## HIJING Monte Carlo Model

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Perturbative QCD predicts jet production from parton scatterings in high energy hadronic interactions. It is therefore expected that hard or semihard parton scatterings with transverse momentum of a few GeV are expected to dominate high energy heavy ion collisions. A major Monte Carlo model for RHIC physics call HIJING (Heavy Ion Jet INteraction Generator) was developed in the theory group in 1990-1991 with special emphasis on the role of minijets in pp, pA and AA reactions at collider energies. The HIJING code has been widely distributed to experimental groups preparing for RHIC and LHC.

Further testing of HIJING has been completed [1-6]. Detailed systematic comparison of HIJING results with a very wide range of data demonstrates for the first time that a quantitative understanding of the interplay between soft string dynamics and hard QCD interaction has been achieved. This is the first time that a large body of data has been reproduced by a single model. In particular, HIJING explained the observed flavor and multiplicity dependence of the average transverse momentum [2,3]. With the reproduction of many other inclusive spectra, this rules out the possibility that a QGP has been formed in high energy $p\bar{p}$ collision. To further verify the influence of minijets and to disentangle minijets from the underlying soft background experimentally, two particle correlation in the azimuthal angle with different transverse momentum cutoffs was proposed[4].

HIJING is also used to investigate two new effects, gluon shadowing and jet quenching, in heavy ion collisions at RHIC [5]. We showed that a systematic study of both pA and AA was required to disentangle the two effects at RHIC, and that such studies would yield the new physics of the gluon structure of nuclei and the energy loss mechanisms in quark-gluon plasmas. Related to gluon shadowing, QCD evolution of the parton distribution functions in nuclei is also investigated [7]. The main conclusion is that gluon shadowing vanishes much more rapidly with increase of the $p_T$ scale involved than quark shadowing. We plan to use this result to study the possibility that the infrared divergences in pQCD calculation of minijet production will be reduced in nuclear reactions due to this scale dependence of the parton shadowing.

Another interesting application of HIJING was to estimate the background of photons from high $p_T$ $\pi_0$ to see the plausibility of direct photon studies at RHIC [8]. We showed that, unless the initial temperature of the plasma exceeded $2T_c$, the decay photons would overwhelm the direct photon signal. This places a limit on the utility of this observable as a probe of QGP via nuclear reactions.

Since a large number of gluons and quarks are produced in the initial stage of heavy ion interaction as predicted by HIJING, the study of the evolution of this dense partonic system becomes very important. A parton cascade model is ideal for this purpose, but it takes time to develop. Assuming kinetical thermalization by free-streaming, we have estimated the evolution of the partonic system via pQCD branching [9]. The infrared divergences are naturally regulated by the Debye screen mass which can be calculated perturbatively and in term depends on the local parton density. By solving two self-consistent and nonlinear equations, we found that the equilibration time of the partonic system produced in Au+Au collisions at RHIC energy is about 1--2 fm/c. The existence of the produced partons via semihard scatterings will also influence the underlying soft particle production. We showed [10] that the produced partons will damp rapidly the chromoelectric fields in the plasma in analog to ohmic heating. This damping is shown to suppress pair production in the context of flux tube model and decrease transverse energy production. We plan to continue along these lines, but will take into account of flavor dynamics, viscosity and color conductivity.

We also plan to continue investigating a variety of physics topics with HIJING including $pA\rightarrow$ dihadron reactions and back-to-back two-particle correlations. The long range objective is to develop a space-time parton shower program that utilizes HIJING as the initial condition prior to hadronization. This latter development is coupled to both the field theoretic studies of $dE/dx$ described in the next section. The space-time cascade can only be done after a proper understanding of how interference phenomena leading to the Landau - Pomeronchuck - Migdal effect can be included in a classical cascade simulation.

Go to the HIJING Page on how to download the program