Direct photon production and scaling properties in large and small system collisions

Abstract

We present recent measurement results of low momentum direct photons obtained by the PHENIX collaboration from Cu+Cu at 200 GeV and Au+Au at 200 GeV, 62.4 GeV, 39 GeV, as well as from p+p and p+Au at 200 GeV. In these measurements a large excess over the scaled reference p+p yield of direct photons is observed in A+A collisions, and a non-zero excess over the scaled p+p yield is found within systematic uncertainties in central p+Au collisions. Based upon the comparison between this recent data and the PHENIX published Au+Au data at 200 GeV, along with including the ALICE Pb+Pb data at 2760 GeV, PHENIX has found that the direct photon pT-spectra and their integrated yield, dNγ /dy, above 1 GeV/c in pT follow a universal scaling as a function of the charged-hadron multiplicity, (dNch /dρ]) α , with α = 1.25. Within experimental uncertainties, the direct photon scaling properties observed in large collision systems is independent of the colliding species, collision centrality selection and center-of-mass energy. One possible explanation is that the photon radiation sources may have similar properties for all colliding large systems across different energies. There is also another trend coming from integrated direct photon pT-spectra of small systems, which suggests that the QGP turn off/on transition region may exist between large and small system collisions.