1,607 research outputs found
A statistical model analysis of fluctuations in heavy ion collisions
We briefly describe two statistical hadronization models, based respectively
on the presence and absence of light quark chemical equilibrium, used to
analyze particle yields in heavy ion collisions. We then try to distinguish
between these models using fluctuations data. We find that while the
non-equilibrium model provides an acceptable description of fluctuations at top
SPS and RHIC energies, both models considerably under-estimate fluctuations at
low SPS energies.Comment: References updated Poster in QM2006 conference, Shangha
Hadron Resonances and Phase Threshold in Heavy Ion Collisions
We show that a measurement of the reaction energy dependence of relative
hadron resonance yields in heavy ion collisions can be used to study the phase
structure of the dense QCD matter created in these collisions, and investigate
the origin of the trends observed in the excitation functions of certain soft
hadronic observables. We show that presence of chemical nonequilibrium in light
quark abundance imparts a characteristic signature on the energy dependence of
resonance yields, that differs considerably from what is expected in the
equilibrium picture.Comment: In press, Phys. Rev.
Hanbury-Brown--Twiss Analysis in a Solvable Model
The analysis of meson correlations by Hanbury-Brown--Twiss interferometry is
tested with a simple model of meson production by resonance decay. We derive
conditions which should be satisfied in order to relate the measured momentum
correlation to the classical source size. The Bose correlation effects are
apparent in both the ratio of meson pairs to singles and in the ratio of like
to unlike pairs. With our parameter values, we find that the single particle
distribution is too distorted by the correlation to allow a straightforward
analysis using pair correlation normalized by the singles rates. An analysis
comparing symmetrized to unsymmetrized pairs is more robust, but nonclassical
off-shell effects are important at realistic temperatures.Comment: 21 pages + 9 figures (tarred etc. using uufiles, submitted
separately), REVTeX 3.0, preprint number: DOE/ER/40561-112/INT93-00-3
Indications for a Detonating Quark-Gluon Plasma
We propose a mechanism which naturally contains the relation of the hadronic gas produced in heavy-ion collisions at CERN. Our
starting assumption is the existence of a sharp front separating the
quark-gluon plasma phase from the hadronic phase. Energy-momentum conservation
across the front leads to the following consequences for an adiabatic process
a) The baryon chemical potential, , is approximately continuous across
the front. b) The temperature in the hadronic gas is higher than the phase
transition temperature due to superheating. c) In the region covered by the
experiments the velocity of the hadronic gas approximately equals the speed of
sound in the hadronic gas.Comment: Latex file 9 pages + 6 figures available as postscript file
Boundary and Coulomb Effects on Boson Systems in High-Energy Heavy-Ion Collisions
The boundary of a boson system plays an important role in determining the
momentum distribution of the bosons. For a boson system with a cylindrical
boundary, the momentum distribution is enhanced at high transverse momenta but
suppressed at low transverse momenta, relative to a Bose-Einstein distribution.
The boundary effects on systems of massless gluons and massive pions are
studied. For gluons in a quark-gluon plasma, the presence of the boundary may
modify the signals for the quark-gluon plasma. For pions in a pion system in
heavy-ion collisions, Coulomb final-state interactions with the nuclear
participants in the vicinity of the central rapidity region further modify the
momentum distribution at low transverse momenta. By including both the boundary
effect and the Coulomb final-state interactions we are able to account for the
behavior of the transverse momentum spectrum observed in many
heavy-ion experiments, notably at low transverse momenta.Comment: 15 pages Postscript uuencoded tar-comprssed file, 9 Postscript
figures uuencoded tar-compressed fil
Search for the QCD critical point in nuclear collisions at the CERN SPS
Pion production in nuclear collisions at the SPS is investigated with the aim
to search, in a restricted domain of the phase diagram, for power-laws in the
behavior of correlations which are compatible with critical QCD. We have
analyzed interactions of nuclei of different size (p+p, C+C, Si+Si, Pb+Pb) at
158 GeV adopting, as appropriate observables, scaled factorial moments in a
search for intermittent fluctuations in transverse dimensions. The analysis is
performed for pairs with invariant mass very close to the two-pion
threshold. In this sector one may capture critical fluctuations of the sigma
component in a hadronic medium, even if the -meson has no well defined
vacuum state. It turns out that for the Pb+Pb system the proposed analysis
technique cannot be applied without entering the invariant mass region with
strong Coulomb correlations. As a result the treatment becomes inconclusive in
this case. Our results for the other systems indicate the presence of power-law
fluctuations in the freeze-out state of Si+Si approaching in size the
prediction of critical QCD.Comment: 31 pages, 11 figure
Source Dimensions in Ultrarelativistic Heavy Ion Collisions
Recent experiments on pion correlations, interpreted as interferometric
measurements of the collision zone, are compared with models that distinguish a
prehadronic phase and a hadronic phase. The models include prehadronic
longitudinal expansion, conversion to hadrons in local kinetic equilibrium, and
rescattering of the produced hadrons. We find that the longitudinal and outward
radii are surprisingly sensitive to the algorithm used for two-body collisions.
The longitudinal radius measured in collisions of 200 GeV/u sulfur nuclei on a
heavy target requires the existence of a prehadronic phase which converts to
the hadronic phase at densities around 0.8-1.0 GeV/fm. The transverse radii
cannot be reproduced without introducing more complex dynamics into the
transverse expansion.Comment: RevTeX 3.0, 28 pages, 6 figures, not included, revised version, major
change is an additional discussion of the classical two-body collision
algorithm, a (compressed) postscript file of the complete paper including
figures can be obtained from Authors or via anonymous ftp at
ftp://ftp_int.phys.washington.edu/pub/herrmann/pisource.ps.
Charged Particle Production in Proton-, Deuteron-, Oxygen- and Sulphur-Nucleus Collisions at 200 GeV per Nucleon
The transverse momentum and rapidity distributions of net protons and
negatively charged hadrons have been measured for minimum bias proton-nucleus
and deuteron-gold interactions, as well as central oxygen-gold and
sulphur-nucleus collisions at 200 GeV per nucleon. The rapidity density of net
protons at midrapidity in central nucleus-nucleus collisions increases both
with target mass for sulphur projectiles and with the projectile mass for a
gold target. The shape of the rapidity distributions of net protons forward of
midrapidity for d+Au and central S+Au collisions is similar. The average
rapidity loss is larger than 2 units of rapidity for reactions with the gold
target. The transverse momentum spectra of net protons for all reactions can be
described by a thermal distribution with `temperatures' between 145 +- 11 MeV
(p+S interactions) and 244 +- 43 MeV (central S+Au collisions). The
multiplicity of negatively charged hadrons increases with the mass of the
colliding system. The shape of the transverse momentum spectra of negatively
charged hadrons changes from minimum bias p+p and p+S interactions to p+Au and
central nucleus-nucleus collisions. The mean transverse momentum is almost
constant in the vicinity of midrapidity and shows little variation with the
target and projectile masses. The average number of produced negatively charged
hadrons per participant baryon increases slightly from p+p, p+A to central
S+S,Ag collisions.Comment: 47 pages, submitted to Z. Phys.
Phase-space dependence of particle-ratio fluctuations in Pb+Pb collisions from 20A to 158A GeV beam energy
A novel approach, the identity method, was used for particle identification
and the study of fluctuations of particle yield ratios in Pb+Pb collisions at
the CERN Super Proton Synchrotron (SPS). This procedure allows to unfold the
moments of the unknown multiplicity distributions of protons (p), kaons (K),
pions () and electrons (e). Using these moments the excitation function of
the fluctuation measure [A,B] was measured, with A and
B denoting different particle types. The obtained energy dependence of
agrees with previously published NA49 results on the related
measure . Moreover, was found to depend
on the phase space coverage for [K,p] and [K,] pairs. This feature most
likely explains the reported differences between measurements of NA49 and those
of STAR in central Au+Au collisions
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