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ABSTRACT: High-pT hadrons produced in hard collisions and detected inclusively bear
peculiar features: (i) they originate from jets whose initial virtuality and
energy are of the same order; (ii) such jets are rare and have a very biased
energy sharing among the particles, namely, the detected hadron carries the
main fraction of the jet energy. The former feature leads to an extremely
intensive gluon radiation and energy dissipation at the early stage of
hadronization, either in vacuum or in a medium. As a result, a leading hadron
must be produced on a short length scale. Evaluation within a model of
perturbative fragmentation confirms the shortness of the production length.
This result is at variance with the unjustified assumption of long production
length, made within the popular energy loss scenario. Thus we conclude that the
main reason of suppression of high-pT hadrons in heavy ion collisions is the
controlled by color transparency attenuation of a high-pT dipole propagating
through the hot medium. Adjusting a single parameter, the transport
coefficient, we describe quite well the data from LHC and RHIC for the
suppression factor R_{AA} as function of pT, collision energy and centrality.
We observe that the complementary effect of initial state interaction causes a
flattening and even fall of R_{AA} at large pT. The azimuthal anisotropy of
hadron production, calculated with no further adjustment, also agrees well with
data at different energies and centralities.
08/2012;
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ABSTRACT: We present an attempt at a critical overview of the current status of modeling for high-pT processes in nuclei. This review covers several topics including coherence phenomena, in particular gluon shadowing and CGC; nuclear effects related to the restrictions imposed by energy conservation at large xL and xT; spacetime development of hadronization of highly virtual light and heavy partons and the related time scales; and the role of early production and subsequent attenuation of pre-hadrons in a dense medium. We identify several intriguing problems in the current paradigms for high-pT processes and propose solutions for some of them.
Journal of Physics G Nuclear and Particle Physics 03/2011; 38(4):043101. · 4.18 Impact Factor
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ABSTRACT: We present an attempt at a critical overview of the current status of
modeling for high-pT processes in nuclei. The paper covers several topics
including coherence phenomena, in particular gluon shadowing and CGC; nuclear
effects related to the restrictions imposed by energy conservation at large xL
and xT ; space-time development of hadronization of highly virtual light and
heavy partons and the related time scales; and the role of early production and
subsequent attenuation of pre-hadrons in a dense medium. We identify several
intriguing problems in the current paradigms for high-pT processes and propose
solutions for some of them.
09/2010;
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ABSTRACT: We discuss a common feature of all known reactions on nuclear targets - a significant suppression at large x. Simple interpretation of this effect is based on energy conservation restrictions in initial state parton rescatterings. Using the light-cone dipole approach this mechanism is shown to control variety of processes on nuclear targets: high-pT particle production at different rapidities as well as direct and virtual (Drell-Yan) photon production. We demonstrate universality and wide applicability of this mechanism allowing to describe large-x effects also at SPS and FNAL energies too low for the onset of coherent effects or shadowing. Comment: 4 pages, 6 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee. Only minor changes in text
07/2009;
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ABSTRACT: We demonstrate that strong suppression of the relative production rate (d+Au)/(p+p) of inclusive high-pT hadrons at forward rapidities observed at RHIC is due to parton multiple rescatterings in nuclear matter. The light-cone dipole approach-based calculations are in a good agreement with BRAHMS and STAR data. They also indicate a significant nuclear suppression at midrapidities with a weak onset of the coherence effects. This prediction is supported by the preliminary d+Au data from the PHENIX Collaboration. Moreover, since similar suppression pattern is also expected to show up at lower energies where effects of parton saturation are not expected, we are able to exclude from the interpretation of observed phenomena models based on the Color Glass Condensate.
12/2008;
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ABSTRACT: We present an universal treatment for a substantial nuclear suppression representing a common feature of all known reactions on nuclear targets (forward production of high-pT hadrons, production of direct photons, the Drell-Yan process, heavy flavor production, etc.). Such a suppression at large Feynman xF, corresponding to region of minimal light-cone momentum fraction variable x2 in nuclei, is tempting to interpret as a manifestation of coherence or the Color Glass Condensate. We demonstrate, however, that it is actually a simple consequence of energy conservation and takes place even at low energies, where no effects of coherence are possible. We analyze this common suppression mechanism for several processes performing model predictions in the light-cone dipole approach. Our calculations agree with data.
08/2008;
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ABSTRACT: Within a light-cone quantum-chromodynamics dipole formalism based on the Green function technique, we study nuclear shadowing in deep-inelastic scattering at small Bjorken xB < 0.01. Such a formalism incorporates naturally color transparency and coherence length effects. Calculations of the nuclear shadowing for the \bar{q}q Fock component of the photon are based on an exact numerical solution of the evolution equation for the Green function, using a realistic form of the dipole cross section and nuclear density function. Such an exact numerical solution is unavoidable for xB > 0.0001, when a variation of the transverse size of the \bar{q}q Fock component must be taken into account. The eikonal approximation, used so far in most other models, can be applied only at high energies, when xB < 0.0001 and the transverse size of the \bar{q}q Fock component is "frozen" during propagation through the nuclear matter. At xB < 0.01 we find quite a large contribution of gluon suppression to nuclear shadowing, as a shadowing correction for the higher Fock states containing gluons. Numerical results for nuclear shadowing are compared with the available data from the E665 and NMC collaborations. Nuclear shadowing is also predicted at very small xB corresponding to LHC kinematical range. Finally the model predictions are compared and discussed with the results obtained from other models.
06/2008;
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ABSTRACT: We study a strong suppression of the relative production rate (d-Au)/(p-p) for inclusive high-pT hadrons of different species at large forward rapidities (large Feynman xF). The model predictions calculated in the light-cone dipole approach are in a good agreement with the recent measurements by the BRAHMS and STAR Collaborations at the BNL Relativistic Heavy Ion Collider. We predict a similar suppression at large pT and large xF also at lower energies, where no effect of coherence is possible. It allows to exclude the saturation models or the models based on Color Glass Condensate from interpretation of nuclear effects.
06/2008;
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ABSTRACT: The study of color transparency (CT) in elastic electroproduction of vector mesons off nuclei encounters the problem of the onset of coherence length (CL) effects. The problem of CT-CL separation arises especially at medium energies, corresponding to the HERMES experiment, when the coherence length is of the order of the nuclear radius RA. Only at asymptotic high energies, corresponding to large CL, lc≫RA, can the CT-CL mixing be eliminated. On the other hand, the net CT effects can be studied in the kinematic range accessible by the experiment at the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility, since in that case the CL is much smaller than the nuclear radius. Using light-cone QCD dipole formalism, we investigate manifestations of CT effects in the electroproduction of vector mesons. Motivated by expected data from the CLAS experiment, we predict the A and Q2 dependence of nuclear transparency for ρ0 mesons produced incoherently off nuclei. We demonstrate that in the CLAS kinematic region, the CL effects are weak enough to keep the photon energy at such values as to obtain maximal photon virtualities keeping optimal statistics of the data. This has a clear advantage over a standard investigation of net CT effects fixing CL.
Phys. Rev. C. 07/2007; 76(1).
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ABSTRACT: Using the light-cone QCD dipole formalism we investigate manifestations of color transparency (CT) and coherence length (CL) effects in electroproduction of longitudinally (L) and transversally (T) polarized vector mesons. Motivated by forthcoming data from the HERMES experiment we predict both the A and Q^2 dependence of the L/T- ratios, for rho^0 mesons produced coherently and incoherently off nuclei. For an incoherent reaction the CT and CL effects add up and result in a monotonic A dependence of the L/T-ratio at different values of Q^2. On the contrary, for a coherent process the contraction of the CL with Q^2 causes an effect opposite to that of CT and we expect quite a nontrivial A dependence, especially at Q^2 >> m_V^2.
04/2007;
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ABSTRACT: The study of color transparency (CT) in elastic electroproduction of vector mesons off nuclei encounters the problem of the onset of coherence length (CL) effects. The problem of CT-CL separation arises especially at medium energies, corresponding to HERMES experiment, when the coherence length is of the order of the nuclear radius R_A. Only at asymptotic large energies, corresponding to large CL, l_c >> R_A, the CT-CL mixing can be eliminated. On the other hand, the net CT effects can be studied in the kinematic range accessible by the CLAS experiment, since in this case the CL is much smaller than the nuclear radius. Using light-cone quantum chromodynamics (QCD) dipole formalism we investigate manifestations of CT effects in electroproduction of vector mesons. Motivated by expected data from the CLAS experiment at JLab, we predict the A and Q^2 dependence of nuclear transparency for rho^0 mesons produced incoherently off nuclei. We demonstrate that in the CLAS kinematical region the CL effects are weak enough to keep the photon energy at such values as to obtain maximal photon virtualities keeping optimal statistics of the data. This has a clear advantage in comparison with a standard investigation of net CT effects fixing CL.
03/2007;
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ABSTRACT: In-medium production of leading hadrons in hard reactions, carrying the main fraction of the jet momentum, involves two stages: (i) the parton originated from the hard process propagates through the medium radiating gluons due to the initial hard collision, as well as to multiple interactions in the medium; (ii) perturbative color neutralization, e.g. picking up an anti-colored parton produced perturbatively, followed by evolution and attenuation of the (pre)hadron in the medium. The color neutralization (or production) length for leading hadrons is controlled by coherence, energy conservation and Sudakov suppression. The pT-broadening is a sensitive and model independent probe for the production length. The color neutralization time is expected to shrink with rising hard scale. In particular, we found a very fast energy dissipation by a highly virtual parton: half of the jet energy is radiated during the first Fermi. Energy conservation makes the production of leading hadrons at longer times difficult.
09/2006;
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ABSTRACT: Recent measurements by the BRAHMS collaboration of high-pT hadron production at forward rapidities at RHIC found the relative production rate(d-Au)/(p-p) to be suppressed, rather than enhanced. Examining other known reactions (forward production of light hadrons, the Drell-Yan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large xF. Since this is the region where x2 is minimal, it is tempting to interpret this as a manifestation of coherence, or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with data. Comment: 12 pages Latex, 8 figures (only technical corrections in the replacement)
01/2005;
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ABSTRACT: Nuclei are unique analyzers for the early stage of the space-time development of hadronization. DIS at medium energies is especially suitable for this task being sensitive to hadronization dynamics, since the production length is comparable with the nuclear size. This was the driving motivation to propose an experiment at HERMES [1], and provide predictions based on a pQCD model of hadronization. Now the experiment is done and one can compare the predictions with data [2]. The model successfully describes with no adjustment the nuclear effects for various energies, zh, pT, and Q2, for different flavors and different nuclei. It turns out that the main source of nuclear suppression of the hadron production rate is attenuation of colorless pre-hadrons in the medium. An alternative model [11] is based upon an unjustified assumption that the struck parton keeps radiating gluons far beyond the nuclear size and the pre-hadron is produced outside the nucleus. This model has apparent problems attempting to explain certain features of the results from HERMES. A good understanding of the hadronization dynamics is important for proper interpretation of the strong suppression of high-pT hadrons observed in heavy ion collisions at RHIC. We demonstrate that the production length is even shorter in this case and keeps contracting with rising pT.
European Physical Journal A 01/2004; 19:111-127. · 2.19 Impact Factor
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ABSTRACT: Nuclei are unique analyzers for the early stage of the space-time development of hadronization. DIS at medium energies is especially suitable for this task being sensitive to hadronization dynamics, since the production length is comparable with the nuclear size. This was the driving motivation to propose measurements at HERMES using nuclear targets, and to provide predictions based on a pQCD model of hadronization [1]. Now when the first results of the experiment are released [2,3], one can compare the predictions with the data. The model successfully describes with no adjustment the nuclear effects for various energies, zh, pT, and Q2, for different flavors and different nuclei. It turns out that the main source of nuclear suppression of the hadron production rate is attenuation of colorless pre-hadrons in the medium. An alternative model [4] is based upon an ad hoc assumption that the colorless pre-hadron is produced outside the nucleus. This model has apparent problems attempting to explain certain features of the results from HERMES. A good understanding of the hadronization dynamics is important for proper interpretation of the strong suppression of high-pT hadrons observed in heavy ion collisions at RHIC. We demonstrate that the production length is even shorter in this case and keeps contracting with rising pT. Comment: Latex 34 p. Based on talks given by B.Z.K. at the Fourth International Conference on Perspectives in Hadronic Physics, Trieste, Italy, May 12-16, 2003; and at the EuroConference on Hadron Structure Viewed with Electromagnetic Probes, Santorini, Greece, October 7-12, 2003
11/2003;
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J. Nemchik
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ABSTRACT: Within a light-cone QCD formalism based on the Green function technique incorporating color transparency, coherence length effects and gluon shadowing we study electroproduction of vector mesons off nuclei. We found rather large color transparency effects in the range of Q^2 < 10-20 GeV^2. They are stronger at low than at high energies and can be easily identified by HERMES or at JLab. We provide predictions for incoherent and coherent vector meson production for future measurements. Comment: 18 pages including 6 figures. Presented at the International Conference HADRON STRUCTURE '2002, Herl'any, Slovakia, 22-27 September, 2002
11/2002;
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ABSTRACT: Recent data from RHIC for high- pT hadrons raised again the long-standing problem of quantitatively understanding the Cronin effect, i.e., nuclear enhancement of high- pT hadrons. All existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows one to explain data without fitting to them and to provide predictions for pA collisions at RHIC and LHC. We point out that the underlying mechanism drastically changes with energy, from incoherent production of high- pT hadrons on different nucleons at low energies, to an entirely coherent process at very high energies.
Physical Review Letters 07/2002; 88(23):232303. · 7.37 Impact Factor
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J. Nemchik
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ABSTRACT: Within a light-cone QCD formalism which incorporates color transparency (CT), coherence length (CL) effects and gluon shadowing (GS) we study electroproduction of charmonia off nuclei. In contrast to production of light vector mesons (rho^0, phi^0) when at small and medium energies CT and the onset of CL effects are not easily separated, in production of charmonia CT effects dominate. We found rather large CT effects in the range of Q^2 < 20 GeV^2. They are stronger at low than at high energies and can be easily identified by the planned future experiments. Our parameter-free calculations explain well the NMC data for variation with photon energy of the Sn/C ratio of nuclear transparencies. We provide predictions for incoherent and coherent production of charmonia for future measurements. Comment: 26 pages including 7 figures
05/2002;
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ABSTRACT: So far no theoretical tool for the comprehensive description of exclusive electroproduction of vector mesons off nuclei at medium energies has been developed. We suggest a light-cone QCD formalism which is valid at any energy and incorporates formation effects (color transparency), the coherence length and the gluon shadowing. At medium energies color transparency (CT) and the onset of coherence length (CL) effects are not easily separated. Indeed, although nuclear transparency measured by the HERMES experiment rises with Q^2, it agrees with predictions of the vector dominance model (VDM) without any CT effects. Our new results and observations are: (i) the good agreement with the VDM found earlier is accidental and related to the specific correlation between Q^2 and CL for HERMES kinematics; (ii) CT effects are much larger than have been estimated earlier within the two channel approximation. They are even stronger at low than at high energies and can be easily identified by HERMES or at JLab; (iii) gluon shadowing which is important at high energies is calculated and included; (iv) our parameter-free calculations explain well available data for variation of nuclear transparency with virtuality and energy of the photon; (v) predictions for electroproduction of \rho and \phi are provided for future measurements at HERMES and JLab. Comment: Latex 57 pages and 17 figures
07/2001;
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J. Nemchik
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ABSTRACT: In the color dipole gBFKL dynamics we predict a strikingly different Q^2 and energy dependence of the diffraction slope for the elastic production of ground state V(1S) and radially excited V'(2S) light vector mesons. The color dipole model predictions for the diffraction slope for \rho^0 and \phi^0 production are in a good agreement with the data from the fixed target and collider HERA experiments. We present how a different form of anomalous energy and Q^2 dependence of the diffraction slope for V'(2S) production leads to a different position of the node in radial wave function and discuss a possibility how to determine this position from the fixed target and HERA data. Comment: 20 pages and 6 figures. Title changed
03/2000;
