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

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© 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 . We measured direct photons for pT<5GeV/c in minimum bias and 0%-40% most-central events at midrapidity for Cu+Cu collisions at sNN=200GeV. 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<4GeV/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.