Enhanced dilepton radiation at RHIC

Recently, there is growing evidence that a new state of matter is formed in s N N = 200 GeV  Au+Au collisions at RHIC: a strongly coupled Quark Gluon Plasma of partonic degrees of freedom which develops a collective motion. Dilepton spectra are not affected by strong interaction and can therefore probe the whole time evolution of the collision. Thus they may be sensitive to the onset of deconfinement, chiral symmetry restoration, as well as the production of thermal photons. The PHENIX experiment measured the production of e + e − pairs in p + p and Au+Au collisions at s N N = 200 GeV . An enhanced dilepton yield in the mass range 150 < m e e < 750 MeV / c 2 is measured. The excess increases faster with centrality than the number of participating nucleons, and is concentrated at p T   < 1 GeV / c . At higher p T the excess below 300 MeV/c2 has been related to an enhanced production of direct photons, possibly of thermal origin.