449 research outputs found
Baryon number conservation and statistical production of antibaryons
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>1) 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 antiproton/pion ratio in nucleus-nucleus collisions at the CERN SPS energies. However, a consistent picture of antibaryon production within the statistical hadronization model has not yet been achieved. This is because the condition of constant hadronization temperature in the baryon-free systems leads to a contradiction with the data on the antiproton/pion ratio in e+e- interactions
J / psi suppression and enhancement in Au + Au collisions at the BNL RHIC
We consider the production of the J/psi mesons in heavy ion collisions at RHIC energies in the statistical coalescence model with an exact (canonical ensemble) charm conservation. The cc quark pairs are assumed to be created in the primary hard parton collisions, but the formation of the open and hidden charm particles takes place at the hadronization stage and follows the prescription of statistical mechanics. The dependence of the J/psi production on both the number of nucleon participants and the collision energy is studied. The model predicts the J/psi suppression for low energies, whereas at the highest RHIC energy the model reveals the J/psi enhancement
Charm coalescence at relativistic energies
The J/psi yield at midrapidity at the top RHIC (relativistic heavy ion collider) energy is calculated within the statistical coalescence model, which assumes charmonium formation at the late stage of the reaction from the charm quarks and antiquarks created earlier in hard parton collisions. The results are compared to the new PHENIX data and to predictions of the standard models, which assume formation of charmonia exclusively at the initial stage of the reaction and their subsequent suppression. Two versions of the suppression scenario are considered. One of them assumes gradual charmonium suppression by comovers, while the other one supposes that the suppression sets in abruptly due to quark-gluon plasma formation. Surprisingly, both versions give very similar results. In contrast, the statistical coalescence model predicts a few times larger J/psi yield in the most central collisions
Statistical coalescence model analysis of J / psi production in Pb + Pb collisions at 158 A GeV
Production of J/psi mesons in heavy ion collisions is considered within the statistical coalescence model. The model is in agreement with the experi- mental data of the NA50 Collaboration for Pb+Pb collisions at 158 A·GeV in a wide centrality range, including the so called anomalous suppression domain. The model description of the J/ psi data requires, however, strong enhancement of the open charm production in central Pb+Pb collisions. This model prediction may be checked in the future SPS runs
Second cluster integral and excluded volume effects for the pion gas
The quantum mechanical formula for Mayer s second cluster integral for the gas of relativistic particles with hard-core interaction is derived. The proper pion volume calculated with quantum mechanical formula is found to be an order of magnitude larger than its classical evaluation. The second cluster integral for the pion gas is calculated in quantum mechanical approach with account for both attractive and hard-core repulsive interactions. It is shown that, in the second cluster approximation, the repulsive -interactions as well as the finite width of resonances give important but almost canceling contributions. In contrast, an appreciable deviation from the ideal gas of pions and pion resonances is observed beyond the second clus- ter approximation in the framework of the Van der Waals excluded-volume model
Open charm enhancement in Pb + Pb collisions at SPS
The statistical coalescence model for the production of open and hidden charm is considered within the canonical ensemble formulation. The data for the J/psi multiplicity in Pb+Pb collisions at 158 A·GeV are used for the model prediction of the open charm yield. We find a strong enhancement of the open charm production, by a factor of about 2 4, over the standard hard-collision model extrapolation from nucleon-nucleon to nucleus-nucleus collisions. A possible mechanism of the open charm enhancement in A+A collisions at the SPS energies is proposed
Van der Waals excluded volume model for Lorentz contracted rigid spheres
Conventional cluster and virial expansions are generalized to momentum dependent interparticle potentials. The model with Lorentz contracted hard core potentials is considered, e.g. as hadron gas model. A Van der Waals-type model with a temperature dependent excluded volume is derived. Lorentz contraction effects at given temperature are stronger for light particles and make their effective excluded volume smaller than that of heavy ones
Phase transition in hot pion matter
The equation of state for the pion gas is analyzed within the third virial approximation. The second virial coeffcient is found from the pi pi -scattering data, while the third one is considered as a free parameter. The proposed model leads to a first-order phase transition from the pion gas to a more dense phase at the temperature Tpt < 136 MeV. Due to relatively low temperature this phase transition cannot be related to the deconfinement. This suggests that a new phase of hadron matter hot pion liquid may exist
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