Marek Gaździcki

A method to study “equilibration” in nucleus-nucleus collisions

A method to study “equilibration level” of a system created in high energy nucleus-nucleus collisions is proposed. The method is based on the fact that the correlation between momentum distribution and particle multiplicity observed in nucleon-nucleon interactions can be also measured in nucleus-nucleus collisions. It is argued that the magnitude of the residual correlation can be used as a measure of the system “equilibration level”. The method is effective even when changes of the “equilibration level” do not modify the inclusive distribution. The quantitative estimate of the effect to be measured is given. Finally, questions arising in practical application of the method are discussed.

Strangeness in nuclear collisions

Data on the mean multiplicity of strange hadrons produced in minimum bias proton-proton and central nucleus-nucleus collisions at momenta between 2.8 and 400 GeV/c per nucleon have been compiled. The multiplicities for nucleon-nucleon interactions were constructed. The ratios of strange particle multiplicity to participant nucleon as well as to pion multiplicity are larger for central nucleus-nucleus collisions than for nucleon-nucleon interactions at all studied energies. The data at AGS energies suggest that the latter ratio saturates with increasing masses of the colliding nuclei. The strangeness to pion multiplicity ratio observed in nucleon-nucleon interactions increases with collision energy in the whole energy range studied. A qualitatively different behaviour is observed for central nucleus-nucleus collisions: the ratio rapidly increases when going from Dubna to AGS energies and changes little between AGS and SPS energies. This change in the behaviour can be related to the increase in the entropy production observed in central nucleus-nucleus collisions at the same energy range. The results are interpreted within a statistical approach. They are consistent with the hypothesis that the Quark Gluon Plasma is created at SPS energies, the critical collision energy being between AGS and SPS energies.

Fluctuations and deconfinement phase transition in nucleus nucleus collisions

We propose a method to experimentally study the equation of state of strongly interacting matter created at the early stage of nucleus–nucleus collisions. The method exploits the relation between relative entropy and energy fluctuations and equation of state. As a measurable quantity, the ratio of properly filtered multiplicity to energy fluctuations is proposed. Within a statistical approach to the early stage of nucleus-nucleus collisions, the fluctuation ratio manifests a non–monotonic collision energy dependence with a maximum in the domain where the onset of deconfinement occurs.

Chemical freeze-out conditions in central S-S collisions at 200A GeV

We determine the chemical freeze-out parameters of hadronic matter formed in central S-S collisions at 200 A GeV, analyzing data from the NA35 collaboration at CERN. In particular we study the quark (baryon number) and strange quark fugacities, as well as the strange quark phase-space occupancy and the freeze-out temperature. The strange quark chemical potential is found to be consistent with zero, and the strange quark abundance is found to be saturated as would be expected for the deconfined phase. We study the stability of our results against variations in the analysis procedure and with respect to a restriction of the data to the central rapidity region. The same analysis applied to nucleonnucleon data yields significantly different freeze-out parameters, in particular regarding the degree of strangeness saturation.

Characteristics of π−-meson multiplicity distributions in central collisions of12C and16O with nuclei at a momentum of 4.5 GeV/c per incident nucleon

Multiplicities of π−-mesons from central collisions of12C and16O(4.5 GeV/c/n momentum) with several target nuclei were studied in a streamer chamber experiment. The parameter η=(〈n−2〉−〈n−〉2)/〈n−〉 was determined for several samples of events characterized by different values of Θch — the “vetoangle” for emission of the projectile charged fragments. The value of η is shown to decrease when the projectile mass number increases from 12 to 16 and to decrease significantly when Θch increases from 0° to 4°, whereas it remains nearly constant when Θch increases further to 14°. The results are compared with predictions of some theoretical models.

Baryon number conservation and statistical production of antibaryons

Abstract The statistical production of antibaryons is considered within the canonical ensemble formulation. We demonstrate that the antibaryon suppression in small systems due to the exact baryon number conservation is rather different in the baryon-free ( B =0) and baryon-rich ( B ≥2) systems. At constant values of temperature and baryon density in the baryon-rich systems the density of the produced antibaryons is only weakly dependent on the size of the system. For realistic hadronization conditions this dependence appears to be close to B /( B +1) which is in agreement with the preliminary data of the NA49 Collaboration for the p /π ratio in nucleus–nucleus collisions at the CERN SPS energies. The statistical hadronization model is also used to describe the data on the p /π ratio in e + +e − interactions. We find that the assumption of a constant hadronization temperature leads in this case to a contradiction with data and should be substituted by another hadronization criterion.

Strangeness enhancement and strangeness suppression in nuclear collisions at 200 GeV/N

Data on yields of strange and negative hadrons in full phase space produced in proton-nucleus and central S+S collisions at 200 GeV/nucleon are reviewed. The total number of produced strange and nonstrange valence quark-antiquark pairs is determined on the basis of the hadron multiplicity at freezout. It is shown that the strangeness production factor λs is increased by a factor of 2 in central S+S collisions, whereas no increase is observed in proton-nucleus collisions.

Unified approach to theJ/Ψ suppression and strangeness enhancement in high-energy nucleus-nucleus collisions

A picture of nuclear interactions at high energies (pLab≧100 GeV/c) is proposed, where the only essential difference between nucleon-neucleon and central nucleus-nucleus collisions is the size of the interaction zone. In both cases intermediate multi-parton systems are created, which achieve thermodynamical equilibrium with respect to characteristics of light quarks and gluons. The characteristics of heavier quarks are far from equilibrium ones in nucleon-nucleon collisions, but the degree of equilibration is substantially increased in central heavy-ion collisions. Therefore, the multi-parton systems produced in central nucleus-nucleus interactions can manifest themseleves as the quark-gluon plasma. A simple model based on this picture is formulated and confronted with the experimental data concerningJ/Ψ suppression and strangeness yield enhancement, both revealed recently in central nucleus-nucleus collisions.

Evidence for Statistical Production ofJ/ψMesons in Nuclear Collisions at 158–200AGeV

The hypothesis of statistical production of J/psi mesons at hadronization is formulated and checked against experimental data. It explains in the natural way the observed scaling behavior of the J/psi to pion ratio at the CERN SPS energies. Using the multiplicities of J/psi and eta mesons the hadronization temperature T_H = 175 MeV is found, which agrees with the previous estimates of the temperature parameter based on the analysis of the hadron yield systematics.

Test of the Coherent Tube Model approach to relativistic nucleus-nucleus interactions

Coherent Tube Model predictions concerning secondary pion multiplicities and transverse momenta were compared with the experimental data. It was shown that the discrepancies between the model and our experimental data could not be removed by taking into account fragmentation processes.