Detailed measurement of the e+e- pair continuum in p+p and Au+Au collisions at √sNN=200 GeV and implications for direct photon production

摘要

PHENIX has measured the e+e- pair continuum in √sNN=200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e+e- yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the ϕ and the J/ψ meson, the yield is consistent with expectations from correlated cc̄ production, although other mechanisms are not ruled out. In the low-mass region, below the ϕ, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7±0.4stat±1.5syst±0.9model. At low mass (mee<0.3 GeV/c2) and high pT (1T<5 GeV/c) an enhanced e+e- pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in pT, with inverse slope T=221±19stat±19syst MeV. Hydrodynamical models with initial temperatures ranging from Tinit≃300–600 MeV at times of 0.6–0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low pT<1 GeV/c the low-mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T≃100 MeV. Theoretical models underpredict the low-mass, low-pT enhancement.