Hadronic centrality dependence in nuclear collisions

The kaon number density in nucleus+nucleus and p+p reactions is investigated for the first time as a function of the initial energy density $\epsilon$ and is found to exhibit a discontinuity around $\epsilon$=1.3 GeV/fm$^3$. This suggests a higher degree of chemical equilibrium for $\epsilon >$ 1.3 GeV/fm$^3$. It can also be interpreted as reflection of the same discontinuity, appearing in the chemical freeze out temperature (T) as a function of $\epsilon$. The $N^{\alpha \sim 1}$ dependence of (u,d,s) hadrons, whith N the number of participating nucleons, also indicates a high degree of chemical equilibrium and T saturation, reached at $\epsilon >$1.3 GeV/fm$^3$. Assuming that the intermediate mass region (IMR) dimuon enhancement seen by NA50 is due to open charm ($D \bar{D}$), the following observation can be made: a) Charm is not equilibrated. b) $J/\Psi/D \bar{D}$ suppression -unlike $J/\Psi/DY$- appears also in S+A collisions, above $\epsilon$ $\sim$1 GeV/fm$^3$. c) Both charm and strangeness show a discontinuity near the same $\epsilon$. d) $J/\Psi$ could be formed mainly through $c \bar{c}$ coalescence. e) The enhancement factors of hadrons with u,d,s,c quarks may be connected in a simple way to the mass gain of these particles if they are produced out of a quark gluon plasma (QGP). We discuss these results as possible evidence for the QCD phase transition occuring near $\epsilon \sim$1.3 GeV/fm$^3$.Comment: 4 pages, 4 figures, proceedings of Vth International Conference on Strangeness in Quark Matter, 20-25 July 2000, Berkeley, California. To appear in Journal of Physics G: Nuclear and Particle Physic

Mapping out the QCD phase transition in multiparticle production

We analyze multiparticle production in a thermal framework for 7 central nucleus nucleus collisions, $e^+$+ $e^-$ annihilation into hadrons on the Z resonance and 4 hadronic reactions (p+p and p+$\bar{p}$ with partial centrality selec tion), with center of mass energies ranging from $\sqrt{s}$= 2.6 GeV (per nucleon pair) to 1.8 TeV. Thermodynamic parameters at chemical freeze-out (temperature and baryon and strangeness fugacities) are obtained from appropriate fits, generally improving in quality for reactions subjected to centrality cuts. All systems with nonvanishing fugacities are extrapolated along trajectories of equal energy density, density and entropy density to zero fugacities. The so obtained temperatures extrapolated to zero fugacities as a function of initial energy density $\epsilon_{in}$ universally show a strong rise followed by a saturating limit of $T_{lim}$ = 155 $\pm$ 6 $\pm$ 20 MeV. We interpret this behaviour as mapping out the boundary between quark gluon plasma and hadronic phases. The ratio of strange antiquarks to light ones as a function of the initial energy density $\epsilon_{in}$ shows the same behaviour as the temperature, saturating at a value of 0.365 $\pm$ 0.033 $\pm$ 0.07. No distinctive feature of 'strangeness enhancement' is seen for heavy ion collisions relative to hadronic and leptonic reactions, when compared at the same initial energy density

Charm in nuclear reactions in sqrt(s)=17 and 19 GeV

Consequences resulting from the D Dbar excess derived indirectly by the NA50 experiment in S+U and Pb+Pb collisions at sqrt(s)=19, 17 GeV, relevant for the identification of the QCD phase transition in these collisions, are discussed. The dependence of open and closed charm yields in Pb+Pb collisions on the number of participating nucleons (N) indicates non thermal charm production and J/Psi dissociation, stronger than the absorption seen in any other elementary hadron. The J/Psi in central Pb+Pb collisions could originate dominantly from c cbar pair coalescence out of a hadronizing quark and gluon environment. Furthermore, the J/Psi appears to be suppressed in S+U collisions at sqrt(s)=19 GeV, as opposed to current interpretations. A significant change in the (J/Psi)/D Dbar ratio as well as in the number density of kaons is observed above energy density approx. 1 GeV/fm^3, suggesting a change of phase at this energy density, and underlining the importance of direct open charm measurements.Comment: (23 pages, 7 figures

Lambda production in central Pb+Pb collisions at CERN-SPS energies

In this paper we present recent results from the NA49 experiment for $\Lambda$ and $\bar{\Lambda}$ hyperons produced in central Pb+Pb collisions at 40, 80 and 158 A$\cdot$GeV. Transverse mass spectra and rapidity distributions for $\Lambda$ are shown for all three energies. The shape of the rapidity distribution becomes flatter with increasing beam energy. The multiplicities at mid-rapidity as well as the total yields are studied as a function of collision energy including AGS measurements. The ratio $\Lambda/\pi$ at mid-rapidity and in 4$\pi$ has a maximum around 40 A$\cdot$GeV. In addition, $\bar{\Lambda}$ rapidity distributions have been measured at 40 and 80 A$\cdot$GeV, which allows to study the $\bar{\Lambda}$/$\Lambda$ ratio.Comment: SQM proceedings. J. Phys. G: Nucl. Part. Phys.: submitte

An investigation of intermittency in proton-gold, oxygen-gold, sulphur-gold and sulphur-sulphur interactions at 200 GeV per nucleon

The intermittency phenomenon is investigated in proton-gold, oxygen-gold, sulphur-gold and sulphur-sulphur collisions at 200 GeV per nucleon. The data were taken with the NA35 streamer chamber detector at the CERN SPS. The data samples are carefully corrected for double counting of tracks and for contamination from photon conversions, particle decays and secondary interactions. The analysis is carried out in terms of factorial moments, using a new definition, and of correlation integrals. Both methods show the presence of nonstatistical fluctuations ('intermittency'). The main conclusions are: the observation of the effect for pairs of hadrons with negative charges and its near absence for pairs of opposite charges is consistent with the assumption that Bose-Einstein correlations yield the dominant contribution. The 'intermittency' effect is not proportional to (dn/dy)"-"1 which would be expected in superposition models. The measured relation between the second and third factorial moment for negative hadrons indicates that genuine 3-particle correlations are small. The predictions of a Monte Carlo simulation which is based on the Lund Fritiof model with the inclusion of Bose-Einstein correlations agree with the experimental results. (orig.)Available from TIB Hannover: RR 2916(93-27) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Multiplicity distributions in small phase-space domains in central nucleus-nucleus collisions

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Strange particle production in nuclear collisions at 200-GeV per nucleon

Multiplicities and spectra of strange particles ($\Lambda ,\bar \Lambda ,K_S^0 ,K^ +$ andK− produced in central32S+S,32S+Ag and32S+Au collisions at 200 GeV per nucleon are presented and compared with data on strange particle production in protonnucleus and nucleon-nucleon interactions. It is shown that strangeness production in32S+Ag collisions is enhanced by a factor of two, similar to that found previously in central32S+S collisions