Low-momentum direct photon measurement in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV

• Main Authors: Ajitanand, N. N, Alfred, M, Aronson, S. H, Barish, K. N, Bassalleck, B, Bathe, S, Belmont, R, Berdnikov, A, Boer, M, Boissevain, J. G, Chernichenko, S, Cleven, C. R, Csanád, M, Deaton, M. B, Desmond, E. J, Drees, A, Dubey, A. K, Durham, J. M, Durum, A, Enokizono, A, En'yo, H, Fan, W, Fields, D. E, Fokin, S. L, Fraenkel, Z, Frantz, J. E, Fukao, Y, Garg, P, Ge, H, Goto, Y, Greene, S. V, Haslum, E, Hodges, A, Holzmann, W, Homma, K, Ichihara, T, Inaba, M, Isenhower, D, Jacak, B. V, Jia, J, Jin, J, Kajihara, F, Kametani, S, Kikuchi, J, Kim, E, Kinney, E, Klein-Boesing, C, Kochetkov, V, Komkov, B, Kotov, D, Kurihara, N, Kurita, K, Kyle, G. S, Lajoie, J. G, Majoros, T, Mao, Y, Mašek, L, Mignerey, A. C, Mishra, M, Mitrovski, M, Morrison, D. P, Morrow, S. I, Newby, J, Norman, B. E, Novitzky, N, Nyanin, A. S, Okada, K, Koop, J. D. Orjuela, Palounek, A. P. T, Perepelitsa, D. V, Pinkenburg, C, Riabov, Y, Rinn, T, Roche, G, Rosnet, P, Saito, N, Samsonov, V, Sato, S, Sawada, S, Schmoll, B. K, Semenov, V, Shimomura, M, Sugitate, T, Suire, C, Sziklai, J, Takagi, S, Tanaka, Y, Taranenko, A, Torii, H, Tsuchimoto, Y, Vale, C, Virius, M, Wei, F, Winter, D, Xie, W, Yanovich, A, Ying, J, Young, G. R, Younus, I, Zolin, L
• Format: Journal Article
• Language: English
• Published: 10.05.2018
• Subjects: Physics - High Energy Physics - Experiment; Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.1805.04066

Phys. Rev. C 98, 054902 (2018) We have measured direct photons for $p_T<5~$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $e^{+}e^{-}$ contribution from quasi-real 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 $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ 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$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$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 $dN_{\rm ch}/d\eta$ observed in Au$+$Au at the same collision energy.