Thermal radiation and inclusive production in the Kharzeev-Levin-Nardi model for ion-ion collisions

We show that in order to obtain a successful description of the transverse momenta distribution for charged particles in ion-ion collisions, one must include a thermal emission term. The temperature of this emission Tth turns out to be proportional to the saturation scale, Tth=1.8Qs/2π. The formalism for the calculation of the transverse momenta spectra in color glass condensate/saturation approach is developed, in which two stages of the process are seen: creation of the color glass condensate, and hadronization of the gluon jets. Our calculations are based on the observation that even for small values of pT, the main contribution in the integration over the dipole sizes stems from the kinematic region in the vicinity of the saturation momentum, where theoretically, we know the scattering amplitude. Nonperturbative corrections need to be included in the model of hadronization. This model incorporates the decay of a gluon jet with effective mass meff2=2Qsμsoft where μsoft denotes the soft scale, with the fragmentation functions at all values of the transverse momenta. We use the Kharzeev-Levin-Nardi model which, provides a simple way to estimate the cross sections for the different centrality classes. Comparing the results of this paper with the transverse distribution in the proton-proton scattering, we see two major differences. First, a larger contribution of the thermal radiation term is needed, in accord with higher parton densities of the produced color glass condensate. Second, even changing the model for the hadronization, without a thermal radiation term, we fail to describe the pT spectrum. Consequently, we conjecture that the existence of the thermal radiation term is independent of the model of confinement.