EXPANDING QUARK-GLUON PLASMAS - TRANSVERSE FLOW, CHEMICAL EQUILIBRATION, AND ELECTROMAGNETIC-RADIATION

We investigate the chemical equilibration of the parton distributions in collisions of two heavy nuclei. We use initial conditions obtained from a self-screened parton cascade calculation and, for comparison, f rom the HUING model. We consider a one-dimensional as well as a three- dimensional expansion of the parton plasma and find that the onset of the transverse expansion impedes the chemical equilibration. At energi es of 100 GeV/nucleon, the results for one-dimensional and three-dimen sional expansions are quite similar except at large values of the tran sverse radius. At energies of several TeV/nucleon, the plasma initiall y approaches chemical equilibrium, but then is driven away from it, wh en the transverse velocity gradients develop. We find that the total p arton multiplicity density remains essentially unaffected by the flow, but the individual concentrations of quarks, antiquarks, and gluons a re sensitive to the transverse flow. The consequences of the flow are also discernible in the transverse momenta of the partons and in the l epton pair spectra, where the flow causes a violation of the so-called M-T scaling.