Diphoton rates from thermalized matter resulting in ultra-relativistic heavy-ion collisions

Abstract
Diphoton radiation off strongly interacting matter resulting from ultrarelativistic heavy-ion collisions is estimated for SPS and RHIC conditions. At SPS emrgies tha thermal diphoton signal competes strongly with the Drell - Yan like background. For given charged-particle rapidity density a scenario with deconfinement transition predicts a larger diphoton yield than a scenario without phase transition. For RHIC energies we find that radiation from initially undersaturated (but gluon-rich) deconfined matter overwhelms the hadron gas and the Drell - Yan like diphotons in the invariant mass window 1 < M < 2.5 GeW. Due to this the so-called M₁ scaling is approximately obtained and can serve as additional tsol for discriminating radiation from quark-gluon matter.