## HIJING Monte Carlo Model

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.
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.