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Number of participating nucleons (N part ) in the hijing MC simulations for five different E 4<|η|<5.2 T bins and the cumulative distribution without any requirement on E 4<|η|<5.2 T
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Dijet production has been measured in collisions at a nucleon-nucleon centre-of-mass energy of 5.02. A data sample corresponding to an integrated luminosity of 35 was collected using the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet transverse momentum balance, azimuthal angle correlations, and pseudorapidity distributions...
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... acceptance, and misreconstruction corrections as described in Ref. [44] is also included in this table. In order to study the correlation between the collision geometry and forward calorimeter energy, the distributions of number of participating nucleons (N part ) in the hijing Monte Carlo sim- ulation in the five E 4<|η|<5.2 T bins are shown in Fig. 2. While the mean of the N part distribution is found to be increasing monotonically as a function of E 4<|η|<5.2 T , the fluctuation of N part is found to be large in each HF activity ...
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Citations
... The higher-twist effects (causing saturation here) die out in inverse powers of the virtuality Q 2 , so that, at the highest values of p T , factorization is expected to hold. Also the global analysis of nPDFs [100,102,103] and jet production in minimum-bias proton-nucleus collisions [109] indicate this to be the case. Thus, to maintain factorization at the highest values of p T , the list of all candidate dijets in an A + B collision is next ordered in decreasing p T . ...
We present a novel Monte Carlo implementation of the EKRT model, MC-EKRT, for computing partonic initial states in high-energy nuclear collisions. Our new MC-EKRT event generator is based on collinearly factorized, dynamically fluctuating perturbative QCD (pQCD) minijet production, supplemented with a saturation conjecture that controls the low- p T particle production. Previously, the EKRT model has been very successful in describing low- p T observables at midrapidity in heavy-ion collisions at energies available at the CERN Large Hadron Collider (LHC) and the BNL Relativistic Heavy Ion Collider (RHIC). As novel features, our new MC implementation gives a full three-dimensional initial state event by event, includes dynamical minijet-multiplicity fluctuations in the saturation and particle production, introduces a new type of spatially dependent nuclear parton distribution functions, and accounts for the conservation of energy/momentum and valence-quark number. In this proof-of-principle study, we average a large set of event-by-event MC-EKRT initial conditions and compute the rapidity and centrality dependence of the charged hadron multiplicities and elliptic flow for LHC Pb + Pb and RHIC Au + Au collisions using ( 3 + 1 ) D viscous fluid-dynamical evolution. Also event-by-event fluctuations and decorrelations of initial eccentricities are studied. The good agreement with the rapidity-dependent data suggests that the same saturation mechanism that has been very successful in explaining the midrapidity observables works well also at larger rapidities.
Published by the American Physical Society 2025
... Therefore, the magnitude of naive factorization breaking due to Glauber modes can be evaluated by comparing theoretical predictions with future high-precision experimental measurements. On the right side of figure 2, we plot the azimuthal angle decorrelation in pPb collisions at √ s = 5.02 TeV from the CMS collaboration [61]. The data is integrated within the region |y c,d | < 3 and the jets were reconstructed using an anti-k T algorithm with R = 0.3. ...
... The colored bands indicate theoretical uncertainties from the variation of hard and jet scales. Right: a comparison of the dijet azimuthal angle decorrelation in pPb collisions from the CMS collaboration at the LHC[61]. ...
A bstract
We study the azimuthal angular decorrelations of dijet production in both proton-proton (pp) and proton-nucleus (pA) collisions. By utilizing soft-collinear effective theory, we establish the factorization and resummation formalism at the next-to-leading logarithmic accuracy for the azimuthal angular decorrelations in the back-to-back limit in pp collisions. We propose an approach where the nuclear modifications to dijet production in pA collisions are accounted for in the nuclear modified transverse momentum dependent parton distribution functions (nTMDPDFs), which contain both collinear and transverse dynamics. This approach naturally generalizes the well-established formalism related to the nuclear modified collinear parton distribution functions (nPDFs). We demonstrate strong consistency between our methodology and the CMS measurements in both pp and pA collisions, and make predictions for dijet production in the forward rapidity region in pA collisions at LHC kinematics and for mid-rapidity kinematics at sPHENIX. Throughout this paper, we focus on the application of this formalism to a simultaneous fit to both collinear and transverse momentum dependent contributions to the transverse momentum dependent distributions.
... The p+Pb events were recorded in two data-taking periods with proton and lead beam directions swapped in the LHC rings: period A with 'Pb+p' geometry and period B with 'p+Pb' geometry. Regardless of the period, the physics results of p+Pb collisions are presented such that the p-going direction is towards positive rapidity; this is the convention used in previous ATLAS publications as well as in publications of other LHC experiments [30][31][32]. The events were required to either satisfy an MB trigger or one of several jet triggers with different thresholds. ...
A bstract
This paper presents measurements of charged-hadron spectra obtained in pp , p +Pb, and Pb+Pb collisions at s or s NN = 5 . 02 TeV, and in Xe+Xe collisions at s NN = 5 . 44 TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb − 1 , 28 nb − 1 , 0.50 nb − 1 , and 3 μ b − 1 , respectively. The nuclear modification factors R p Pb and R AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor R p Pb shows a moderate enhancement above unity with a maximum at p T ≈ 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p T -dependence with a local maximum at p T ≈ 2 GeV and a local minimum at p T ≈ 7 GeV. This dependence is more distinguishable in more central collisions. No significant | η |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe R AA better in central collisions and in the p T range from about 10 to 100 GeV.
... Experimental measurements of the azimuthal angular decorrelations in proton-proton and proton-lead (pPb) collisions at the LHC were performed in [60,61], respectively; while in [61,62] the integrated dijet azimuthal angle decorrelation in the region ∆ϕ > 2π/3 was measured. The first phenomenological studies of these data have been used to further constrain the nuclear modified collinear PDFs, see for instance in [12,63,64], by ap- Figure 1. ...
... Experimental measurements of the azimuthal angular decorrelations in proton-proton and proton-lead (pPb) collisions at the LHC were performed in [60,61], respectively; while in [61,62] the integrated dijet azimuthal angle decorrelation in the region ∆ϕ > 2π/3 was measured. The first phenomenological studies of these data have been used to further constrain the nuclear modified collinear PDFs, see for instance in [12,63,64], by ap- Figure 1. ...
... Nonetheless, the magnitude of naive factorization breaking due to Glauber modes can be evaluated by comparing theoretical predictions with future high-precision experimental measurements. On the right side of Fig. 2, we plot the azimuthal angle decorrelation in pPb collisions at √ s = 5.02 TeV from the CMS collaboration [61]. The data is integrated within the region |y c,d | < 3 and the jets were reconstructed using an anti-k T algorithm with R = 0.3. ...
We study the azimuthal angular decorrelations of dijet production in both proton-proton (pp) and proton-nucleus (pA) collisions. By utilizing soft-collinear effective theory, we establish the factorization and resummation formalism at the next-to-leading logarithmic accuracy for the azimuthal angular decorrelations in the back-to-back limit in pp collisions. We propose an approach where the nuclear modifications to dijet production in pA collisions are accounted for in the nuclear modified transverse momentum dependent parton distribution functions (nTMDPDFs), which contain both collinear and transverse dynamics. This approach naturally generalizes the well-established formalism related to the nuclear modified collinear parton distribution functions (nPDFs). We demonstrate strong consistency between our methodology and the CMS measurements in both pp and pA collisions, and make predictions for dijet production in the forward rapidity region in pA collisions at LHC kinematics and for mid-rapidity kinematics at sPHENIX. Throughout this paper, we focus on the application of this formalism to a simultaneous fit to both collinear and transverse momentum dependent contributions to the transverse momentum dependent distributions.
... The +Pb events were recorded in two data-taking periods with proton and lead beam directions swapped in the LHC rings: period A with 'Pb+ ' geometry and period B with ' +Pb' geometry. Regardless of the period, the physics results of +Pb collisions are presented such that the -going direction is towards positive rapidity; this is the convention used in previous ATLAS publications as well as in publications of other LHC experiments [30][31][32]. The events were required to either satisfy an MB trigger or one of several jet triggers with different thresholds. ...
This paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at or TeV, and in Xe+Xe collisions at TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb, 28 nb, 0.50 nb, and 3 b, respectively. The nuclear modification factors and are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor shows a moderate enhancement above unity with a maximum at GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct -dependence with a local maximum at GeV and a local minimum at GeV. This dependence is more distinguishable in more central collisions. No significant -dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe better in central collisions and in the range from about 10 to 100 GeV.
... In particular, the relatively small final-state effects (e.g. jet quenching) in pPb collisions support the idea of using jets as probes for nPDF fits [7]. ...
We quantify the impact of LHC dijet production measurements on the nNNPDF2.0 nuclear PDFs in twofold. First, from a proton baseline based on NNPDF3.1 and augmented by pp dijet production measurements from ATLAS and CMS at 7 and 8 TeV. Second, from a new nNNPDF2.0 global analysis including the ratio of pPb to pp dijet spectra from CMS at 5 TeV. We show that as opposed to the CMS at 5 TeV absolute pp and pPb dijet spectra, the pPb/pp ratio is well described in a nPDFs fit and provides strong constraints on the gluon of lead.
... In the EPPS16 analysis, we used the first CMS 5 TeV singledifferential dijet pPb data [58] in the form of a forward-tobackward ratio. Now, a double-differential analysis [3] of the same data sample has become available and this is what we use in the present analysis. ...
We present an updated global analysis of collinearly factorized nuclear parton distribution functions (PDFs) at next-to-leading order in perturbative QCD. In comparison to our previous fit, EPPS16, the present analysis includes more data from proton-lead collisions at the Large Hadron Collider: 5 TeV double-differential CMS dijet and LHCb D-meson data, as well as 8 TeV CMS W ± data. These new data lead to significantly better-constrained gluon distributions at small and intermediate values of the momentum fraction x , confirming the presence of shadowing and antishadowing for gluons in large nuclei. In addition, we include Jefferson Lab measurements of deeply inelastic scattering which probe nuclear PDFs at large x and low virtualities. For the first time within the Hessian framework, we now also explore the uncertainties of nuclear PDFs due to the errors in the baseline proton PDFs. We release the results of our analysis as a new public parametrization of nuclear PDFs called EPPS21.
... The first CMS dijet measurement [57] in pPb collisions has been already used in EPPS16 [25]. The more recent CMS dijet measurement includes more p T bins with improved precision and the corresponding measurement in pp collisions used for normalization [53]. ...
... Initially, there were concerns about the use of dijet data in nPDF fits due to the possibility of jet quenching in pPb collisions. The experimental investigations in [46,57] have not found any evidence of jet quenching. They confirm the dominance of nPDF effects in the observed jet kinematics comparison between pp and pPb collisions. ...
... Currently, no sign of jet quenching has been observed in pPb collisions, based on high-transverse momentum particle suppression in single hadron spectra in proton-nucleus collisions compared to pp collisions [574][575][576][577][578] or dijet momentum imbalance [57]. There are two conceptual problems to address parton energy loss in collisions involving light nuclei and protons as initial states. ...
The Quark-Gluon Plasma (QGP), a QCD state of matter created in ultra-relativistic heavy-ion collisions, has remarkable properties as a low shear viscosity over entropy ratio. Through the detection of multi-particle production, the bulk debris of the collision, these so-called soft probes have provided quantitative insight into the created matter. However, its fast evolution and thermalization properties remain elusive to the soft sector. Only the usage of high momentum objects as probes of the QGP can unveil its constituents at the different wavelengths. In this review, we attempt to provide a comprehensive picture of what was, so far, possible to withdraw given our current theoretical understanding of jets, heavy-flavor, and quarkonia. We will bridge the resulting qualitative picture to the experimental observations done at both the LHC and RHIC. We will focus on the phenomenological description of experimental observations, provide a brief analytical summary of the description of hard probes, and an outlook towards the main difficulties we will need to surpass in the following years. To benchmark QGP-related effects, we will also address nuclear modifications to the initial state and hadronization effects.
... This would result in a gross violation of the geometrical estimate of the distribution over ν (even including fluctuations around the average value given by σ in ). Such a violation was observed at the LHC [338,339] and RHIC [340] in pA and dA collisions, where the production of jets originating from large x quarks was studied as a function of the centrality of the collision. This pattern was explained in the model, where λ(x) = σ(x) /σ in drops with an increase of x reaching λ(0.5) ∼ 0.6, [341,342]. ...
We review a broad range of phenomena in diffraction in the context of hadron-hadron, hadron-nucleus collisions and deep inelastic lepton-proton/nucleus scattering focusing on the interplay between the perturbative QCD and non-perturbative models. We discuss inclusive diffraction in DIS, phenomenology of dipole models, resummation and parton saturation at low x, hard diffractive production of vector mesons, inelastic diffraction in hadron-hadron scattering, formalism of color fluctuations, inclusive coherent and incoherent diffraction as well as soft and hard diffraction phenomena in hadron-hadron/nucleus and photon-nucleus collisions. For each topic we review key results from the past and present experiments including HERA and the LHC. Finally, we identify the remaining open questions, which could be addressed in the continuing experiments, in particular in photon-induced reactions at the LHC and the future Electron-Ion Collider (EIC) in the US, Large Hadron electron Collider (LHeC) and Future Circular Collider (FCC) at CERN.
... In the EPPS16 analysis, we used the first CMS 5 TeV single-differential dijet pPb data [57] in the form of a forward-to-backward ratio. Now, a double-differential analysis [3] of the same data sample has become available and this is what we use in the present analysis. ...
We present an updated global analysis of collinearly factorized nuclear parton distribution functions (PDFs) at next-to-leading order in perturbative QCD. In comparison to our previous fit, EPPS16, the present analysis includes more data from proton-lead collisions at the Large Hadron Collider: 5TeV double-differential CMS dijet and LHCb D-meson data, as well as 8TeV CMS W data. These new data lead to significantly better-constrained gluon distributions at small and intermediate values of the momentum fraction x, confirming the presence of shadowing and antishadowing for gluons in large nuclei. In addition, we include Jefferson Lab measurements of deeply inelastic scattering which probe nuclear PDFs at large x and low virtualities. For the first time within the Hessian framework, we now also explore the uncertainties of nuclear PDFs due to the errors in the baseline proton PDFs. We release the results of our analysis as a new public parametrization of nuclear PDFs called EPPS21.
