7,921 research outputs found
Parton cascade description of relativistic heavy-ion collisions at CERN SPS energies ?
We examine Pb+Pb collisions at CERN SPS energy 158 A GeV, by employing the
earlier developed and recently refined parton-cascade/cluster-hadronization
model and its Monte Carlo implementation. This space-time model involves the
dynamical interplay of perturbative QCD parton production and evolution, with
non-perturbative parton-cluster formation and hadron production through cluster
decays. Using computer simulations, we are able to follow the entwined
time-evolution of parton and hadron degrees of freedom in both position and
momentum space, from the instant of nuclear overlap to the final yield of
particles. We present and discuss results for the multiplicity distributions,
which agree well with the measured data from the CERN SPS, including those for
K mesons. The transverse momentum distributions of the produced hadrons are
also found to be in good agreement with the preliminary data measured by the
NA49 and the WA98 collaboration for the collision of lead nuclei at the CERN
SPS. The analysis of the time evolution of transverse energy deposited in the
collision zone and the energy density suggests an existence of partonic matter
for a time of more than 5 fm.Comment: 16 pages including 7 postscript figure
Out of Equilibrium Non-perturbative Quantum Field Dynamics in Homogeneous External Fields
The quantum dynamics of the symmetry broken lambda (Phi^2)^2 scalar field
theory in the presence of an homogeneous external field is investigated in the
large N limit. We choose as initial state the ground state for a constant
external field J .The sign of the external field is suddenly flipped from
J to - J at a given time and the subsequent quantum dynamics calculated.
Spinodal instabilities and parametric resonances produce large quantum
fluctuations in the field components transverse to the external field. This
allows the order parameter to turn around the maximum of the potential for
intermediate times. Subsequently, the order parameter starts to oscillate near
the global minimum for external field - J, entering a novel quasi-periodic
regime.Comment: LaTex, 30 pages, 12 .ps figures, improved version to appear in Phys
Rev
Analysis of reaction dynamics at RHIC in a combined parton/hadron transport approach
We introduce a transport approach which combines partonic and hadronic
degrees of freedom on an equal footing and discuss the resulting reaction
dynamics. The initial parton dynamics is modeled in the framework of the parton
cascade model, hadronization is performed via a cluster hadronization model and
configuration space coalescence, and the hadronic phase is described by a
microscopic hadronic transport approach. The resulting reaction dynamics
indicates a strong influence of hadronic rescattering on the space-time pattern
of hadronic freeze-out and on the shape of transverse mass spectra. Freeze-out
times and transverse radii increase by factors of 2 - 3 depending on the hadron
species.Comment: 10 pages, 4 eps figures include
Heavy resonance production in high energy nuclear collisions
We estimate freezeout conditions for , , and quarks in high energy
nuclear collisions. Freezeout is due either to loss of thermal contact, or to
particles ``wandering'' out of the region of hot matter. We then develop a
thermal recombination model in which both single-particle (quark and antiquark)
and two-particle (quark-antiquark) densities are conserved. Conservation of
two-particle densities is necessary because quarks and antiquarks are always
produced in coincidence, so that the local two-particle density can be much
larger than the product of the single-particle densities. We use the freezeout
conditions and recombination model to discuss heavy resonance production at
zero baryon density in high energy nuclear collisions.Comment: revtex, 15 pages, no figures, KSUCNR-009-9
Isoscalar-isovector mass splittings in excited mesons
Mass splittings between the isovector and isoscalar members of meson nonets
arise in part from hadronic loop diagrams which violate the Okubo-Zweig-Iizuka
rule.
Using a model for these loop processes which works qualitatively well in the
established nonets, I tabulate predictions for the splittings and associated
isoscalar mixing angles in the remaining nonets below about 2.5 GeV, and
explain some of their systematic features.
The results for excited vector mesons compare favorably with experiment.Comment: 8 RevTeX pages, including 1 LaTeX figure.
CMU-HEP93-23/DOE-ER-40682-4
MoodBar: Increasing new user retention in Wikipedia through lightweight socialization
Socialization in online communities allows existing members to welcome and
recruit newcomers, introduce them to community norms and practices, and sustain
their early participation. However, socializing newcomers does not come for
free: in large communities, socialization can result in a significant workload
for mentors and is hard to scale. In this study we present results from an
experiment that measured the effect of a lightweight socialization tool on the
activity and retention of newly registered users attempting to edit for the
first time Wikipedia. Wikipedia is struggling with the retention of newcomers
and our results indicate that a mechanism to elicit lightweight feedback and to
provide early mentoring to newcomers improves their chances of becoming
long-term contributors.Comment: 9 pages, 5 figures, accepted for presentation at CSCW'1
Inclusive Particle Spectra at RHIC
A simulation is performed of the recently reported data from PHOBOS at
energies of 56 and 130 A GeV using the relativistic heavy ion cascade LUCIFER
which had previously given a good description of the NA49 inclusive spectra at
E=17.2 A GeV. The results compare well with these early measurements at RHIC.Comment: 4 pages, 2 figure
Hydrogen-like Atoms from Ultrarelativistic Nuclear Collisions
The number of hydrogen-like atoms produced when heavy nuclei collide is
estimated for central collisions at the Relativistic Heavy Ion Collider using
the sudden approximation of Baym et al. As first suggested by Schwartz, a
simultaneous measurement of the hydrogen and hadron spectra will allow an
inference of the electron or muon spectra at low momentum where a direct
experimental measurement is not feasible.Comment: 6 pages, 4 figure
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