Transverse-momentum distribution of dileptons in different scenarios for the QCD phase transition

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Abstract
The transverse-momentum distribution of lepton pairs produced in ultrarelativistic nuclear collisions is analyzed in different scenarios for the phase transition from quarks and gluons to hadrons. A first-order phase transition at equilibrium is characterized by an intermediate phase where quarks, gluons, and hadrons coexist for an appreciable amount of proper time. The subsequent transition from one phase to the other is therefore very smooth. This is reflected in the dilepton production rate by a smooth change from the small- to the large-dilepton-invariant-mass region. A detonation is characterized by substantial supercooling of the quark-gluon-plasma phase with a subsequent superheating of the hadronic phase. The superheating is so intense that the dilepton rate arises almost completely from the hadronic phase, leaving almost no trace from the quark-gluon phase. A second-order phase transition is characterized by an instantaneous transition from one phase to the other. This reflects itself in a clean separation in the invariant mass in the dilepton production rate. It is suggested that experimental measurement of the average transverse momentum as a function of the invariant mass might help considerably in determining the dilepton production mechanism and through it reveal part of the evolution of the quark-gluon-plasma phase to the hadronic phase.