Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions

We study the induced photon bremsstrahlung from a fast quark produced in AA-collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions the induced photon spectrum is sharply peaked at photon energy close to the initial quark energy. In this region the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses [4, 5, 6, 7] our results show that at RHIC and LHC energies the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production, and even may enhance it at pT ∼ 5− 15 GeV. 1. In resent years much attention has been attracted to the direct photon production in AA-collisions at RHIC and LHC energies (see, for example, [1, 2, 3] and references therein). It is expected that for sufficiently small transverse momenta (pT ∼ < 3− 4 GeV) the dominating source of the direct photons at RHIC and LHC is radiation from quarkgluon plasma (QGP), and at higher pT it is hard partonic mechanisms (Compton-process, quark-antiquark annihilation, and bremsstrahlung from fast quarks (antiquarks) produced in hard reactions) [1, 2]. Nuclear effects should modify the pQCD partonic contribution to the direct photons in AA-collisions as compared to that in pp interaction. For the Compton and annihilation processes which occur at small distances this modification, related to the initial state interaction (ISI) effects (nuclear shadowing and Cronin effect), is relatively small. However, one can expect strong final-state interaction (FSI) effects in the QGP for the bremsstrahlung which occurs at large distances. Investigation of the influence of the FSI on photon bremsstrahlung is of great interest from the point of view using the direct photons as a probe for the QGP at RHIC and LHC. It is especially important for LHC energies where the bremsstrahlung component is the dominating partonic mechanism [1, 4, 5]. It was suggested [4, 5, 6, 7] that at RHIC and LHC the quark energy loss in the QGP phase due to the induced gluon radiation related to the multiple scattering should suppress strongly the bremsstrahlung contribution. At LHC energies it is equivalent to strong suppression of the total contribution of the pQCD mechanisms. In [4, 5, 6, 7] it was assumed that the only effect of QGP on photon bremsstrahlung comes from the shift of the quark energy to a smaller value due to gluon emission before photon radiation. However, the analyses [4, 5, 6, 7] missed two essential points. First of all, multiple scattering which fast quarks undergo in the QGP must enhance photon radiation due to the induced photon emission. Also, the assumption that all gluons are radiated before photon emission is not justified since the formation lengths for gluon and photon radiation are of the same order. In this case the gluons radiated after the photon do not suppress photon emission. For this reason one can expect that analyses [4, 5, 6, 7] overestimate suppression of photon bremsstrahlung. In the present paper we address the photon bremsstrahlung at RHIC and LHC energies accounting for the induced photon radiation and the effect of finite gluon formation length. 2. The contribution of the bremsstrahlung mechanism to the cross section of photon production can be written as [8, 9] dσ γ (pT ) dydpT = 1