Z. Phys. C 61, 109-114 (1994) ZEITSCHRIFT FOR PHYSIK C 9 Springer-Verlag 1994 Transverse momentum dependence of J] suppression in an equilibrium hadron gas* Dariusz Prorok and Ludwik Turko Institute of Theoretical Physics, University of Wroctaw, P1.Maksa Borna 9, PL-50-204 Wroctaw, Poland Received 5 July 1993 Abstract. We consider an ideal gas of massive hadrons in thermal and chemical equilibrium. The gas expands lon- gitudinally in accordance with the Bjorken law. We show that J/T suppression in such an environment is essentially the same as in a pion gas of the equal initial energy density and agrees with the NA38 data. 1 Introduction Since the paper of Matsui and Satz appeared [1], there have been some doubts about J/7~ suppression as a unique signal of a quark-gluon plasma (QGP) appear- ance during a heavy-ion collision. Many authors have argued that the observed suppression is caused by J/T absorption in a hadron gas which, instead of the plasma, appears in the central rapidity region (CRR) of the colli- sion [-2-5]. When combined with initial state scattering of primordial gluons, this model indeed explains the data [-6-83. In the simplest case the hadronic matter is supposed to be made of noninteracting massless pions which expand longitudinally according to a hydrodynamicat evolution. However an assumption of a thermodynamical equilib- rium leads to the conclusion that a system under consid- eration should consist of all species of hadrons allowed by conditions of the equilibrium. We are going to consider a process of J/7~ absorption in this more realistic case as compared to the absorption in an oversimplified pion gas model (some preliminary estimates of j/tp absorption in a p and ~o gas has been delivered in [-3, 4]). There are few reasons for different behaviour of both systems. J/T is absorbed in dense hadronic matter through interactions of the type J/TJ + h + D § X, (1) * Work partially supported by the Polish Committee for Scientific Research under contract KBN-200579101 where h denotes a hadron, D is a charm meson and X means a particle which is necessary to conserve the charge, baryon number or strangeness. Threshold effects are much more important for a pion gas than for a system where higher resonances are taken into account. Cross sections for reaction (1) are different for mesons and nuc- leons. We assume that the absorption cross section for mesons, o'm, is 2/3 of the corresponding cross section for baryons, ab, which is due to the quark counting. To describe J/7 ~ absorption in the expanding hadronic gas we generalize a picture of the CRR in heavy ion collisions as given in [5, 9]. A thermodynamical equilibrium is reached at some initial temperature To and we assume further that a hadronic gas undergoes a one dimensional uniform expansion. The effect of absorption cuts off when the temperature reaches the freez-out temperature Tf.o after some cooling time At. An assumption about a one dimen- sional expansion allows to solve hydrodynamical equa- tions to find time dependencies of physical quantities [9]. We consider as input parameters the freez-out temper- ature and experimental results on the baryon number density in the CCR. Values of the initial temperature To, of the evolution time At, and of chemical potentials depend on the structure of the hadronic gas and these are parameters of a model. We shall see that the initial temper- ature for the gas of pions is much higher than for the hadronic gas with heavy particles and resonances taken into account. But the effective time of absorption for the gas of pions is roughly the same as in the case of hadrons. So we obtain both factors increasing a fraction of surviving J/7j particles in the gas of hadrons as well as factors which decrease this fraction. The lower initial temperature makes absorption more difficult but higher cross sections and weaker overall threshold effects increase an absorp- tion. 2 J/~ absorption in hadronic matter For an ideal hadron gas in thermal and chemical equi- librium, which consists of l species of particles, energy