Volume 147B, number 1,2,3 PHYSICS LETTERS 1 November 1984 PRODUCTION OF J/ql RESONANCES IN A QUARK-GLUON PLASMA WITH BJORKEN COOLING J. CLEYMANS Department of Theoretical Physics, Universitiit Bielefeld, D-4800 Bielefeld 1, Fed. Rep. Germany and C. VANDERZANDE 1 lnstituut voor Theoretische Fysika, Universiteit Leu yen, B-3030 Leu yen, Belgium Received 10 July 1984 The rapidity distribution of J/ff resonances produced in a quark-gluon plasma undergoing Bjorken cooling is calculated. Our results illustrate that a drastic change in the lepton pair spectrum, as found in usual hadronic collisions, is to be expected for large invariant masse~ High energy heavy ion collisions have been the sub- ject of much interest recently [1 ] due to the possibili- ty of producing a quark-gluon plasma (QGP) in such reactions. Asymptotic freedom predicts that such a QGP will behave like a free gas at very high densities and/or temperatures. In this limit the space-time evo- lution will be governed by the relativistic hydrody- namic equations describing a perfect fluid. This de- scription was used by Bjorken [2] in the central region of rapidity space where the net baryon number density is small and the inclusive nucleon-nucleon cross aec- tions show approximately a plateau independent of rapidity. Bjorken's solution for the energy density and for the temperature is characterized by a cooling law given by: T ~- To(ro/O 1/3 , (1) where T O is the initial temperature and r is the proper time of the fluid element. Bjorken's analysis was later generalised by Kajantie, Raitio, McLerran and Ruus- kanen [3] to the case where source terms are present in the hydrodynamic equations. One of the results of their analysis is that the highest energy densities will be reached in the central region of rapidity space. z Work supported by the Belgian IIKW. Whatever energy density is present initially in the frag- mentation regions will be dampened exponentially as the proper time increases. For this reason we have re- peated and extended a previous analysis [4], about the production of heavy resonances in a quark-gluon plas- ma. Such an analysis is of relevance for experiments which have been proposed e.g. at CERN to look at the spectrum of lepton pairs produced in ion collisions. In usual hadronic collisions resonances like the ¢ or the J/~k play a prominent role in such a spectrum. For this purpose we briefly recall the basic physical aspects of the calculations. Charmed particles, because of their heavy mass, will only very rarely be produced in a plas- ma. This is not a priori a handicap because the experi- ment triggers only on rare events like lepton pairs. Be- cause of their low production rate, there will be no chemical equilibrium for charmed quarks unless the QGP has an unusually long lifetime. Next, because of the long mean free path of charmed particles (due to their hadronic cross sections being much smaller) they have a great probability to escape from the QGP with- out further collisions. Thus there will not be thermo- dynamical equilibrium either. These arguments parallel those given for the production of lepton pairs through electromagnetic annihilation of quarks and antiquarks [5]. This is the reason why lepton pairs and charmed 186