurements from CCOR and PHENIX as well as to P ythia 8 and Herwig 7 simulations.
... To have sensitivity to possible factorization breaking and modified TMD evolution effects, a particular observable must be sensitive to a small scale on the order of Λ QCD and measured over a range of hard scales. Nearly back-to-back dihadron production has long been used as a proxy for measuring initial-state partonic transverse momentum k T [45][46][47][48], which is defined in Fig. 1. First used in predictions by Ref. [49] as a method for understanding large differences in hard scattering cross sections between theory and data, nearly back-to-back twoparticle and dijet angular correlations have since been used to measure k T over a large range of center of mass energies [45,47,50,51]. ...
... Nearly back-to-back dihadron production has long been used as a proxy for measuring initial-state partonic transverse momentum k T [45][46][47][48], which is defined in Fig. 1. First used in predictions by Ref. [49] as a method for understanding large differences in hard scattering cross sections between theory and data, nearly back-to-back twoparticle and dijet angular correlations have since been used to measure k T over a large range of center of mass energies [45,47,50,51]. Direct photon-hadron correlations are of particular interest because the photon comes directly from the partonic hard scattering, and thus carries initial-state information without any final-state fragmentation effects. ...
... where p assoc T is the p T of the associated hadron and ∆φ is the azimuthal angular separation between the trigger and associated particle as shown in Fig. 1. Reference [45] has shown that the root mean square of p out and k T are related by ...
Article
Full-text available
Dihadron and isolated direct photon-hadron angular correlations are measured in p+p collisions at s=510 GeV. Correlations of charged hadrons of 0.7<pT<10 GeV/c with π0 mesons of 4<pT<15 GeV/c or isolated direct photons of 7<pT<15 GeV/c are used to study nonperturbative effects generated by initial-state partonic transverse momentum and final-state transverse momentum from fragmentation. The nonperturbative behavior is characterized by measuring the out-of-plane transverse momentum component pout perpendicular to the axis of the trigger particle, which is the high-pT direct photon or π0. Nonperturbative evolution effects are extracted from Gaussian fits to the away-side inclusive-charged-hadron yields for different trigger-particle transverse momenta (pTtrig). The Gaussian widths and root mean square of pout are reported as a function of the interaction hard scale pTtrig to investigate possible transverse-momentum-dependent evolution differences between the π0−h± and direct photon-h± correlations and factorization breaking effects. The widths are found to decrease with pTtrig, which indicates that the Collins-Soper-Sterman soft factor is not driving the evolution with the hard scale in nearly back-to-back dihadron and direct photon-hadron production in p+p collisions. This behavior is in contrast to Drell-Yan and semi-inclusive deep-inelastic scattering measurements.
... The correlations are constructed similarly to previous PHENIX two-particle correlation analyses; see, e.g., Refs. [31][32][33]. Per-trigger yields are constructed for a given observable, such as φ, which show the yield of charged hadrons per-trigger π 0 and are defined by ...
... However, the away-side correlations are sensitive to both initial and final-state transverse momentum. Because the initialstate k T is much larger than final-state j T (see, e.g., [31,32]), this leads to a broader p out distribution on the away side than the near side. Nonetheless, a nonperturbative Gaussian region can still be identified on the near side as shown in Fig. 3, similarly to the away side, with a power-law spectrum at larger p out that is not well described by the Gaussian fit. ...
Article
Full-text available
The PHENIX collaboration has measured high-pT dihadron correlations in p+p, p+Al, and p+Au collisions at sNN=200 GeV. The correlations arise from inter- and intrajet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of pout, the transverse-momentum component of the associated hadron perpendicular to the trigger hadron, are sensitive to initial- and final-state transverse momenta. These distributions are measured multidifferentially as a function of xE, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side pout widths, sensitive to fragmentation transverse momentum, show no significant broadening between p+Au, p+Al, and p+p. The away-side nonperturbative pout widths are found to be broadened in p+Au when compared to p+p; however, there is no significant broadening in p+Al compared to p+p collisions. The data also suggest that the away-side pout broadening is a function of Ncoll, the number of binary nucleon-nucleon collisions, in the interaction. The potential implications of these results with regard to initial- and final-state transverse-momentum broadening and energy loss of partons in a nucleus, among other nuclear effects, are discussed.
... The latter assumption is called the Zero-Yield-At-Minimum (ZYAM) method [119]. Early studies of di-hadron correlations fit the data near ∆φ ≈ 1 to determine the background level [111,[120][121][122]. Later studies typically use a few points around the minimum [116,123,124]. ...
... Figure from[122]. Compilation of pT pair = √ 2kT measurements where kT is the acoplanarity momentum vector. Dimuons and dijets measurements are from fixed target experiments and the diphoton measurements are from the Tevetron. ...
Article
A hot, dense medium called a Quark Gluon Plasma (QGP) is created in ultrarelativistic heavy ion collisions. Hard parton scatterings generate high momentum partons that traverse the medium, which then fragment into sprays of particle called jets. Experimental measurements from high momentum hadrons, two particle correlations, and full jet reconstruction at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) continue to improve our understanding of energy loss in the QGP. Run 2 at the LHC recently began and there is a jet detector at RHIC under development. Now is the perfect time to reflect on what the experimental measurements have taught us so far, the limitations of the techniques used for studying jets, how the techniques can be improved, and how to move forward with the wealth of experimental data such that a complete description of energy loss in the QGP can be achieved. Measurements of jets to date clearly indicate that hard partons lose energy. Detailed comparisons of the nuclear modification factor between data and model calculations led to quantitative constraints on the opacity of the medium to hard probes. While there is substantial evidence for softening and broadening jets through medium interactions, the difficulties comparing measurements to theoretical calculations limit further quantitative constraints on energy loss mechanisms. We call for an agreement between theorists and experimentalists on the appropriate treatment of the background, Monte Carlo generators that enable experimental algorithms to be applied to theoretical calculations, and a clear understanding of which observables are most sensitive to the properties of the medium, even in the presence of background. This will enable us to determine the best strategy for the field to improve quantitative constraints on properties of the medium in the face of these challenges.
... Previously j T has been measured for example by correlating the particles inside a jet cone with the reconstructed jet axis [1,2,3] or by calculating it from the azimuthal correlation function [4]. Only one component for j T is extracted in these studies, describing the whole time evolution of the jet. ...
... The final j 2 T results are calculated from the obtained fit parameters for narrow and wide components. Based on earlier results [1,4] the hadronization component is expected to be universal, meaning that it is √ s independent and that similar jets in different p Tt (same x bin) give the same j 2 T results. It can be seen from Figure 5 that the narrow component results show a flat trend as a function of p Tt and that there is no difference between results from pp and p-Pb data. ...
Article
QCD color coherence phenomena, like angular ordering, can be studied by looking at jet fragmentation. As the jet is fragmenting, it is expected to go through two different phases. First, there is QCD branching that is calculable in perturbative QCD. Next, the produced partons hadronize in a non-perturbative way later in a hadronization process. The jet fragmentation can be studied using the method of two particle correlations. A useful observable is the jet transverse fragmentation momentum jTj_{\mathrm{T}}, which describes the angular width of the jet. In this contribution, a differential study will be presented in which separate jTj_{\mathrm{T}} components for branching and hadronization will be distinguished from the data measured by the ALICE experiment. The pTtp_{\mathrm{Tt}} dependence of the hadronization component <jT2>\sqrt{\left<j_{\mathrm{T}}^{2}\right>} is found to be rather flat, which is consistent with universal hadronization assumption. However, the branching component shows slightly rising trend in pTtp_{\mathrm{Tt}}. The s=7TeV\sqrt{s} = 7\,\mathrm{TeV} pp and sNN=5.02TeV\sqrt{s_{\mathrm{NN}}} = 5.02\,\mathrm{TeV} p-Pb data give the same results within error bars, suggesting that this observable is not affected by cold nuclear matter effects in p-Pb collisions. The measured data will also be compared to the results obtained from PYTHIA8 simulations.
... The near-side peak is relatively narrow for high-p T trigger particles due to the kinematic bias made when a trigger particle is selected. In contrast, the away-side peak is typically wider due to the relatively softer particles composing the recoil jet and to jet acoplanarity, predominantly originating from initialstate and/or final-state radiation [49]. In addition, the shape of the correlation functions is sensitive to the hadronization and particle decays which alter the kinematics of the detected jet particles relative to the parent parton. ...
Preprint
Full-text available
We report on the measurement of inclusive, non-prompt, and prompt J/ψ\psi-hadron correlations by the ALICE Collaboration at the CERN Large Hadron Collider in pp collisions at a center-of-mass energy of 13 TeV. The correlations are studied at midrapidity (y<0.9|y| < 0.9) in the transverse momentum ranges pT<40 GeV/cp_{\rm T} < 40~\text{GeV}/c for the J/ψ\psi and 0.15<pT<100.15 < p_{\rm T} < 10 GeV/cc and η<0.9|\eta|<0.9 for the associated hadrons. The measurement is based on minimum bias and high multiplicity data samples corresponding to integrated luminosities of Lint=34 nb1L_{\text{int}} = 34~\text{nb}^{-1} and Lint=6.9 pb1L_{\text{int}} = 6.9~\text{pb}^{-1}, respectively. In addition, two more data samples are employed, requiring, on top of the minimum bias condition, a threshold on the tower energy of E=4E = 4 and 9 GeV9~\text{GeV} in the ALICE electromagnetic calorimeters, which correspond to integrated luminosities of Lint=0.9 pb1L_{\text{int}} = 0.9~\text{pb}^{-1} and Lint=8.4 pb1L_{\text{int}} = 8.4~\text{pb}^{-1}, respectively. The results are presented as associated hadron yields per J/ψ\psi trigger as a function of the azimuthal angle difference between the associated hadrons and J/ψ\psi mesons. The integrated near-side and away-side correlated yields are also extracted as a function of the J/ψ\psi transverse momentum. The measurements are discussed in comparison to PYTHIA calculations.
... Previously, j T has been studied using two-particle correlations where j T is calculated for particles with respect to the highest transverse momentum particle in each event instead of reconstructed jet. The study using the correlation method was done by the CCOR collaboration at ISR in pp collisions at centre-of-mass energies √ s = 31, 45 and 63 GeV [15] and by the PHENIX collaboration at RHIC in pp collisions at √ s = 200 GeV [16] and d-Au collisions at a center-of-mass energy per nucleon pair √ s NN = 200 GeV [17]. The results showed no clear dependence on the transverse momentum (p T ) of the trigger particle. ...
Preprint
Full-text available
Jet fragmentation transverse momentum (jTj_{\rm T}) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV with the ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-kTk_{\rm T} algorithm with resolution parameter R=0.4R=0.4 in the pseudorapidity range η<0.25|\eta|<0.25. The jTj_{\rm T} values are calculated for charged particles inside a fixed cone with a radius R=0.4R = 0.4 around the reconstructed jet axis. The measured jTj_{\rm T} distributions are compared with a variety of parton-shower models. Herwig and PYTHIA 8 based models describe the data well for the higher jTj_{\rm T} region, while they underestimate the lower jTj_{\rm T} region. The jTj_{\rm T} distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher jTj_{\rm T} values (called the "wide component"), related to the perturbative component of the fragmentation process, and with a Gaussian for lower jTj_{\rm T} values (called the "narrow component"), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation.
... The dijet acollinearity can be estimated from the widths of Δf=0 and Δf=π peaks in the measured g 2 (Δf), following a method first used in particle physics by the CCOR collaboration [3] about 40 years ago, and more recently by the PHENIX collaboration [4]. This method relies on a Gaussian approximation for the jet transverse spread and basic trigonometry to obtain an average angle of dijet acollinearity. ...
Article
Full-text available
This letter explains features of the emission of jets by driven Bose-Einstein condensates, discovered by Clark et al. (Nature 551, 356359), by drawing analogies with particle and nuclear physics. The widening of the Δϕ=π\Delta\phi=\pi peak in the angular correlation function is due to a dijet acollinearity caused by the uncertainty principle, like in parton-parton scattering with "intrinsic kTk_{T}". The constant linear width of the jets follows from quantum diffusion with high jet momentum, a phenomena that is an ingredient of the "color transparency" effect in nuclear collisions. I propose new "jet tomography" studies of exotic forms of matter formed by driven ultra-cold atoms by analogy to studies of the quark-gluon plasma.
Article
The transverse momentum jTj_{\textrm{T}}-dependent jet fragmentation functions have been investigated in proton+proton (p ++ p) and proton+lead (p ++ Pb) collisions at sNN=5.02 TeV\sqrt{s_{\textrm{NN}}} = 5.02~\textrm{TeV} with a multiphase transport model containing both a simple quark coalescence mechanism and a new hybrid hadronization mechanism with coalescence and fragmentation processes. Hadronized by the new hadronization mechanism, the AMPT model achieves a quantitative description of the jTj_{\textrm{T}}-dependent jet fragmentation functions measured by ALICE. Besides, no obvious jet-medium interaction and cold nuclear matter effects on the jTj_{\textrm{T}}-dependent jet fragmentation functions in p ++ Pb collisions were observed. We found the jTj_{\textrm{T}}-dependent jet fragmentation functions are dominated by the quark coalescence contribution for the new hadronization mechanism, which can be decomposed into narrow and wide parts. The root mean square value of the wide part depends on the jet radius R and jet transverse momentum pT, jetp_{\textrm{T, jet}}, which is sensitive to different hadronization mechanisms and their components. Therefore, the jTj_{\textrm{T}}-dependent jet fragmentation functions are proposed as a sensitive probe to study the non-perturbative hadronization effect of jets in small colliding systems.
Article
A prevalence of production of twisted (vortex) particles in noncentral heavy-ion collisions is shown. In such collisions, photons emitted due to the rotation of charges are highly twisted. Charged particles are produced in nonspreading multiwave states and have significant orbital angular momenta. It can be expected that an emission of any twisted particles manifesting themselves in specific effects is rather ubiquitous.
Article
Full-text available
Results from Relativistic Heavy Ion Collider Physics in 2018 and plans for the future at Brookhaven National Laboratory are presented.
Article
High transverse momentum ( ) particle production is suppressed owing to the parton (jet) energy loss in the hot dense medium created in relativistic heavy-ion collisions. Redistribution of energy at low-to-modest has been difficult to measure, owing to large anisotropic backgrounds. We report a data-driven method for background evaluation and subtraction, exploiting the away-side pseudorapidity gaps, to measure the jetlike correlation shape in Au+Au collisions at GeV in the STAR experiment. The correlation shapes, for trigger particles and various associated particle ranges within , are consistent with Gaussians, and their widths increase with centrality. The results indicate jet broadening in the medium created in central heavy-ion collisions.
Article
Full-text available
Dihadron and isolated direct photon-hadron angular correlations are measured in p + p collisions at √ s = 200 GeV . The correlations are sensitive to nonperturbative initial-state and final-state transverse momenta k T and j T in the azimuthal nearly back-to-back region Δ ϕ ∼ π . To have sensitivity to small transverse momentum scales, nonperturbative momentum widths of p out , the out-of-plane transverse-momentum component perpendicular to the trigger particle, are measured. In this region, the evolution of p out can be studied when several different hard scales are measured. These widths are used to investigate possible effects from transverse-momentum-dependent factorization breaking. When accounting for the longitudinal-momentum fraction of the away-side hadron with respect to the near-side trigger particle, the widths are found to increase with the hard scale; this is qualitatively similar to the observed behavior in Drell-Yan and semi-inclusive deep-inelastic scattering interactions, where factorization is predicted to hold. The momentum widths are also studied as a function of center-of-mass energy by comparing to previous measurements at √ s = 510 GeV . The nonperturbative jet widths also appear to increase with √ s at a similar x T , which is qualitatively consistent to similar measurements in Drell-Yan interactions. Future detailed global comparisons between measurements of processes where transverse-momentum-dependent factorization is predicted to hold and be broken will provide further insight into the role of color in hadronic
Article
A hot, dense medium called a quark gluon plasma (QGP) is created in ultrarelativistic heavy ion collisions. Early in the collision, hard parton scatterings generate high momentum partons that traverse the medium, which then fragment into sprays of particles called jets. Understanding how these partons interact with the QGP and fragment into final state particles provides critical insight into quantum chromodynamics. Experimental measurements from high momentum hadrons, two particle correlations, and full jet reconstruction at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) continue to improve our understanding of energy loss in the QGP. Run 2 at the LHC recently began and there is a jet detector at RHIC under development. Now is the perfect time to reflect on what the experimental measurements have taught us so far, the limitations of the techniques used for studying jets, how the techniques can be improved, and how to move forward with the wealth of experimental data such that a complete description of energy loss in the QGP can be achieved. Measurements of jets to date clearly indicate that hard partons lose energy. Detailed comparisons of the nuclear modification factor between data and model calculations led to quantitative constraints on the opacity of the medium to hard probes. However, while there is substantial evidence for softening and broadening jets through medium interactions, the difficulties comparing measurements to theoretical calculations limit further quantitative constraints on energy loss mechanisms. Since jets are algorithmic descriptions of the initial parton, the same jet definitions must be used, including the treatment of the underlying heavy ion background, when making data and theory comparisons. An agreement is called for between theorists and experimentalists on the appropriate treatment of the background, Monte Carlo generators that enable experimental algorithms to be applied to theoretical calculations, and a clear understanding of which observables are most sensitive to the properties of the medium, even in the presence of background. This will enable us to determine the best strategy for the field to improve quantitative constraints on properties of the medium in the face of these challenges.
Article
A revolution in elementary particle physics occurred during the period from the ICHEP1968 to the ICHEP1982 with the advent of the parton model from discoveries in Deeply Inelastic electron-proton Scattering at SLAC, neutrino experiments, hard-scattering observed in p++p collisions at the CERN ISR, the development of QCD, the discovery of the J/Ψ\Psi at BNL and SLAC and the clear observation of high transverse momentum jets at the CERN SPS pˉ+p\bar{p}+p collider. These and other discoveries in this period led to the acceptance of QCD as the theory of the strong interactions. The desire to understand nuclear physics at high density such as in neutron stars led to the application of QCD to this problem and to the prediction of a Quark-Gluon Plasma (QGP) in nuclei at high energy density and temperatures. This eventually led to the construction of the Relativistic Heavy Ion Collider (RHIC) at BNL to observe superdense nuclear matter in the laboratory. This article discusses how experimental methods and results which confirmed QCD at the first hadron collider, the CERN ISR, played an important role in experiments at the first heavy ion collider, RHIC, leading to the discovery of the QGP as a perfect liquid as well as discoveries at RHIC and the LHC which continue to the present day.
Article
Full-text available
The azimuthal width of the di-hadron correlations in p++p collisons, beyond the fragmentation transverse momentum, jTj_T, is dominated by kTk_T, the so-called intrinsic transverse momentum of a parton in a nucleon, which can be measured. The predicted azimuthal broadening in A++A collisions should produce a larger kTk_T than in p++p collisions. The present work introduces the observation that the kTk_T measured in p++p collisions for di-hadrons with pTtp_{Tt} and pTap_{Ta} must be reduced to compensate for the energy loss of both the trigger and away parent partons when comparing to the kTk_T measured with the same di-hadron pTtp_{Tt} and pTap_{Ta} in Au++Au collisions. This idea is applied to a recent STAR di-hadron measurement, with result q^L=3.5±1.4\langle{\hat{q}L}\rangle=3.5\pm 1.4 GeV2^2. This is more precise but in agreement with a theoretical calulation of q^L=1414+42\langle{\hat{q}L}\rangle=14^{+42}_{-14} GeV2^2 using the same data. Assuming a length L7\langle{L}\rangle\approx 7 fm for central Au++Au collisions the present result gives q^=0.5±0.2\hat{q}=0.5\pm 0.2 GeV2^2/fm, in fair agreement with the JET collaboration result of q^1.2±0.3\hat{q}\approx 1.2\pm 0.3 GeV2^2/fm at sNN=200\sqrt{s_{NN}}=200 GeV.
Article
A leading charged particle correlation analysis was performed on p + p data measured by the ALICE experiment at √s = 7 TeV. The main emphasis was on the xE distributions for a given charged hadron trigger momentum pTt. It was observed that dN/dxE is driven by the imbalance function at low xE. At high xE it shows exponential behavior and the extracted slope can be related to hzti of the trigger particle.
Article
Full-text available
The transverse momentum spectrum of η\eta meson in relativistic heavy-ion collisions is studied at the next-to-leading-order (NLO) within the perturbative QCD, where the jet quenching effect in the QGP is incorporated with the effectively medium-modified η\eta fragmentation functions using the higher-twist approach. We show that the theoretical simulations could give nice descriptions of PHENIX data on η\eta meson in both p+p\rm p+p and central Au+Au\rm Au+Au collisions at the RHIC, and also provide numerical predictions of η\eta spectra in central Pb+Pb\rm Pb+Pb collisions with sNN=2.76\sqrt{s_{NN}}=2.76~TeV at the LHC. The ratios of η/π0\eta/\pi^0 in p+p\rm p+p and in central Au+Au\rm Au+Au collisions at 200200~GeV are found to overlap in a wide pTp_T region, which matches well the measured ratio η/π0\eta / \pi^0 by PHENIX. We demonstrate that, at the asymptotic region when pTp_{T} \rightarrow \infty the ratios of η/π0\eta/\pi^{0} in both Au+Au\rm Au+Au and p+p\rm p+p are almost determined only by quark jets fragmentation and thus approach to the one in e+ee^{+} e^{-} scattering; in addition, the almost identical gluon (quark) contribution fractions to η\eta and to π\pi result in a rather moderate variation of η/π0\eta/\pi^{0} distribution at intermediate and high pTp_T region in A+A\rm A+A relative to that in p+p\rm p+p; while a slightly higher η/π0\eta/\pi^{0} at small pTp_T in Au+Au\rm Au+Au can be observed due to larger suppression of gluon contribution fraction to π0\pi^{0} as compared to the one to η\eta. The theoretical prediction for η/π0\eta / \pi^0 at the LHC has also been presented.
Article
Full-text available
The production of direct photons in pp collisions via Compton scattering and q annihilation leading order processes is of great interest to test pQCD predictions and also to study parton fragmentation. The fragmentation of the recoil scattered parton can be studied using correlation between the photon and the charged particles emitted in the opposite direction. We present the study of the parton fragmentation in pp collisions at = 7 TeV measuring the imbalance parameter . Furthermore, a first uncorrected direct photon spectrum measured with the ALICE electromagnetic calorimeter (EMCal) is shown. These measurements open the way to a straightforward comparison between pp and Pb-Pb collisions concerning parton fragmentation and direct photon production.
Article
Full-text available
The many different theoretical studies of energy loss of a quark or gluon traversing a medium have one thing in common: the transport coefficient of a gluon in the medium, q^\hat{q}, which is defined as the mean 4-momentum transfer2^2, <q2>\left<q^2\right>, by a gluon to the medium per gluon mean free path, λmfp\lambda_{\rm mfp}. In the original BDMPSZ formalism, the energy loss of an outgoing parton, dE/dx-dE/dx, per unit length (xx) of a medium with total length LL, due to coherent gluon bremsstrahlung, is proportional to the <q2>\left< q^2\right> and takes the form: dE/dxαs<q2(L)>=αsμ2L/λmfp=αsq^L {-dE/dx }\simeq \alpha_s \left<{q^2(L)}\right>=\alpha_s\, \mu^2\, L/\lambda_{\rm mfp} =\alpha_s\, \hat{q}\, L\ , where μ\mu, is the mean momentum transfer per collision. Thus, the total energy loss in the medium goes like L2L^2. Additionally, the accumulated momentum2^2, <k2>\left<{k_{\perp}^2}\right>, transverse to a gluon traversing a length LL in the medium is well approximated by <k2><q2(L)>=q^L\left<{k_{\perp}^2}\right>\approx\left<{q^2(L)}\right>=\hat{q}\, L. A simple estimate shows that the <k2>q^L\left<{k_{\perp}^2}\right>\approx\hat{q}\,L should be observable at RHIC at sNN=200\sqrt{s_{NN}}=200 GeV via the broadening of di-hadron azimuthal correlations resulting in an azimuthal width 2\sim\sqrt{2} larger in Au++Au than in p+pp+p collisions . Measurements relevant to this issue will be discussed as well as recent STAR jet results presented at QM2014. Other topics to be discussed include the danger of using forward energy to define centrality in p(d)+p(d)+A collisions for high pTp_T measurements, the danger of not using comparison p+pp+p data at the same s\sqrt{s} in the same detector for RAAR_{AA} or lately for RpAR_{pA} measurements.
Article
Full-text available
Direct photon tagged jets, in the form of photon-hadron correlations, are well suited to provide unique insight into how jets interact with the quark gluon plasma. Since photons do not interact strongly with medium produced at RHIC, at leading order the measured photon momentum approximately balances that of the away-side parton. The modification to the effective fragmentation function can be measured by comparing integrated away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. By varying the away-side integration range, the angular dependence of the observed modification can also be studied. Direct photon-hadron correlations have been measured with PHENIX in p+p and Au+Au using a statistical subtraction technique to remove the decay photon contribution from the inclusive photon-hadron correlations, with an additional isolation cut applied in p+p to reduce uncertainties. Recently published results showing a significant modification to the fragmentation function with an azimuthal angle dependence indicative of broadening are discussed in the context of related theoretical models and complementary results from other experiments.
Article
Jet-medium interactions are studied via a multihadron correlation technique (called “2+1”), where a pair of back-to-back hadron triggers with large transverse momentum is used as a proxy for a di-jet. This work extends the previous analysis for nearly symmetric trigger pairs with the highest momentum threshold of trigger hadron of 5 GeV/c with the new calorimeter-based triggers with energy thresholds of up to 10 GeV and above. The distributions of associated hadrons are studied in terms of correlation shapes and per-trigger yields on each trigger side. In contrast with di-hadron correlation results with single triggers, the associated hadron distributions for back-to-back triggers from central Au+Au data at sNN=200 GeV show no strong modifications compared to d+Au data at the same energy. An imbalance in the total transverse momentum between hadrons attributed to the near-side and away-side of jetlike peaks is observed. The relative imbalance in the Au+Au measurement with respect to d+Au reference is found to increase with the asymmetry of the trigger pair, consistent with the expectation from medium-induced energy-loss effects. In addition, this relative total transverse momentum imbalance is found to decrease for softer associated hadrons. Such evolution indicates that the energy missing at higher associated momenta is converted into softer hadrons.
Article
Full-text available
In the standard pQCD picture particles are produced via the parton jet fragmentation process. However, there are also other production mechanisms like higher twist (HT) processes. A usual example of a HT process is a direct production of an outgoing hadron, where the hadron is produced in the hard subprocess without fragmentation. We study the HT phenomena using a shape analysis (xT scaling) of the inclusive invariant cross sections of charged hadrons, measured by the ALICE collaboration at center-of-mass energies = 2.76 TeV and 7 TeV. The data is compared to PYTHIA8 event generator and to a phenomenological model for HT. Using PYTHIA8, we explore a possible enhancement of HT phenomena for isolated particles, by comparing the shapes of the isolated distributions to inclusive distributions. The results from the standard PYTHIA8, without HT, is compared to a PYTHIA8 where we had included a HT process. Finally, we found out that the effects observed in the xT spectra originate from kinematic biases posed by the isolation cuts, rather than from an enrichment of the HT hadrons at the observed cross sections. A more detailed data analysis is ongoing.
Article
Full-text available
The analysis is presented for the first moments of collective observable distributions in two-jet events for various interaction types and for wide initial energy range. These observables include sphericity, thrust, components of transverse particle momentum, alignment and planarity. Database of experimental results created in the framework of the paper includes data for all interactions. Energy dependencies of average values for collective observables except components of transverse momentum show universal behavior for various interactions. Particle transverse momentum as well as its components increase faster for e⁺e⁻ interaction with growth of s\sqrt{s}, than that for other interactions. Empirical analytical functions are suggested for description of energy dependence for all collective observables under study with exception of infrared-stable thrust variable. Energy dependence for average thrust is compared with QCD predictions including perturbative part and analytical phenomenological corrections which account for nonperturbative effects. Dispersive model and single dressed gluon approximation are considered for description of energy dependence of first moment of thrust distribution and estimation of strong coupling constant for various interactions as well as for joint sample. The dispersive model allow to describe average thrust versus initial energy in wide range of s\sqrt{s} down to strongly nonperturbative domain s23 GeV\sqrt{s} \sim 2\hbox{--}3~{\rm GeV} at qualitative level at least. Study of event shape observables allows to obtain estimations of αS(MZ) which are in reasonable agreement both with world average value and with results extracted in the framework of other methods. Using suggested analytical fitted functions some estimations of values of collective parameters under study have been obtained for present and future facilities. In TeV energy domain average values of collective observables either depend on s\sqrt{s} weakly or do not depend on initial energy at all within errors. Thus, the TeV scale can be considered as an estimation of the low boundary of asymptotic region for traditional collective parameters. Usually, multiplicity dependence of collective observables under consideration agree with power function in energy domain s<12 GeV\sqrt{s} < 12~{\rm GeV} at qualitative level at least. Behavior of sphericity versus multiplicity and comparison of experimental results with model calculations allow to suggest that the universal estimation of the low energy boundary for experimental appearance of event jet structure in multiparticle production processes is sc3 GeV\sqrt{s_{c}} \sim 3~{\rm GeV}.
Article
Hard-scattering of point-like constituents (or partons) in p-p collisions was discovered at the CERN-ISR1 in 1972 by measurements utilizing inclusive single or pairs of hadrons with large transverse momentum (pT). Due to the steeply falling power-law pT spectrum of the hard-scattered partons, the inclusive single particle (e.g. π0) pTt spectrum from parton fragmentation to a jet is dominated by trigger fragments with large zt ~ 0.7–0.8, where zt = pTt/pTjet is the fragmentation variable. It was generally assumed, following Feynman, Field and Fox,2 as shown by data from the CERN-ISR experiments, that the pTa distribution of away side hadrons from a single particle trigger [with pTt], corrected for zt, would be the same as that from a jet-trigger and follow the same fragmentation function as observed in e+e− or DIS. PHENIX3 attempted to measure the fragmentation function from the away side xE ~ PTa/pTt distribution of charged particles triggered by a π0 in p−p collisions at RHIC and showed by explicit calculation that the xE distribution is actually quite insensitive to the fragmentation function. Illustrations of the original arguments and ISR results will be presented. Then the lack of sensitivity to the fragmentation function will be explained, and an analytic formula for the xE distribution given, in terms of incomplete Gamma functions, for the case where the fragmentation function is exponential. The away-side distribution in this formulation has the nice property that it both exhibits xE scaling and is directly sensitive to the ratio of the away jet to that of the trigger jet, , and thus can be used, for example, to measure the relative energy loss of the two jets from a hard-scattering which escape from the medium in A+A collisions. Comparisons of the analytical formula to RHIC measurements will be presented, including data from STAR4,5 and PHENIX,3,6 leading to some interesting conclusions.
Article
Full-text available
Highlights from Brookhaven National Laboratory (BNL) and experiments at the BNL Relativistic Heavy Ion Collider (RHIC) are presented for the years 2011–2013. This review is a combination of lectures which discussed the latest results each year at a three year celebration of the 50th anniversary of the International School of Subnuclear Physics in Erice, Sicily, Italy. Since the first collisions in the year 2000, RHIC has provided nucleus–nucleus and polarized proton–proton collisions over a range of nucleon–nucleon center-of-mass energies (sNN)(\sqrt{s_{NN}}) from 7.7 GeV to 510 GeV with nuclei from deuterium to uranium, most often gold. The objective was the discovery of the Quark Gluon Plasma, which was achieved, and the measurement of its properties, which were much different than expected, namely a "perfect fluid" of quarks and gluons with their color charges exposed rather than a gas. Topics including quenching of light and heavy quarks at large transverse momentum, thermal photons, search for a QCD critical point as well as measurements of collective flow, two-particle correlations and J/Ψ suppression are presented. During this period, results from the first and subsequent heavy ion measurements at the Large Hadron Collider (LHC) at CERN became available. These confirmed and extended the RHIC discoveries and have led to ideas for new and improved measurements.
Article
Full-text available
At RHIC, a suppression, RAA0.2R_{AA}\approx 0.2 relative to binary-scaling, for π0\pi^0 with 5pT205\leq p_T\leq 20 GeV/c was discovered in central Au+Au collisions at sNN=200\sqrt{s_{NN}}=200 GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at sNN=2.76\sqrt{s_{NN}}=2.76 TeV. In this pTp_T range, the LHC results for pions nearly overlap the RHIC results but the flatter spectrum at LHC implies that the energy loss in the medium must be 40\sim 40% larger than at RHIC. At LHC, the unique and beautiful measurement of the fractional transverse momentum imbalance 1p^T2/p^T11-\langle{\hat{p}_{T_2}/\hat{p}_{T_1}}\rangle of di-jets in Pb+Pb collisions relative to p-p collisions, shows 15%\approx 15\% for jets with 120p^T1360120\leq\hat{p}_{T_1}\leq 360 GeV/c. This is a much smaller fractional jet imbalance than the 45%\approx 45\% derived from two-particle correlations of di-jet fragments at RHIC corresponding to jet p^T1020\hat{p}_T\approx 10-20 GeV/c. This presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a q^\hat{q} effect by observation of momentum transferred to the medium by outgoing partons. An implied hard scattering component for the soft physics multiplicity distributions in A+A collisions based on a popular formula, dNchAA/dη=[(1x)NpartdNchpp/dη/2+xNcolldNchpp/dη]{dN_{\rm ch}^{AA}/d\eta}= [(1-x) \langle{N_{\rm part}}\rangle {dN_{\rm ch}^{pp}/d\eta}/2 + x\, \langle{N_{\rm coll}}\rangle {dN_{\rm ch}^{pp}/d\eta}], seems to be an unphysical way to understand the deviation from NpartN_{\rm part} scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.
Article
We review results of PHENIX two-particle correlations for both di-hadrons and direct photon hadron pairs. These measurements can reveal details of QGP induced jet energy loss in heavy ion collisions. The spectra of per-trigger yields on the awayside from trigger photons in the photon-hadron correlations or fragmentation function for direct photons of greater than 5 GeV/c in transverse momentum appear to show some modification compared to the same measurement in p+p collisions, including a suppression at high z (> ~0.4), and much less suppression, possibly even enhancement at low z. A framework for calculating parton or jet level observables from the PHENIX 2-particle yields results that can be compared to recent LHC jet reconstruction observables. Under certain assumptions, one of these observables, the mean fractional energy loss, appears to be larger for lower jet energies (~10 GeV) that the PHENIX data is sensitive to, than for much higher energy jets (~100 GeV) at the LHC. However the current PHENIX di-hadron measurements for this comparison which may have inherent biases need to be followed up by higher statistics direct photon-hadron measurements before definite statements could be made.
Article
Full-text available
Jet quenching has been observed at both RHIC and LHC energies, indicating that partons lose energy as they traverse the medium. To probe the effects of this partonic energy loss, measurements of the angular correlations between fully reconstructed jets and charged tracks in Pb-Pb collisions are studied. Fully reconstructing a jet provides access to the kinematics of the initial hard scattering while allowing us to study the distribution of hadrons on the away side from the modified recoil jet. Here we present first measurements of jet-hadron correlations in pp collisions at =2.76 TeV and an outlook for Pb-Pb collisions. The jets in this analysis are reconstructed from the 2011 data set using both charged tracks and neutral energy measured in the ALICE tracking system and the electromagnetic calorimeter respectively.
Article
The novel phenomena observed in particle angular correlations are reviewed. They include the double-peak away-side azimuthal correlations in relativistic heavy-ion collisions and the long-range pseudorapidity near-side (ridge) correlations in heavy-ion as well as in proton-induced collisions. The collision system and energy dependence of these phenomena are examined, wherever possible and most abundantly for the ridge correlations. Their possible theoretical interpretations and what might be learned about the properties of the collision systems from theoretical comparisons are discussed. Prospective future measurements and theoretical undertakings are outlined that might help further the understanding of the physics mechanisms underlying these phenomena.
Article
Hard-scattering in p-p collisions was discovered in 1972 at the CERN-ISR, the first hadron collider. Techniques were developed and several hard-processes were discovered which form the basis for many of the measurements made in p-p and Au+Au collisions at RHIC. Recent measurements of hard-scattering and related reactions by the PHENIX experiment at RHIC are presented in this context.
Article
Angular correlations involving energetic particles associated with partonic jet fragmentation provide an important opportunity to study the hot nuclear matter produced in A+ A collisions, particularly when compared against a p+ p reference. Recent results from di-hadron correlations suggest that jet observables from A+ A and p+ p differ considerably at low p T but approach comparable values as p T increases above 7 GeV/ c, providing clues on the nature of partonic energy loss and the medium response. The most recent pi 0-triggered jet correlation results from the PHENIX experiment are presented, as well as a quantitative study of the jet-peak widths and shapes.
Article
In the Relativistic Heavy Ion Collider era the high-pT particle emerging from hard scattering became an important tool of exploration of excited nuclear medium formed in ultra-relativistic heavy ion collisions. Discovery of anomalous suppression of high-pT particle yield [1] together with an observation of disappearance of back-to-back hadron correlation in central Au+Au collisions in 2002 [2] were the key results interpreted as a manifestation of the deconfined QCD medium in heavy ion collisions. Analysis of the high-pT particle and jet production is already a standard experimental technique providing a test bench for pQCD description of the point-like constituent scattering in p+p collisions and sensitive probes of the excited nuclear medium in A+A collisions.
Article
The hard scattering processes appear as back to back jets in pp collisions. The effective transverse momentum of the two hard scattered partons (kt) arises due to the intrinsic transverse momentum of the partons and its broadening. Azimuthal correlations of the neutral clusters (mostly from pi0) with transverse energy Et=6.5-18.5 GeV as trigger particles and the charged tracks as associated particles have been measured by the STAR experiment in pp and dAu collisions at &surd; {sNN}= 200 {GeV}. Using the di-hadron correlation technique the effect of the cold nuclear matter on &surd; {kt2} is discussed. A simulation study from PYTHIA using simple di-jet like clustering is also discussed in context of results of &surd; {kt2} from di-hadron correlations and di-jets.
Article
In the first LHC 2009 and 2010 runs the ALICE experiment collected about 10 μb−1 at s=900GeV and more than 20 nb1 at s=7TeV bench for the pQCD description of hard scattering phenomena at the highest center of mass energy p + p collisions ever recorded. These data are used to study the universality of the charged hadron invariant yields measured by the ALICE experiment in terms of xT scaling. Leading particle associated yield is used to study the angular correlations. A comparison of per-trigger yield in s=900GeV and s=7TeV data samples is presented. An increasing role of the soft QCD radiation is deduced from the slope of the associated particle yield in the region perpendicular to the leading particle.
Article
The energy loss of partons leaving the hot media created in the heavy ion collisions has been at the center of the interest of the experimental and theoretical communities of the field. Hence an increase in the transverse momentum imbalance and acoplanarity of dijets may be a way to study the parton energy loss. In order to study the effect, we have made simulations of dijet events where the intrinsic k T of the partons, the initial and final state radiation are taken into account. The results of these simulations for p + p and Pb + Pb collisions, as a function of the center of mass energy of collision will be presented.
Article
Full-text available
We compare results of the kt-factorization approach and the next-to-leading-order collinear-factorization approach for dijet correlations in proton-proton collisions at RHIC energies. We discuss correlations in azimuthal angle as well as correlations in two-dimensional space of transverse momenta of two jets. Some kt-factorization subprocesses are included for the first time in the literature. Different unintegrated gluon/parton distributions are used in the kt-factorization approach. The results depend on unintegrated gluon distribution functions (UGDF)/unintegrated parton distribution function (UPDF) used. For the collinear next-to-leading order (NLO) case, the situation depends significantly on whether we consider correlations of any two jets or correlations of leading jets only. In the first case, the 2-->2 contributions associated with soft radiations summed up in the kt-factorization approach dominate at varphi˜pi and at equal moduli of jet transverse momenta. The collinear NLO 2-->3 contributions dominate over kt-factorization cross section at small relative azimuthal angles as well as for asymmetric transverse-momentum configurations. In the second case, the NLO contributions vanish at small relative azimuthal angles and/or large jet transverse-momentum disbalance due to simple kinematical constraints. There are no such limitations for the kt-factorization approach. All this makes the two approaches rather complementary. The role of several cuts is discussed and quantified.
Article
The suppression of high transverse momentum (P{sub T}) inclusive hadron spectra in heavy-ion collisions as compared to the scaled expectation of high P{sub T} hadron production in p-p collisions is the most direct manifestation of the interaction of hard partons with the soft bulk medium produced in heavy-ion collisions which is absent in p-p collisions. Yet the measured nuclear suppression factor R{sub AA} is a very averaged quantity and hence only a limited amount of information about the medium evolution and the nature of the interaction with the medium can be deduced from R{sub AA}. Measurements of hard back-to-back hadron correlations in 200A GeV Au-Au collisions at RHIC have opened a new window to study the energy loss of partons in a medium in a more differential way and for a different distribution of in-medium path lengths than in the case of R{sub AA}. In this work, we present an extrapolation of our results for back-to-back yields at RHIC energies to 5.5 TeV Pb-Pb collisions at the CERN LHC. We also discuss differences and similarities between the measurement at RHIC.
Article
Full-text available
During the first year of data taking ALICE has collected large data samples from pp collisions at the LHC (s=900 GeV and 7 TeV). They allowed us to have a first look at high-pT phenomena in a new energy regime. We give an overview of first results from the study of jet-like correlations and neutral meson production.
Article
Analysis of the leading particle correlations in p+p data taken by the ALICE experiment at centre of mass energies s = 0.9 and 7 TeV is presented. Per trigger normalized associated particle yield as a function of pair relative azimuthal angle Δϕ is used to determine the jet-fragmentation transverse momentum jT. The resulting values are compared to similar measurements at lower centre of mass energies done at RHIC and ISR. Furthermore we study the distribution of the transverse momentum component of associated particles with respect to leading particle, pout. This quantity is related to di-jet acoplanarity. We present an analytic formula which estimates a background component of measured pout coming from uncorrelated pairs.
Article
We compare results of the kt-factorization approach and the next-to-leading order collinear-factorization approach for photon-jet correlations in pp and pp̅ collisions at Relativistic Heavy Ion Collider, Tevatron, and Large Hadron Collider energies. We discuss correlations in the azimuthal angle as well as in the two-dimensional space of transverse momentum of photon and jet. Different unintegrated parton distributions (UPDF) are included in the kt-factorization approach. The results depend on UPDFs used. The standard collinear approach gives a cross section comparable to the kt-factorization approach. For correlations of the photon and any jet the next-to-leading order (NLO) contributions dominate at relatively small azimuthal angles as well as for asymmetric transverse momenta. For correlations of the photon with the leading jet (the one having the biggest transverse momentum) the NLO approach gives zero contribution at ϕ-<π/2, which opens a possibility to study higher-order terms and/or UPDFs in this region.
Article
Azimuthal angle (Δϕ) correlations are presented for a broad range of transverse momentum (0.4<pT<10 GeV/c) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at √sNN=200 GeV. With increasing pT, the away-side Δϕ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p+p data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at Δϕ~π, and an enhanced “shoulder” region centered at Δϕ~π±1.1. The pT spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger pT. The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in Δϕ and Δη, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to pT≲4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for pT≳5 GeV/c and a medium-induced component which is important for pT≲4 GeV/c. We also quantify the role of jets at intermediate and low pT through the yield of jet-induced pairs in comparison with binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the pT magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower pT.
Article
The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at √s=200 GeV, for inclusive π0 production at midrapidity. Unpolarized cross section results are given for transverse momenta pT=0.5 to 20 GeV/c, extending the range of published data to both lower and higher pT. The cross section is described well for pT<1 GeV/c by an exponential in pT, and, for pT>2 GeV/c, by perturbative QCD. Double helicity asymmetries ALL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.
Article
Full-text available
Inclusive transverse momentum spectra of η mesons in the range pT≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at √sNN=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π0π+π- decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, RdAu(pT)≈1.0–1.1, suggests at most only modest pT broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with RAuAu(pT)≈0.2. The ratio of η to π0 yields is approximately constant as a function of pT for the three colliding systems in agreement with the high-pT world average of Rη/π0≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (√sNN≈3–1800 GeV) as well as, for high scaled momentum xp, in e+e- annihilations at √s=91.2 GeV. These results are consistent with a scenario where high-pT η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π0,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.
Article
Transverse momentum spectra of electrons (pTe) from semileptonic weak decays of heavy-flavor mesons in the range of 0.3<pTe<9.0 GeV/c have been measured at midrapidity (|y|<0.35) by the PHENIX experiment at the Relativistic Heavy Ion Collider in p+p and Au+Au collisions at √sNN=200 GeV. In addition, the azimuthal anisotropy parameter v2 has been measured for 0.3<pTe<5.0 GeV/c in Au+Au collisions. The substantial modification in the pTe spectra in Au+Au compared with p+p collisions as well as the nonzero v2 indicate substantial interactions and flow of heavy quarks in traversing the produced medium. Comparisons of these observables with detailed theoretical calculations can be used to identify the nature of these interactions and to quantify their extent.
Article
Measurements of electrons from the decay of open-heavy-flavor mesons have shown that the yields are suppressed in Au+Au collisions compared to expectations from binary-scaled p+p collisions. These measurements indicate that charm and bottom quarks interact with the hot dense matter produced in heavy-ion collisions much more than expected. Here we extend these studies to two-particle correlations where one particle is an electron from the decay of a heavy-flavor meson and the other is a charged hadron from either the decay of the heavy meson or from jet fragmentation. These measurements provide more detailed information about the interactions between heavy quarks and the matter, such as whether the modification of the away-side-jet shape seen in hadron-hadron correlations is present when the trigger particle is from heavy-meson decay and whether the overall level of away-side-jet suppression is consistent. We statistically subtract correlations of electrons arising from background sources from the inclusive electron-hadron correlations and obtain two-particle azimuthal correlations at √sNN=200 GeV between electrons from heavy-flavor decay with charged hadrons in p+p and also first results in Au+Au collisions. We find the away-side-jet shape and yield to be modified in Au+Au collisions compared to p+p collisions.
Article
Correlations of charged hadrons of 1<pT<10 Gev/c with high pT direct photons and π0 mesons in the range 5<pT<15 Gev/c are used to study jet fragmentation in the γ+jet and dijet channels, respectively. The magnitude of the partonic transverse momentum, kT, is obtained by comparing to a model incorporating a Gaussian kT smearing. The sensitivity of the associated charged hadron spectra to the underlying fragmentation function is tested and the data are compared to calculations using recent global fit results. The shape of the direct photon-associated hadron spectrum as well as its charge asymmetry are found to be consistent with a sample dominated by quark-gluon Compton scattering. No significant evidence of fragmentation photon correlated production is observed within experimental uncertainties.
Article
Full-text available
We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, IAA, in central Au+Au collisions, is 0.32±0.12stat±0.09syst for hadrons of 3<pTh<5 in coincidence with photons of 5<pTγ<15 GeV/c. The suppression is comparable to that observed for high-pT single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, zT≡pTh/pTγ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.
Article
Full-text available
We calculate the distribution of secondary particles C in processes A+B-->C+anything at very high energies when (1) particle C has transverse momentum pT far in excess of 1 GeV/c, (2) the basic reaction mechanism is presumed to be a deep-inelastic electromagnetic process, and (3) particles A, B, and C are either leptons (l), photons (γ), or hadrons (h). We find that such distribution functions possess a scaling behavior, as governed by dimensional analysis. Furthermore, the typical behavior even for A, B, and C all hadrons, is a power-law decrease in yield with increasing pT, implying measurable yields at NAL of hadrons, leptons, and photons produced in 400- GeV pp collisions even when the observed secondary-particle pT exceeds 8 GeV/c. There are similar implications for particle yields from e+/--e- colliding-beam experiments and for hadron yields in deep-inelastic electro-production (or neutrino processes). Among the processes discussed in some detail are ll-->h, γγ-->h, lh-->h, γh-->h, γh-->l, as well as hh-->l, hh-->γ, hh-->W, and W-->h, where W is the conjectured weak-interaction intermediate boson. The basis of the calculation is an extension of the parton model. The new ingredient necessary to calculate the processes of interest is the inclusive probability for finding a hadron emerging from a parton struck in a deep-inelastic collision. This probability is taken to have a form similar to that generally presumed for finding a parton in an energetic hadron. We study the dependence of our conclusions on the validity of the parton model, and conclude that they follow mainly from kinematics, duality arguments ala Bloom and Gilman, and the crucial assumption that multiplicities in such reactions grow slowly with energy. The picture we obtain generalizes the concept of deep-inelastic process, and predicts the existence of "multiple cores" in such reactions. We speculate on the possibility of strong, nonelectromagnetic deep-inelastic processes. If such processes exist, our predictions of particle yields for hh-->h could be up to 4 orders of magnitude too low, and for γh-->h and hh-->γ up to 2 orders of magnitude too low.
Article
Full-text available
The invariant differential cross section for inclusive neutral-pion production in [Formula presented] collisions at [Formula presented] has been measured at midrapidity ([Formula presented]) over the range [Formula presented] by the PHENIX experiment at the Relativistic Heavy Ion Collider. Predictions of next-to-leading order perturbative QCD calculations are consistent with these measurements. The precision of our result is sufficient to differentiate between prevailing gluon-to-pion fragmentation functions.
Article
Full-text available
Since large-pT particles in high-energy hadronic or nuclear collisions come from jet fragmentation, jet quenching due to parton energy loss in dense matter will cause the suppression of large-pT hadron spectra in high-energy heavy-ion collisions. Assuming an effective energy loss dE/dx for the high-ET partons, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed. Using such effective fragmentation functions, high-pT hadron spectra and particle suppression factors relative to pp collisions are estimated in central high-energy nuclear collisions with a given range of the assumed dE/dx. It is found that the suppression factors are very sensitive to the value of the effective energy loss. Systematic nuclear and flavor dependence of the hadron spectra are also studied.
Article
Full-text available
Symmetric three-jet events are selected from hadronic Z0 decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For thek ⊥ jet finder withy cut=0.02, we find\begin{gathered} \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle b quark}} = 1.089 \pm 0.024(stat.) \pm 0.024(syst.) \hfill \\ \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle uds quark}} = 1.390 \pm 0.038(stat.) \pm 0.032(syst.) \hfill \\ \end{gathered} as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.
Article
Full-text available
A study has been made of the production characteristics of W and Z bosons produced at the CERN p Collider. The event sample consists of 251 W→ev decays, and 39 Z→e+e− decays, identified by the UA2 detector, and corresponds to an integrated luminosity of 142 nb−1 at √s = 546 GeV, and 768 nb−1 at √s = 630 GeV. Measurements of W and Z production cross sections, of longitudinal and transverse momentum distributions, and of associated jet production, are presented. These measurements are compared with theoretical expectations, which include higher order QCD effects.
Article
Full-text available
We extend the Gyulassy–Levai–Vitev reaction operator approach to multiple elastic scattering of fast partons traversing dense nuclear matter to take into account the leading power corrections due to the medium recoil and to derive the change in the partons' longitudinal momentum. We employ a boost-invariant formalism to generalize previous treatments of the problem, which were specific to the target rest frame. We apply the transverse momentum diffusion results in a simple analytic model to evaluate the broadening of the back-to-back di-hadron correlation function in d+Au reactions.
Article
Full-text available
The observed suppression of high-pT hadron spectra, finite azimuthal anisotropy, disappearance of jet-like back-to-back correlations, and their centrality dependence in Au+Au collisions at RHIC are shown to be quantitatively described by jet quenching within a pQCD parton model. The difference between h± and π0 suppression in intermediate pT is consistent with the observed (K+p)/π enhancement which should disappear at pT>6 GeV/c. The suppression of back-to-back correlations is shown to be directly related to the medium modification of jet fragmentation functions (FF) similar to direct-photon triggered FF's.
Article
Full-text available
The structure of hadron–hadron correlations is investigated in proton–proton (pp) collisions. We focus on the transmission of the initial transverse momenta of partons (“intrinsic kT”) to the hadron–hadron correlations. Values of the intrinsic transverse-momentum obtained from experimental correlations are compared to the results of a model with partially randomized parton transverse momenta at ISR and RHIC energies. Procedures for extracting the correlations from data are discussed.
Article
Full-text available
Azimuthal anisotropy (v2v_2) and two-particle angular correlations of high pTp_T charged hadrons have been measured in Au+Au collisions at sNN\sqrt{s_{NN}}=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high pTp_T partons. The monotonic rise of v2(pT)v_2(p_T) for pT<2p_T<2 GeV/c is consistent with collective hydrodynamical flow calculations. At \pT>3 GeV/c a saturation of v2v_2 is observed which persists up to pT=6p_T=6 GeV/c.
Article
Full-text available
The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the DELPHI detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy \mbox{e}^+\mbox{e}^- annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio CA/CF is measured to be: \begin{eqnarray*}\frac{C_A}{C_F} = 2.26 \pm 0.09_{stat.} \pm 0.06_{sys.} \pm 0.12_{clus.,scale}\, . \end{eqnarray*}
Article
Full-text available
It is shown that if, in a calculation of high-transverse-momentum (p⊥) meson production in hadron-hadron collisions, one includes not only the scale-breaking effects that might be expected from asymptotically free theories but also the effects due to the transverse momentum of quarks in hadrons and further adds contributions from quark-gluon and gluon-gluon scattering to those of quark-quark scattering then the results are not inconsistent with the data. The approach yields the correct magnitude and an apparent approximate 1/p⊥8 behavior in accord with single-particle data for the energy range currently observed. Two-particle correlations are examined. Because of scale-breaking effects and the presence of gluons, the theory does not have the problem of predicting too many away-side hadrons at large p⊥ as did an earlier quark-quark scattering "black-box" approach. We conclude that the quantum-chromodynamics approach is in reasonable accord with the data although theoretical uncertainties (especially at low p⊥) make incontrovertible conclusions impossible at present. Crucial tests of the theory require higher p⊥ than are now available; estimates for this region are made.
Article
Full-text available
The transverse momentum distributions are studied for inclusive proton-proton interactions. The results are interpreted in terms of the parton model and quantum chromodynamics. (AIP)
Article
Full-text available
Inclusive transverse momentum distributions of charged hadrons within 0.2<p(T)<6.0 GeV/c have been measured over a broad range of centrality for Au+Au collisions at sqrt[s(NN)]=130 GeV. Hadron yields are suppressed at high p(T) in central collisions relative to peripheral collisions and to a nucleon-nucleon reference scaled for collision geometry. Peripheral collisions are not suppressed relative to the nucleon-nucleon reference. The suppression varies continuously at intermediate centralities. The results indicate significant nuclear medium effects on high-p(T) hadron production in heavy-ion collisions at high energy.
Article
Full-text available
Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudorapidity range and full azimuth in Au+Au and p+p collisions at sqrt[s(NN)]=200 GeV. The small-angle correlations observed in p+p collisions and at all centralities of Au+Au collisions are characteristic of hard-scattering processes previously observed in high-energy collisions. A strong back-to-back correlation exists for p+p and peripheral Au+Au. In contrast, the back-to-back correlations are reduced considerably in the most central Au+Au collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.
Article
Full-text available
We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high p(T)) in minimum bias and central d+Au collisions at sqrt[s(NN)]=200 GeV. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, while the two-particle azimuthal distributions are very similar to those observed in p+p collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high p(T) previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.
Article
Full-text available
We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25<p(T)<6.0 GeV/c, in a pseudorapidity range of 0.2<eta<1.4 in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to p+pmacr; collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in Au+Au collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-p(T) hadrons observed in Au+Au collisions.
Article
Full-text available
Transverse momentum spectra of neutral pions in the range 1<p(T)<10 GeV/c have been measured at midrapidity by the PHENIX experiment at BNL RHIC in Au+Au collisions at sqrt[s(NN)]=200 GeV. The pi(0) multiplicity in central reactions is significantly below the yields measured at the same sqrt[s(NN)] in peripheral Au+Au and p+p reactions scaled by the number of nucleon-nucleon collisions. For the most central bin, the suppression factor is approximately 2.5 at p(T)=2 GeV/c and increases to approximately 4-5 at p(T) approximately 4 GeV/c. At larger p(T), the suppression remains constant within errors. The deficit is already apparent in semiperipheral reactions and increases smoothly with centrality.
Article
The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy e+e{\rm e}^+{\rm e}^- annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio CA/CFC_A/C_F is measured to be:
Article
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured charged hadron yields at midrapidity over a wide range of transverse momenta (0.5<pT<10GeV∕c) in Au+Au collisions at sNN=200GeV. The data are compared to π0 measurements from the same experiment. For both charged hadrons and neutral pions, the yields per nucleon-nucleon collision are significantly suppressed in central compared to peripheral and nucleon-nucleon collisions. The suppression sets in gradually and increases with increasing centrality of the collisions. Above 4–5GeV∕c in pT, a constant and almost identical suppression of charged hadrons and π0’s is observed. The pT spectra are compared to published spectra from Au+Au at sNN=130 in terms of xT scaling. Central and peripheral π0 as well as peripheral charged spectra exhibit the same xT scaling as observed in p+p data.
Article
Measurements of the invariant cross section Ed3σd3p are presented for the production of hadrons (π, K, p, and p¯) at large transverse momentum (p⊥) by 200-, 300-, and 400-GeV protons incident on H2, D2, Be, Ti, and W targets. The measurements were made at a laboratory angle of 77 mrad, which corresponds to angles near 90° in the c.m. system of the incident proton and a single nucleon at rest. The range in p⊥ for the data is 0.77<=p⊥<=6.91 GeV/c, corresponding to values of the scaling variable x⊥=2p⊥s from 0.06 to 0.64. For p-p collisions, the pion cross sections can be represented in the region x⊥>35 by the form (1p⊥n)(1-x⊥)b, with n=8 and b=9. The ratio of π+ to π- production grows as a function of x⊥ to a value larger than 2 at x⊥>~0.5. The ratios of the production of K+ and protons to π+ and of K- and antiprotons to π- also scale with x⊥ for p-p collisions. The K+/-, p, and p¯ fitted values for n and b are given. Particle ratios are also presented for D2, Be, Ti, and W targets and the dependences on atomic weight (A) are discussed.
Article
The shape of jets produced in (quasi-) real photon-photon collisions as well as in e+e- annihilation process has been studied with a cone jet finding algorithm, using the data taken with the TOPAZ detector at the TRISTAN e+e- collider at an average center-of-mass energy (see) of 58 GeV. The results are presented in terms of the jet width as a function of the jet transverse energy (ETjet) as well as a scaled transverse jet energy, xT (=2ETjet/sqrt(s)). The jet width narrows as EjetT increases; however, at the same value of ETjet the jet width in gammagamma collisions at TRISTAN is significantly narrower than that in gammap collisions at HERA. By comparing our results with the data in other reactions, it has been shown that the jet width in gammagamma, gammap, pp¯ collisions as well as the e+e- annihilation process has an approximate scaling behavior as a function of xT.
Article
A measurement of the direct production of photons with high transverse momentum from pˉp\bar pp collisions at s=630\sqrt s = 630 GeV is presented. The structure of events containing a high transverse momentum photon is studied. The results support predictions from QCD theory.
Article
Invariant cross-sections are presented for the inclusive reaction p + p → πo + anything, Measurements of large transverse momentum πo's (2.5 GeV/c<p⊥<9 GeV/c) were made near 90° at the CERN ISR at five centre-of-mass energies (√s = 23.5, 30.6, 44.8, 52.7 and 62.4 GeV. At large p⊥, the invariant cross-sections are seem to vary with s and p⊥, in good agreement with a fit of the form Ap⊥−nF(p⊥/√s), with n≈8 and F(p⊥/√s)≈exp(−26p⊥/√s).
Article
We present measurements from events with two isolated prompt photons in [ital [bar p]p] collisions at [radical][ital s] =1.8 TeV. The differential cross section, measured as a function of transverse momentum ([ital P][sub [ital T]]) of each photon, is about 3 times what next-to-leading-order QCD calculations predict. The cross section for photons with [ital P][sub [ital T]] in the range 10--19 GeV is 86[plus minus]27(stat)[sub [minus]23][sup +32](syst) pb. We also study the correlation between the two photons in both azimuthal angle and [ital P][sub [ital T]]. The magnitude of the vector sum of the transverse momenta of both photons, [ital K][sub [ital T]]=[vert bar][bold P][sub [ital T]1]+[bold P][sub [ital T]2][vert bar], has a mean value of [l angle][ital K][sub [ital T]][r angle]=5.1[plus minus]1.1 GeV.
Article
The average transverse momentum of pions as a function of their longitudinal momentum is studied in a number of reactions produced by 4.25 GeV/ac negative kaons or by 7.3 GeV/c antiprotons on protons. The observed seagull effects are attributed to the dominance of different reaction mechanisms at different values of the longitudinal momentum of the pions.
Article
The anisotropy parameter (v2), the second harmonic of the azimuthal particle distribution, has been measured with the PHENIX detector in Au+Au collisions at sNN=200 GeV for identified and inclusive charged particle production at central rapidities (|η|<0.35) with respect to the reaction plane defined at high rapidities (|η|=3–4 ). We observe that the v2 of mesons falls below that of (anti)baryons for pT>2 GeV/c, in marked contrast to the predictions of a hydrodynamical model. A quark-coalescence model is also investigated.
Article
We use a calculation based on the lowest order in the perturbation series for quantum chromodynamics to obtain an estimate for the contribution of hard-scattering processes involving vector gluons to the production of hadrons at large transverse momentum. Some simple models for the distribution of gluons in a proton and for the distribution of hadrons within a hard gluon jet are presented and used to calculate the process pp→π0X. At √s=53 GeV we find that the contribution of the subprocess qV→qV is comparable to that of qq→qq. The resulting cross sections are rather close to the CERN ISR data in magnitude. It is possible that small corrections arising, for example, from higher-order terms in the perturbation expansion might lead to a detailed fit to these data. At higher energies, such as those to be obtained in proposed new experimental facilities, our results indicate that the mechanisms VV→VV and qV→qV may dominate over qq→qq in much of the accessible kinematic regime. We briefly consider some experimental consequences of possible gluon dominance.
Article
High-pT single-particle inclusive cross section calculations are presented for the CERN ISR and ISABELLE energy ranges, taking into account all lowest-order hard-scattering subprocesses required by quantum chromodynamics (QCD). The input quark and gluon distribution and fragmentation functions were determined from analyses of deep-inelastic lepton data and were subject to various theoretical constraints such as sum rules and SU(3) symmetry. We thoroughly discuss the effects of the individual contributions from fermionic and gluonic subprocesses, as well as those effects stemming from QCD scaling violations in parton distributions and/or fragmentation functions. In particular, the inclusion of the large elastic gluon-gluon and gluon-quark scattering terms has a profound effect on both the normalization and the pT dependence of the predictions. The pT and θ dependences of single-π0 production are shown to be in good agreement with available data in the region pT≳4.5 GeV/c and √s≳50 GeV. In addition, we predict and discuss various ratios for inclusive single-particle production of π+, π-, K+, and K-, which also turn out to be in excellent agreement with presently available experiments.
Article
Calculations based on the idea of asymptotic freedom enable one to estimate an effective quark-gluon coupling from experimental observations of scaling violations in deep-inelastic scattering, from Okubo-Zweig-Iizuka-rule-forbidden decays of the ψ and the ψ′ and from corrections to the parton model for σ(e+e-→hadrons). These estimates then make it possible to normalize the one-gluon-exchange Born term for quark-quark elastic scattering and, within the context of hard-scattering models for hadronic collisions, obtain a lower limit on large-pT inclusive cross sections. The theoretical lower limit for the cross section is found to be slightly below CERN ISR data on pp→π0X at √s=53 GeV, pT≥6 GeV. Further measurements at the CERN ISR or at projected new accelerators may be sensitive to the presence of this mechanism. Such measurements will be extremely important in testing the underlying unity of diverse reactions within the general framework of quark-gluon dynamics. They may also provide the most direct determination of the effective hadronic coupling constant.
Article
Events with a charged particle of high transverse momentum were studied in pp collisions at √s = 52.5 GeV with the Split Field magnet (SFM) at the CERN-ISR, triggering on a charged particle of pT > 2 GeV/c, in the c.m. angular region θ ≅ 20° and θ ≅ 45°. The results provide further evidence for jet-like structure in high pT reactions. One of two produced jets is associated with the triggering hadron whereas the other is produced at an azimuthal angle that differs from that of the trigger by roughly 180°. The main features of the data are found to agree with the predictions of a simple parton-parton hard scattering model with vector gluon exchange, provided that one allows for partons in the proton to have transverse momentum. The ingredients of the model are all derived from data on deep inelastic lepton-proton scattering and from hadron production in e+e- annihilation. The model retains a simple pT-4 dependence for the parton-parton scattering. However, for pT <~ 4GeV/c, the transverse movement of partons yields an approximate pT-8 scaling for the inclusive hadron distribution in agreement with observations. For the understanding of our data, it is found to be essential to take into account the soft component arising from the fragmentation of the spectator partons into hadrons.
Article
The effect of multiple scattering on bremsstrahlung and pair production is considered. The probability of these processes decreases considerably at energies >~1013 ev. The calculations are carried out with the aid of the density matrix. The formulas thus obtained yield the probability of pair production and bremsstrahlung for arbitrary electron and photon energies.
Article
A large solid-angle apparatus consisting of a superconducting solenoid magnet, cylindrical drift chambers and two arrays of lead-glass counters was used to examine particles associated with a high transverse momentum trigger in p-p collisions with three √s values at the CERN ISR. The trigger was given by energy deposition in lead-glass arrays centred at 90°. The trigger transverse momentum range covered was 3 < pT trig < 11 GeV/c. Results are given for pout for both individual charged particles, and also for the sum of charged particle momenta in the hemisphere opposite to the trigger. Mean values are then deduced for the parton transverse momentum kT, and for the jet fragmentation momentum jT. Research supported in part by the US Department of Energy.
Article
Previous calculations have shown that quantum chromodynamics can successfully describe the behavior of high-pT single-particle inclusive processes in the region s>~50 GeV and pT>~5 GeV/c. Here it is shown that this region can be enlarged significantly by including effects due to parton transverse momenta.
Article
The theoretical framework for describing the production of direct photons in hadronic collisions is reviewed. A detailed comparison between the theoretical predictions and existing data is presented along with a critical evaluation of the various sources of theoretical uncertainty. The information available from direct-photon experiments is contrasted with that learned from jet or single-hadron production. Prospects for new types of measurements in future experiments are also presented.
Article
A rigorous quantum treatment of the Landau-Pomeranchuk-Migdal effect in QED and QCD is given for the first time. The rate of photon (gluon) radiation by an electron (quark) in a medium is expressed in terms of the Green’s function of a two-dimensional Schrödinger equation with an imaginary potential. In QED this potential is proportional to the dipole cross section for scattering of an e + e − pair off an atom, while in QCD it is proportional to the cross section of interaction of the color singlet quark-antiquark-gluon system with a color center.
Article
Symmetric three-jet events are selected from hadronic Z0 decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For the k ⊥ jet finder with y cut=0.02, we find «nglench.»nglegluoner«nglench.»nglebquark=1.089pm0.024(stat.)pm0.024(syst.){«ngle n^{⤪ ch.}»ngle {⤪ gluon}⩈er «ngle n^{⤪ ch.}»ngle {⤪ b} {⤪ quark}}=1.089pm 0.024 ({⤪ stat.})pm0.024 ({⤪ syst.}) «nglench.»nglegluoner«nglench.»ngleudsquark=1.390pm0.038(stat.)pm0.032(syst.){«ngle n^{⤪ ch.}»ngle {⤪ gluon}⩈er «ngle n^{⤪ ch.}»ngle {⤪ uds} {⤪ quark}}=1.390pm 0.038 ({⤪ stat.})pm0.032 ({⤪ syst.}) as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.
Article
The PHENIX Electromagnetic Calorimeter (EMCal) is used to measure the spatial position and energy of electrons and photons produced in heavy ion collisions. It covers the full central spectrometer acceptance of 70°⩽θ⩽110° with two walls, each subtending 90° in azimuth. One wall comprises four sectors of a Pb-scintillator sampling calorimeter and the other has two sectors of Pb-scintillator and two of a Pb-glass Cherenkov calorimeter. Both detectors have very good energy, spatial and timing resolution, while the Pb-scintillator excels in timing and the Pb-glass in energy measurements. Also, having two detectors with different systematics increases the confidence level of the physics results. Design and operational parameters of the Pb-scintillator, Pb-glass and special readout electronics for EMCal are presented and running experience during the first year of data taking with PHENIX is discussed. Some examples of data taken during the first run are shown.
Article
The PHENIX tracking system consists of Drift Chambers (DC), Pad Chambers (PC) and the Time Expansion Chamber (TEC). PC1/DC and PC2/TEC/PC3 form the inner and outer tracking units, respectively. These units link the track segments that transverse the RICH and extend to the EMCal. The DC measures charged particle trajectories in the r–φ direction to determine pT of the particles and the invariant mass of particle pairs. The PCs perform 3D spatial point measurements for pattern recognition and longitudinal momentum reconstruction and provide spatial resolution of a few mm in both r–φ and z. The TEC tracks particles passing through the region between the RICH and the EMCal. The design and operational parameters of the detectors are presented and running experience during the first year of data taking with PHENIX is discussed. The observed spatial and momentum resolution is given which imposes a limitation on the identification and characterization of charged particles in various momentum ranges.
Article
The Ring-Imaging Cherenkov (RICH) and the Time-of-Flight (ToF) systems provide identification of charged particles for the PHENIX central arm. The RICH is located between the inner and outer tracking units and is one of the primary devices for identifying electrons among the very large number of charged pions. The ToF is used to identify hadrons and is located between the most outer pad chamber (PC3) and the electromagnetic calorimeter. A Time Zero (T0) counter that enhances charged particle measurements in p–p collisions is described. Details of the construction and performance of both the RICH, ToF and T0 are given along with typical results from the first PHENIX data taking run.
Article
The PHENIX detector is designed to perform a broad study of A–A, p–A, and p–p collisions to investigate nuclear matter under extreme conditions. A wide variety of probes, sensitive to all timescales, are used to study systematic variations with species and energy as well as to measure the spin structure of the nucleon. Designing for the needs of the heavy-ion and polarized-proton programs has produced a detector with unparalleled capabilities. PHENIX measures electron and muon pairs, photons, and hadrons with excellent energy and momentum resolution. The detector consists of a large number of subsystems that are discussed in other papers in this volume. The overall design parameters of the detector are presented.
Article
We continue to investigate consequences of the assumption that the high transverse momentum particles seen in hadron-hadron collisions are produced by a single, hard, large-angle elastic scattering of quarks; one from the target and one from the beam. A large p⊥ event consists of four jets (collection of hadrons moving in roughly the same direction). The two outgoing quarks fragment into two jets of hadrons with roughly equal and opposite transverse momentum. The other two jets have small transverse momentum and result from the break up of the beam and target hadrons. We compare the model with existing data on the correlations among particles on the same and opposite (away) side of a large p⊥ trigger. If we allow the quarks within the initial hadrons to have a large mean transverse momentum, 〈k⊥〉h→q, then good agreement with data is found. We have taken 〈k⊥〉h→q = 500 MeV; however, even larger values are suggested by certain ISR experiments. A large cross section for producing a jet (quark) is predicted. For instance, σ(pp → jet + X)/σ(pp → π0 + X) = 370 at x⊥ = 0.4 and θc.m. = 90°. This is consistent with a recent FNAL jet trigger experiment. Predictions for future FNAL and ISR experiments are made.
Article
The interchange theory for inclusive scattering at large transverse momentum is extended into kinematic regions where Regge effects are important. Hadronic bremsstrahlung is shown to lead inevitably to Reggeization of the fundamental production process as s goes to infinity for fixed, large, PT. The cross sections are shown to rise to their ultimate scaling limit. This transition zone connects smoothly with the Feynman scaling region at low pT, and the deep scale-invariant region at large pT ∼ O(√s). The inclusive results of the interchange theory have the form of the standard fragmentation, triple Regge, and pionization formulae in their respective regions, but in addition predict the behavior of the associated residues and trajectories at large t or p2T.
Article
Soft gluon radiation induced by multiple scattering of a fast quark or gluon propagating through QCD matter is discussed. After revisiting the Landau-Pomeranchuk-Migdal effect in QED we show that large formation times of bremsstrahlung quanta determine the QCD radiation intensity (in analogy to QED) and derive the gluon energy spectrum. Coherent suppression takes place as compared to the Bethe-Heither situation of independent emissions. As a result the energy loss of fast partons in a QCD medium depends on the incident energy E similarly to QED, -.
Article
We present a study of the final state structure in proton-proton collisions (√s = 53 GeV) where a large transverse momentum π0 (pt > 2 GeV/c) is produced at an angle of 90°. Charged secondaries have been detected and momentum analysed in the split field magnet detector at the CERN Intersecting Storage Rings. The large angular coverage of this detector extends over ±2.5 units of rapidity and ±30° of azimuth with respect to the trigger π0, both towards and away from it. In each of these directions, where we observe similar strong correlations, we present charged particle distributions, in rapidity and momentum. In the hemisphere containing the trigger π0 we have measured the cross section for inclusive production of large transverse momentum ϱ± mesons. In the opposite hemisphere the data exhibit several features predicted by hard scattering quark-parton models: coplanarity and short-range rapidity correlation for the large transverse momentum secondaries as well as a transverse momentum sharing distribution similar to that observed in deep inelastic electro-production and in e+e− collisions.
Article
The predictions of a QCD-based hardscattering model for large pT hadronic processes are investigated. Not only quark-quark scattering but all contributions in lowest order are considered. At low xT, gluon-gluon and quark-gluon scattering are found to dominate the quark-quark term. At present energies the QCD predictions lie below the data but already come very close at the highest ISR energy. Predictions for higher pT values and higher energies are made.
Article
We study applications of QCD soft-gluon resummations to electroweak annihilation cross sections. We focus on a formalism that allows to resum logarithmic corrections arising near partonic threshold and at small transverse momentum simultaneously.
Article
Transverse momentum spectra for charged hadrons and for neutral pions in the range 1 GeV/c<p(T)<5 GeV/c have been measured by the PHENIX experiment at RHIC in Au+Au collisions at root square[s(NN)] = 130 GeV. At high p(T) the spectra from peripheral nuclear collisions are consistent with scaling the spectra from p+p collisions by the average number of binary nucleon-nucleon collisions. The spectra from central collisions are significantly suppressed when compared to the binary-scaled p+p expectation, and also when compared to similarly binary-scaled peripheral collisions, indicating a novel nuclear-medium effect in central nuclear collisions at RHIC energies.