Scaling of particle production with number of participants in high-energy A+A collisions in the parton-cascade model

In view of the recent WA98 data of pi0 spectra from central Pb+Pb collisions at the CERN SPS, we analyze the production of neutral pions for A+$collisions across the periodic table at sqrt(s)=17 AGeV and 200 AGeV within the framework of the parton-cascade model for relativistic heavy ion collisions. The multiplicity of the pions (having pT > 0.5 GeV/c) in the central rapidity region, is seen to scale as \sim (N_part)^alpha, where N_part$ is the number of participating nucleons, which we have approximated as 2A for central collisions of identical nuclei. We argue that the deviation of \alpha (\simeq 1.2) from unity may have its origin in the multiple scattering suffered by the partons. We also find that the constant of proportionality in the above scaling relation increases substantially in going from SPS to RHIC energies. This would imply that the (semi)hard partonic activity becomes a much cleaner signal above the soft particle production at the higher energy of RHIC, and thus much less dependent on the (lack of) understanding of the underlying soft physics background.Comment: 4 pages including 2 postscript figure

Signatures of Parton Exogamy in e+ e- -> W+ W- -> hadrons

We propose possible signatures of `exogamous' combinations between partons in the different W+ and W- hadron showers in e+e- -> W+W- events with purely hadronic final states. Within the space-time model for hadronic shower development that we have proposed previously, we find a possible difference of about 10 % between the mean hadronic multiplicity in such purely hadronic final states and twice the hadronic multiplicity in events in which one W decays hadronically and the other leptonically, i.e., \ne 2 , associated with the formation of hadronic clusters by `exogamous' pairs of partons. We discuss the dependence of this possible difference in multiplicity on the center-of-mass energy, on the hadron momenta, and on the angular separation between the $W^{\pm}$ dijets. If it were observed, any such multiplicity difference would indicate that the W's do not hadronize independently, and hence raise questions about the accuracy with which the W mass could be determined from purely hadronic final states.Comment: 14 pages including 5 postscript figure

Interplay of parton and hadron cascades in nucleus-nucleus collisions at the CERN SPS and RHIC

We introduce a Monte Carlo space-time model for high-energy collisions with nuclei, involving the dynamical interplay of perturbative QCD parton production and evolution, with non-perturbative parton-cluster formation and `afterburner' cascading of formed pre-hadronic clusters plus hadron excitations. This approach allows us to trace the space-time history of parton and hadron degrees of freedom of nuclear collisions on the microscopical level of parton and hadron cascades in both position and momentum space, from the instant of nuclear overlap to the final yield of particles. In applying this approach, we analyze Pb+Pb collisions at the CERN SPS with beam energy 158 GeV (sqrt{s}/A = 17 GeV) and Au+Au collisions at RHIC with collider energy \sqrt{s}/A = 200 GeV. We find that the perturbative QCD parton production and cascade development provides an important contribution to particle production at central rapidities, and that the `afterburner' cascading of pre-hadronic clusters and formed hadrons emerging from the parton cascade is essential. The overall agreement of our model calculations including the `afterburner' cascading with the observed particle spectra at the CERN SPS is fairly good, whereas the neglect of the final-state interactions among hadronic excitations deviates significantly.Comment: 20 pages including 11 postscript figure

Multiple parton interactions in high-density QCD matter

Multiple interactions of quarks and gluons in high-energy heavy-ion collisions may give rise to interesting phemomena of color charges propagating in high-density QCD matter. We study the dynamics of multi-parton systems produced in nucleus-nucleus collisions at energies corresponding the the CERN SPS and the future BNL RHIC experiments. Due to the complexity of the multi-particle dynamics we choose to employ the parton cascade model in order to simulate the development of multiple parton scatterings and associated stimulated emision processes. Our results indicate a non-linear increase with nuclear mass A of, e.g., parton multiplicity, energy density, strangeness, and contrast a linear A-scaling as in Glauber-type approaches. If multiple interactions are suppressed and only single parton scatterings (no re-interactions) are considered, we recover such a linear behavior. It remains to be studied whether these results on the parton level can be experimentally seen in final-state observables, such as the charged particle multiplicity, the magnitude of produced transverse energy, or the number of produced strange hadrons.Comment: 15 pages including 9 postscript figure

Spectra of produced particles at CERN SPS heavy-ion collisions from a parton-cascade model

We evaluate the spectra of produced particles (pions, kaons, antiprotons) from partonic cascades which may develop in the wake of heavy-ion collisions at CERN SPS energies and which may hadronize by formation of clusters which decay into hadrons. Using the experimental data obtained by NA35 and NA44 collaborations for S+S and Pb+Pb collisions, we conclude that the Monte Carlo implementation of the recently developed parton-cascade/cluster-hadronization model provides a reasonable description of the distributions of the particles produced in such collisions. While the rapidity distribution of the mid-rapidity protons is described reasonably well, their transverse momentum distribution falls too rapidly compared to the experimental values, implying a significant effect of final state scattering among the produced hadrons neglected so far