287 research outputs found
Pion Interferometry: Recent results from SPS
Recent results from the CERES experiment at the SPS are reviewed. Emphasis is
put on the centrality and beam energy dependence, and the results are put into
perspective with findings at lower and higher beam energies. The rather weak
beam energy dependence of the HBT radii maybe understood in terms of a
transition from baryon to pion dominated freeze-out. The observed short
lifetimes end emission durations are presently in contradiction to results from
model calculations.Comment: Invited talk at the International Workshop XXX on Gross Properties of
Nuclei and Nuclear Excitations - Ultrarelativistic Heavy-Ion Collisions, Jan.
13-19, 2002, Hirschegg, Austria. 11 pages, 8 figure
Directed and Elliptic Flow in 158 GeV/Nucleon Pb + Pb Collisions. Draft 2.2.
The directed and elliptic flow of protons and charged pions has been observed from the semi-central collisions of a 158 GeV/nucleon Pb beam with a Pb target. The rapidity and transverse momentum dependence of the flow has been measured. The directed flow of the pions is opposite to that of the protons but both exhibit negative flow at low pt. The elliptic flow of both is fairly independent of rapidity but rises with pt. PACS numbers: 25.75.-q, 25.75.L
Xi and Xi-bar production in 158 GeV/nucleon Pb + Pb collisions
We report measurements of Xi and Xi-bar hyperon absolute yields as a function of rapidity in 158 GeV/c Pb+Pb collisions. At midrapidity, dN/dy = 2.29 +/- 0.12 for Xi, and 0.52 +/- 0.05 for Xi-bar, leading to the ratio of Xi-bar/Xi = 0.23 +/- 0.03. Inverse slope parameters fitted to the measured transverse mass spectra are of the order of 300 MeV near mid-rapidity. The estimated total yield of Xi particles in Pb+Pb central interactions amounts to 7.4 +/- 1.0 per collision. Comparison to Xi production in properly scaled p+p reactions at the same energy reveals a dramatic enhancement (about one order of magnitude) of Xi production in Pb+Pb central collisions over elementary hadron interactions
Hadronic expansion dynamics in central Pb + Pb collisions at 158 GeV per nucleon
Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to extract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced
Baryon stopping and charged particle distributions in central Pb+Pb collisions at 158 GeV per nucleon
Net proton and negative hadron spectra for central \PbPb collisions at 158 GeV per nucleon at the CERN SPS were measured and compared to spectra from lighter systems. Net baryon distributions were derived from those of net protons, utilizing model calculations of isospin contributions as well as data and model calculations of strange baryon distributions. Stopping (rapidity shift with respect to the beam) and mean transverse momentum \meanpt of net baryons increase with system size. The rapidity density of negative hadrons scales with the number of participant nucleons for nuclear collisions, whereas their \meanpt is independent of system size. The \meanpt dependence upon particle mass and system size is consistent with larger transverse flow velocity at midrapidity for \PbPb compared to \SS central collisions
Two-proton correlations from 158 AGeV Pb+Pb central collisions
The two-proton correlation function at midrapidity from Pb+Pb central collisions at 158 AGeV has been measured by the NA49 experiment. The results are compared to model predictions from static thermal Gaussian proton source distributions and transport models RQMD and VENUS. An effective proton source size is determined by minimizing CHI-square/ndf between the correlation functions of the data and those calculated for the Gaussian sources, yielding 3.85 +-0.15(stat.) +0.60-0.25(syst.) fm. Both the RQMD and the VENUS model are consistent with the data within the error in the correlation peak region
Event-by-event fluctuations of average transverse momentum in central Pb+Pb collisions at 158 GeV per nucleon
We present first data on event-by-event fluctuations in the average transverse momentum of charged particles produced in Pb+Pb collisions at the CERN SPS. This measurement provides previously unavailable information allowing sensitive tests of microscopic and thermodynamic collision models and to search for fluctuations expected to occur in the vicinity of the predicted QCD phase transition. We find that the observed variance of the event-by-event average transverse momentum is consistent with independent particle production modified by the known two-particle correlations due to quantum statistics and final state interactions and folded with the resolution of the NA49 apparatus. For two specific models of non-statistical fluctuations in transverse momentum limits are derived in terms of fluctuation amplitude. We show that a significant part of the parameter space for a model of isospin fluctuations predicted as a consequence of chiral symmetry restoration in a non-equilibrium scenario is excluded by our measurement
Modification of jet-like correlations in Pb-Au at the SPS
A high statistics study of high- two-particle azimuthal correlations in
Pb-Au at GeV, performed by the CERES experiment at the
CERN-SPS, is presented. A broad away-side correlation with significant dip at
is observed. The shape and magnitude of the correlation
is similar to measurements at RHIC. In comparison to PYTHIA calculations, we
observe a significant excess of soft particles at the away-side. A study of
charge correlations between trigger and associated particles disfavors vacuum
fragmentation of the away-side jet and suggests significant energy transfer of
the hard-scattered parton to the medium.Comment: Proceedings to Quark Matter 2009 Conference, Knoxville, Tennessee,
US
Kinetic Freeze-out and Radial Flow in 11.6 A GeV Au+Au Collisions
We study the kinetic freeze-out conditions of hadrons in Au+Au collisions at
11.6 A GeV/c using different parametrizations of an expanding thermal fireball.
We take into account the available double differential momentum spectra of a
variety of particle species, covering a large fraction of the total momentum
space. The overall fit to the data is very good and indicates a relatively low
kinetic freeze-out temperature of about 90 MeV with an average transverse
expansion velocity at midrapidity of about 0.5 c.Comment: 5 pages ReVTeX, incl. 3 figures and 2 tables. Revised version with
improved discussion of model and added references. Accepted by Phys. Lett.
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