Sensitivity of Pion versus Parton-Jet Nuclear Modification Factors to the Path-Length Dependence of Jet-Energy Loss at RHIC and LHC

We compare the jet-path length and beam-energy dependence of the pion nuclear modification factor and a patton-jet nuclear modification factor at RHIC and LHC, and contrast the predictions based on a linear pQCD and a highly non-linear hybrid AdS holographic model of jet-energy loss. It is found that both models require a reduction of the jet-medium coupling from RHIC to LHC to account for the measured pion nuclear modification factor. In the case of the parton-jet nuclear modification factor, however, which serves as a lower bound for the LO jet nuclear modification factor of reconstructed jets, the extracted data can be characterized without a reduced jet-medium coupling at LHC energies. It is concluded that when the reconstructed jets are sensitive to both quarks and gluons and thus provide more information than the pion nuclear modification factor, their information regarding the jet-medium coupling is limited due to the superposition with NLO and medium effects. Hence, a detailed description of the underlying physics requires both the leading hadron and the reconstructed jet nuclear modification factor. Unfortunately, the results for both the pion and the parton-jet nuclear modification factor are insensitive to the jet-path dependence of the models considered.

Study of the behavior of the nuclear modification factor in freeze-out state

One of the latest trends in the advancement of experimental high-energy physics is to identify the quark gluon plasma （QGP） predicted qualitatively by quantum chromodynamics （QCD）. We discuss whether nuclear transparency effect which is considered an important phenomenon, connected with dynamics of hadron-nuclear and nuclear-nuclear interactions could reflect some particular properties of the medium. FASTMC is used for Au-Au collision at RHIC energies. Critical change in the transparency is considered a signal on the appearance of new phases of strongly interacting matter and the QGP.

Nuclear modification factor for J/ψ production in nucleus-nucleus collisions

The STAR Collaboration has offered an eminent nuclear modification factor of J/ψ at high p T and midrapidity produced in Cu-Cu collisions at sNN^（1/2） = 200 GeV. Recalling a prediction, we can understand that the feature of high-pT nuclear modification factor is related to cc produced by 2 → 1 and 2 → 2 partonic processes in deconfined matter, particularly in the prethermal stage and to the recombination of c and c. The nuclear modification factor at high p T is sensitive to the earliest form of deconfined matter that does not have a temperature.

Study on open charm hadron production and angular correlation in high-energy nuclear collisions

We study the production and angular correlationof charm hadrons in hot and dense matter produced in high-energy nuclear-nuclear collisions within a multiphasetransport model(AMPT).By triggering additional charm-anticharm quark pair production in the AMPT,the modeldescribes the D^0 nuclear modification factor in the low andintermediate pr regions in Au+Au collisions at√VSNN=200 GeV reasonably well.Further exploration of the D^0 pair azimuthal angular correlation for different centralitiesshows clear evolution from low-multiplicity to high-mul-tiplicity events,which is associated with the number ofcharm quark interactions with medium partons duringAMPT transport.

Coherent photoproduction of J/ψ and γ mesons in pp and PbPb ultraperipheral collisions from dynamical gluon distributions

We present calculations of coherent photoproduction of vector mesons（J/ψ and Υ） with leading-order parton distribution functions to check new kinds of corrections of the DGLAP equations and nuclear modifications.The input gluon distribution of the proton is the dynamical parton model from the DGLAP equations with GLR-MQZRS（Gribov-Levin-Ryskin,Mueller-Qiu,Zhu-Ruan-Shen） modifications.From comparison between several other gluon distribution models,we find that the dynamical gluon distribution fits with the results of meson photoproduction experiments in the high energy region.The calculation of the differential cross sections using dynamical and other gluon distributions is compared with the experimental data from the HERA,ZEUS and LHCb Collaborations.Although there is little data for the rapidity distribution of vector meson photoproduction near zero rapidity,the dynamical gluon distribution works well with the data in the large rapidity region.

Jet production at next-to-leading order in p+Au collisions at the RHIC

We calculate jet productions in p＋Au collisions at the RHIC at next-to-leading order with perturbative QCD. Inclusive jet transverse energy spectrum, dijet invariant mass spectrum, dijet angular distribution, and corresponding nuclear modification factors for the three observables in p＋Au collisions at √s=200 GeV are given, where the initial-state cold nuclear matter （CNM） effects are included by taking advantage of four parametrization sets of nuclear parton distribution functions （nPDFs） - EPS, nCTEQ, HKN and DS. We demonstrate that inclusive jet transverse energy （ET） spectrum, dijet invariant mass （MJJ） spectrum with all 4 nPDFs are increased at low ET or MJJ, whereas at high ET or MJJ large deviation of results with different nPDFs is observed. It is found that the dijet angular distributions in p＋Au collisions do not vary relative to those in p＋p collisions for all 4 nPDFs.

Centrality dependence of pT spectra for identified hadrons in Au+Au and Cu+Cu collisions at (~SNN)^(1/2)=200 GeV

The centrality dependence of transverse momentum spectra for identified hadrons at midrapidityin Au＋Au collisions at √sNN=200GeV is systematically studied in a quark combination model. The PTspectra of π^±, K^±, p（p） and A（A） in different centrality bins and the nuclear modification factors （Rcp） forthese hadrons are calculated. The centrality dependence of the average collective transverse velocity （β （r）） for the hot and dense quark matter is obtained in Au＋Au collisions, and it is applied to a relative smallerCu＋Cu collision system. The centrality dependence of PT spectra and the Rcp for π^0, Ks^0 and A in Cu-bCucollisions at √sNN = 200 GeV are well described. The results show that （β （r）〉is only a function of the numberof participants Npart and it is independent of the collision system.

Study of K^(*)(892)^(0)andФ(1020)meson production in proton–proton and Pb–Pb collisions at√S_(NN)=2.76 TeV

In this paper,we describe a study of charged particle yield as a function of pT for K^(*)(892)0 and phgr(1020)mesons in proton–proton(pp)and Pb–Pb collisions at√S_(NN)=2.76 TeV in the central rapidity region of|y|<0.5,in a pT range of 0<pT<15 GeV c^(−1)in pp collisions and in a pT range of 0<pT<20 GeV c−1 in Pb-Pb collisions.We also investigated a very important ratio,the nuclear modification factor,to study the effects of the medium in the most central region,i.e.0%–5%centrality.For data simulation,we used the EPOS-LHC and EPOS-1.99 models.To check the validity of these models'simulations,we compared the data obtained from these Monte Carlo simulation programs with ALICE experimental data for√S_(NN)=2.76 TeV.It was concluded that the models'predictions for the phgr-meson in pp and for the most central Pb-Pb collisions disagreed with the ALICE data,and that the difference increased with pT.This may be connected with the essential role of collective parton behaviors which could not have been taken into account by the models.For K^(*)0 mesons,both programs gave almost the same predictions,and with pT in the interval pT>3 GeV c^(−1),the predictions were very close to the experimental data.Both models gave higher predictions for the soft pT interval and lower predictions for the hard interactions.The values of the RAA distributions were lower than unity and both models were very close to the ALICE data.It is very interesting that the models were not able to describe the pT distributions,but they gave good predictions for their ratios.This may possibly be due to parton collective behaviors.We observed some additional suppression of K^(*)0 at low values of pT with respect to phgr-mesons,which may be related to the role of the masses of the particles in soft interactions.The rising trend for R_(AA)in the region from pT=10 GeV c^(−1)to 20 GeV c^(−1) observed by the ALICE experiment was absent for theФ-mesons.