Abstrakt
The measurement of direct photons from Au + Au collisions at ,/sNN = 39 and 62.4 GeV in the transverse-momentum range 0.4 < pT < 3 Gev/c is presented by the PHENIX collaboration at the BNLRelativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems.
A universal scaling is observed when the direct-photon pT spectra for different center-of-mass energies and for different centrality selections at ,/sNN = 62.4 GeV is scaled with (dNch/d eta)alpha for alpha = 1.21 +/- 0.04. This scaling also holds true for direct-photon spectra from Au + Au collisions at ,/sNN = 200 GeV measured earlier by PHENIX, as well as the spectra from Pb + Pb at ,/sNN = 2760 GeV published by ALICE.
The scaling power alpha seems to be independent of pT, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to pT of 2 Gev/c.
The spectra have a local inverse slope Teff increasing with pT of 0.174 +/- 0.018 Gev/c in the range 0.4 < pT < 1.3 Gev/c and increasing to 0.289 +/- 0.024 Gev/c for 0.9 < pT < 2.1 Gev/c. The observed similarity of low-pT direct-photon production from ,/sNN = 39 to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.
