Quark Energy Loss In Cold Nuclear Matter

The parton energy loss in high energy collisions has attracted an increasing amount of attention from both the nuclear and particle physics communities. The wealth of experimental data on jet-quenching from RHIC and LHC reflect clearly the energy loss of fast partons while traversing this hot and dense medium in high energy nucleus-nucleus collisions. However, a detailed understanding of the parton energy loss in hot and dense medium requires the good investigation into the fast parton propagation in cold nuclear matter because there are common elements between the two mediums. The nuclear Drell-Yan process and semi-inclusive deep inelastic scattering on nuclear targets provide ideal enviorment for studying quark energy loss effect of an incoming quark and outgoing quark in nuclear medium, respectively.Firstly, a systematic analysis of the incoming quark energy loss is given in cold nuclear matter by using the nuclear Derll-Yan process. By means of three different parametrizations of quark energy loss and four typical sets of nuclear parton distribution functions, a leading order phenomenological analysis is performed on the nuclear Drell-Yan differential cross section ratio as a function of the quark momentum fraction. We obtain some conclusions as following:The obtained values of quark energy loss are smaller from the global fit to the relative experimental data by HKN07, nDS and EPS09nuclear effects than that by HKM nuclear effects. The reason is that HKM nuclear corrects to sea quark distribution are different from other three sets. HKN07, nDS and EPS09nuclear effects include the nuclear Drell-Yan data except for HKM parameterizations.The existing experimental data on nuclear Drell-Yan differential cross section ratio do not distinguish between the linear (△x1=α<L>A/Eh) or quadratic (αx1=β<L>A2/Eh) dependence of quark energy loss.It is found that with independence on the nuclear modification of parton distributions, the existing experimental data from lower incident beam energy rule out the incident-parton momentum fraction quark energy loss (△x1=kx1A1/3).Our results show that the incoming quark energy loss per unit path length dE/dL=1.21±0.09GeV/fm by means of the global fit of all selected data and HKM nuclear parton distribution functions determined only by means of the existing experimental data on nuclear structure functions.Secondly, the quark energy loss for an outgoing quark is investigated in cold nuclear matter by using the semi-inclusive deep inelastic scattering on nuclei. By means of the short hadron formation time, the HERMES and EMC experimental data with quark hadronization occurring outside the nucleus are picked out. With the vacuume fragmentation fubctions modified only by quark energy loss effect owing to multiple scattering and gluon radiation while propagating through the target nucleus, the leading-order computations for hadron multiplicity ratios are presented and compared with the HERMES and EMC data for the quarks hadronization occurring outside the nucleus. It is demonstrated that there are not nuclear effects on the parton distribution functions in semi-inclusive deep inelastic scattering.Our results show that the theoretical results with the fragmentation functions modified due to quark energy loss are in good agreement with the experimental data for hadrons produced outside the nucleus.The calculated results from the linear and quadratic quark energy loss are in favor of the attenuation of hadron multiplicity ratios. Therefore, the hadron multiplicity ratios can not determine whether the quark energy loss is linear or quadratic with the path length. We obtain the energy loss per unit length dE/dL=0.38±0.03GeV/fm for an outgoing quark by the global fit to all selected data. By combining our previous discussion on the nuclear Drell-Yan process, the obtained energy loss of an incoming quark and outgoing quark is not in support of the theoretical prediction (dE/dL)out=3(dE/dL)in.Thirdly, the atomic mass dependence of hadron production is studied in semi-inclusive deep inelastic lepton-nucleus scattering. We employ the so called two dimensional data from HERMES on the multiplicities for the production of pions on helium, neon, krypton, and xenon targets relative to those for deuterium. The experimental data with quark hadronization occurring outside the nucleus are selected by means of the hadron formation time. It is found that the atomic mass number dependence of hadron attenuation is theoretically and experimentally in good agreement with the A2/3power law.Furthermore, a leading-order computations for hadron multiplicity ratios are presented by using a model including quark energy loss and nuclear absorption and the comparison is made with the selected HERMES pions production data with the quark hadronization occurring inside the nucleus by means of the hadron formation time. It is shown that with increasing values of the virtual-photon energy (the fraction of this energy carried by the hadron), the influence of quark energy loss on hadron multiplicity ratio RA/Dπ increases (decreases) while the suppression on hadron multiplicity ratio RA/Dπ from nuclear absorption gets smaller (bigger). It is found that when hadronization occurs inside the nucleus, the nuclear absorption is the dominant mechanism which causes a reduction of the hadron yield. The atomic mass dependence of hadron attenuation for quark hadronization starting inside the nucleus is obtained theoretically to be proportional to A1/3.