251 research outputs found
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+ 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 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
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