Direct photons in WA98

Lebedev A, Aggarwal MM, Agnihotri A, Ahammed Z, Angelis ALS, Antonenko V, Arefiev V, Astakhov V, Avdeitchikov V, Awes TC, Baba PVKS, Badyal SK, Barlag C, Bathe S, Batiounia B, Bernier T, Bhalla KB, Bhatia VS, Blume C, Bock R, Bohne EM, Borocz Z, Bucher D, Buijs A, Busching H, Carlen L, Chalyshev V, Chattopadhyay S, Cherbatchev R, Chujo T, Claussen A, Das AC, Decowski MP, Delagrange H, Djordjadze V, Donni P, Doubovik I, Dutt S, Majumdar MRD, El Chenawi K, Eliseev S, Enosawa K, Foka P, Fokin S, Ganti MS, Garpman S, Gavrishchuk O, Geurts FJM, Ghosh TK, Glasow R, Gupta SK, Guskov B, Gustafsson HA, Gutbrod HH, Higuchi R, Hrivnacova I, Ippolitov M, Kalechofsky H, Kamermans R, Kampert KH, Karadjev K, Karpio K, Kato S, Kees S, Klein-Bosing C, Knoche S, Kolb BW, Kosarev I, Koutcheryaev I, Krumpel T, Kugler A, Kulinich P, Kurata M, Kurita K, Kuzmin N, Langbein I, Lebedev A, Lee YY, Lohner H, Luquin L, Mahapatra DP, Manko V, Martin M, Martinez G, Maximov A, Mgebrichvili G, Miake Y, Mir MF, Mishra GC, Miyamoto Y, Mohanty B, Morrison D, Mukhopadhyay DS, Naef H, Nandi BK, Nayak SK, Nayak TK, Neumaier S, Nianine A, Nikitine V, Nikolaev S, Nilsson P, Nishimura S, Nomokonov P, Nystrand J, Obenshain FE, Oskarsson A, Otterlund I, Pachr M, Pavliouk S, Peitzmann T, Petracek V, Pinganaud W, Plasil F, von Poblotzki U, Purschke ML, Rak J, Raniwala R, Raniwala S, Ramamurthy VS, Rao NK, Retiere F, Reygers K, Roland G, Rosselet L, Roufanov I, Roy C, Rubio JM, Sako H, Sambyal SS, Santo R, Sato S, Schlagheck H, Schmidt HR, Schutz Y, Shabratova G, Shah TH, Sibiriak I, Siemiarczuk T, Silvermyr D, Sinha BC, Slavine N, Soderstrom K, Solomey N, Sood G, Sorensen SP, Stankus P, Stefanek G, Steinberg P, Stenlund E, Stuken D, Sumbera M, Svensson T, Trivedi MD, Tsvetkov A, Tykarski L, Urbahn J, Van der Pijll EC, von Eijndhoven N, von Nieuwenhuizen GJ, Vinogradov A, Viyogi YP, Vodopianov A, Voros S, Wyslouch B, Yagi K, Yokota Y, Young GR

Abstract

A measurement of direct photon production in Pb-208+Pb-208 collisions at 158 A GeV has been carried out in the CERN WA98 experiment. Invariant yield of direct photons was extracted as a function of transverse momentum in the interval 0.5 < P-T < 3.3 GeV/c for peripheral, and 0.5 < P-T < 4.0 GeV/c for central collisions. Direct photon yield was extracted on a statistical basis as the difference between the measured inclusive spectrum of photons, and the calculated spectrum of photons which result from all hadrons with significant radiative decay contributions. While no direct photon excess was observed for the peripheral event sample, a significant direct photon signal, compared to statistical and systematical errors, was seen in central collisions at P-T > 1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions.