Low-momentum direct-photon measurement in Cu + Cu collisions at √sNN = 200 GeV

Abstract

We measured direct photons for pT < 5 GeV/c in minimum bias and 0%–40% most-central events at midrapidity for Cu+Cu collisions at √sNN = 200 GeV. The e+e− contribution from quasireal direct virtual photons has been determined as an excess over the known hadronic contributions in the e+e− mass distribution. A clear enhancement of photons over the binary scaled p+p fit is observed for pT < 4 GeV/c in Cu+Cu data. The pT spectra are consistent with the Au+Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the p+p baseline are 285 ± 53(stat) ± 57(syst) MeV/c and 333 ± 72(stat) ± 45(syst) MeV/c for minimum bias and 0%–40% most-central events, respectively. The rapidity density, dN/dy, of photons demonstrates the same power law as a function of dNch/dη observed in Au+Au at the same collision energy.