800 research outputs found
Multiplicity Fluctuations in Limited Segments of Momentum Space in Statistical Models
Multiplicity fluctuations in limited segments of momentum space are
calculated for a classical pion gas within the statistical model. Results for
the grand canonical, canonical, and micro-canonical ensemble are obtained,
compared and discussed. We demonstrate that even in the large volume limit
correlations between macroscopic subsystems due to energy and momentum
conservation persist. Based on the micro-canonical formulation we make
qualitative predictions for the rapidity and transverse momentum dependence of
multiplicity fluctuations. The resulting effects are of similar magnitude as
the predicted enhancement due to a phase transition from a quark-gluon plasma
to a hadron gas phase, or due to the critical point of strongly interacting
matter, and qualitatively agree with recently published preliminary
multiplicity fluctuation data of the NA49 SPS experiment.Comment: 23 pages, 4 figure
Multiplicity fluctuations in relativistic nuclear collisions: statistical model versus experimental data
The multiplicity distributions of hadrons produced in central nucleus-nucleus
collisions are studied within the hadron-resonance gas model in the large
volume limit. The microscopic correlator method is used to enforce conservation
of three charges - baryon number, electric charge, and strangeness - in the
canonical ensemble. In addition, in the micro-canonical ensemble energy
conservation is included. An analytical method is used to account for resonance
decays. The multiplicity distributions and the scaled variances for negatively,
positively, and all charged hadrons are calculated along the chemical
freeze-out line of central Pb+Pb (Au+Au) collisions from SIS to LHC energies.
Predictions obtained within different statistical ensembles are compared with
the preliminary NA49 experimental results on central Pb+Pb collisions in the
SPS energy range. The measured fluctuations are significantly narrower than the
Poisson ones and clearly favor expectations for the micro-canonical ensemble.
Thus this is a first observation of the recently predicted suppression of the
multiplicity fluctuations in relativistic gases in the thermodynamical limit
due to conservation laws.Comment: 27 pages, 9 figures, corrected reference
Multiplicity fluctuations in relativistic nuclear collisions
Multiplicity distributions of hadrons produced in central nucleus-nucleus
collisions are studied within the hadron-resonance gas model in the large
volume limit. In the canonical ensemble conservation of three charges (baryon
number, electric charge, and strangeness) is enforced. In addition, in the
micro-canonical ensemble energy conservation is included. An analytical method
is used to account for resonance decays. Multiplicity distributions and scaled
variances for negatively charged hadrons are presented along the chemical
freeze-out line of central Pb+Pb (Au+Au) collisions from SIS to LHC energies.
Predictions obtained within different statistical ensembles are compared with
preliminary NA49 experimental results on central Pb+Pb collisions in the SPS
energy range. The measured fluctuations are significantly narrower than a
Poisson reference distribution, and clearly favor expectations for the
micro-canonical ensemble.Comment: 6 pages, 3 figure
Particle number fluctuations in nuclear collisions within excluded volume hadron gas model
The multiplicity fluctuations are studied in the van der Waals excluded
volume hadron-resonance gas model. The calculations are done in the grand
canonical ensemble within the Boltzmann statistics approximation. The scaled
variances for positive, negative and all charged hadrons are calculated along
the chemical freeze-out line of nucleus-nucleus collisions at different
collision energies. The multiplicity fluctuations are found to be suppressed in
the van der Waals gas. The numerical calculations are presented for two values
of hard-core hadron radius, fm and 0.5 fm, as well as for the upper
limit of the excluded volume suppression effects.Comment: 19 pages, 4 figure
Multiplicity Fluctuations in Nucleus-Nucleus Collisions: Dependence on Energy and Atomic Number
Event-by-event multiplicity fluctuations in central C+C, S+S, In+In, and
Pb+Pb as well as p+p collisions at bombarding energies from 10 to 160 AGeV are
studied within the HSD and UrQMD microscopic transport approaches. Our
investigation is directly related to the future experimental program of the
NA61 Collaboration at the SPS for a search of the QCD critical point. The
dependence on energy and atomic mass number of the scaled variances for
negative, positive, and all charged hadrons is presented and compared to the
results of the model of independent sources. Furthermore, the nucleus-nucleus
results from the transport calculations are compared to inelastic proton-proton
collisions for reference. We find a dominant role of the participant number
fluctuations in nucleus-nucleus reactions at finite impact parameter . In
order to reduce the influence of the participant numbers fluctuations on the
charged particle multiplicity fluctuations only the most central events have to
be selected. Accordingly, the samples of the 1% most central nucleus-nucleus
collisions with the largest numbers of the projectile participants are studied.
The results are compared with those for collisions at zero impact parameter. A
strong influence of the centrality selection criteria on the multiplicity
fluctuations is pointed out. Our findings are essential for an optimal choice
of colliding nuclei and bombarding energies for the experimental search of the
QCD critical point.Comment: 26 pages, 12 figures, extended version, to be published in Phys. Rev.
Energy dependence of transverse mass spectra of kaons produced in p+p and p+pbar interactions.A compilation
The data on m_T spectra of K0S K+ and K- mesons produced in all inelastic p+p
and p+pbar interactions in the energy range sqrt(s)NN=4.7-1800GeV are compiled
and analyzed. The spectra are parameterized by a single exponential function,
dN/(m_T*dm_T)=C exp(-m_T/T), and the inverse slope parameter T is the main
object of study. The T parameter is found to be similar for K0S, K+ and K-
mesons. It increases monotonically with collision energy from T~30MeV at
sqrt(s)NN=4.7GeV to T~220MeV at sqrt(s)NN=1800GeV. The T parameter measured in
p+p and p+pbar interactions is significantly lower than the corresponding
parameter obtained for central Pb+Pb collisions at all studied energies. Also
the shape of the energy dependence of is different for central Pb+Pb
collisions and p+p(pbar) interactions.Comment: more differential analysis adde
Strangeness production time and the K+/pi+ horn
We construct a hadronic kinetic model which describes production of strange
particles in ultrarelativistic nuclear collisions in the energy domain of SPS.
We test this model on description of the sharp peak in the excitation function
of multiplicity ratio K+/pi+ and demonstrate that hadronic model reproduces
these data rather well. The model thus must be tested on other types of data in
order to verify the hypothesis that deconfinement sets in at lowest SPS
energies.Comment: proceedings of Hot Quarks 0
Strangeness and threshold of phase changes
We explore entropy and strangeness as signature of QGP for top AGS energy and
the energy scan at SPS. We find that the hadronization dynamics changes between
20 and 30 GeV projectile energy. The high energy results are consistent
with QGP.Comment: Presented at SQM07, to appear in JPG special issu
Energy Dependence of Multiplicity Fluctuations in Heavy Ion Collisions at the CERN SPS
Multiplicity fluctuations of positively, negatively and all charged hadrons
in the forward hemisphere were studied in central Pb+Pb collisions at 20A, 30A,
40A, 80A and 158A GeV. The multiplicity distributions and their scaled
variances are presented in dependence of collision energy as well as of
rapidity and transverse momentum. The distributions have bell-like shape and
their scaled variances are in the range from 0.8 to 1.2 without any significant
structure in their energy dependence. No indication of the critical point in
fluctuations are observed. The string-hadronic model UrQMD significantly
overpredicts the mean, but approximately reproduces the scaled variance of the
multiplicity distributions. The predictions of the statistical hadron-resonance
gas model obtained within the grand-canonical and canonical ensembles disagree
with the measured scaled variances. The narrower than Poissonian multiplicity
fluctuations measured in numerous cases may be explained by the impact of
conservation laws on fluctuations in relativistic systems.Comment: 26 pages, 34 figures, updated version including referee comment
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
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