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Beam energy and centrality dependence of direct-photon emission from ultrarelativistic heavy-ion collisions

Adare, A.*; Hasegawa, Shoichi   ; Imai, Kenichi; Nagamiya, Shoji*; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 666 of others*

The PHENIX collaboration presents first measurements of low-momentum (0.4 $$< p_T <$$ 3 GeV/$$c$$) direct-photon yields from Au$$+$$Au collisions at $$sqrt{s_{_{NN}}}$$ = 39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$$+$$Au collisions at $$sqrt{s_{_{NN}}}$$ = 200. Analyzing the photon yield as a function of the experimental observable $$dN_{rm ch}/deta$$ reveals that the low-momentum ($$>$$1 GeV/$$c$$) direct-photon yield $$dN_{gamma}^{rm dir}/deta$$ is a smooth function of $$dN_{rm ch}/deta$$ and can be well described as proportional to $$(dN_{rm ch}/deta)^alpha$$ with $$alpha approx$$ 1.25. This new scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and Large Hadron Collider, for centrality selected samples, as well as for different, $$A+A$$ collision systems. At a given beam energy the scaling also holds for high $$p_T$$ ($$>$$5 GeV/$$c$$) but when results from different collision energies are compared, an additional $$sqrt{s_{_{NN}}}$$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.



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Category:Physics, Multidisciplinary



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