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Transverse Mass Spectra at RHIC

The transverse mass spectra of J/ψ and ψ ′ mesons and Ω hyperons produced in central Au+Au collisions at RHIC energies are discussed within a statistical model used successfully for the interpretation of the SPS results. The comparison of the presented model with the future RHIC data should serve as a further crucial test of the hypothesis of statistical production of charmonia at hadronization. Finally, in case of validity, the approach should allow to estimate the mean transverse flow velocity at the quark gluon plasma hadronization. The concepts of chemical (hadron multiplicities) and kinetic (hadron momentum spectra) freeze–outs were introduced to interpret data on hadron production in relativistic nucleus–nucleus (A+A) collisions. The equilibrium hadron gas (HG) model describes remarkably well the light hadron multiplicities measured in A+A collisions at the SPS [1] and RHIC [2] energies, where the creation of a quark–gluon plasma (QGP) is expected. Recently it was found [3] that also charmonium yield systematics in nuclear collisions at the SPS [4] follow the pattern predicted by the hadron gas model. The hadronization temperature parameter extracted from the fits to the hadron multiplicities is similar for 1 both energies: T H = 170 ± 10 MeV. It is close to an estimate of the temperature T C for the QGP–HG transition at zero baryonic density. Experimental results on inclusive hadron spectra and correlations show evidence for a hydrodynamic expansion of the matter created in heavy ion collisions. Strong transverse flow effects in Pb+Pb collisions at the SPS (average collective transverse velocity is approximately 0.5 ÷ 0.6) are firmly established from the combined analysis [5,6] of the pion transverse mass spectra and correlations. The kinetic ('thermal') freeze–out of pions and nucleons seems to occur at a rather late stage of an A+A reaction. The thermal freeze–out temperature parameter of pions measured by the NA49 Collaboration [7] for central Pb+Pb collisions at the SPS is T f ∼ = 120 MeV. Further exploration of idea of the statistical J/ψ production [3] led to the formulation of the hypothesis that the kinetic freeze–out of J/ψ and ψ ′ mesons takes place directly at hadronization [8]. This means that chemical and thermal freeze–outs occur simultaneously for those mesons and they, therefore, carry information on the flow velocity of strongly interacting matter just after the transition to the HG phase. A possible influence of the effect of rescattering in the hadronic phase on the …