Transverse-massM⊥dependence of dilepton emission from preequilibrium and quark-gluon plasma in high energy nucleus-nucleus collisions

The dependence on invariant mass M and transverse momentum ${\mathit{q}}_{\mathrm{\perp }}$ of lepton pairs emitted by quarks during the early stage of central collisions between gold nuclei at √s =200A GeV is investigated within the parton cascade model. For fixed transverse mass ${\mathit{M}}_{\mathrm{\perp }}$ between 2 and 3 GeV, the spectrum is thermal shaped at low ${\mathit{q}}_{\mathrm{\perp }}$ and exhibits a power law behavior at larger ${\mathit{q}}_{\mathrm{\perp }}$. A suspected ${\mathit{M}}_{\mathrm{\perp }}$ scaling is not observed in the calculated distribution, but the significant shape of the spectrum and the large yield from preequilibrium emission may nevertheless serve as a characteristic signal for a parton plasma formation.