# Thermal radiation and inclusive production in the KLN model for ion-ion collisions

## TL;DR

This paper develops a CGC/saturation-based formalism for ion-ion collisions, demonstrating that including a thermal emission term proportional to the saturation scale is essential for accurately describing transverse momentum distributions.

## Contribution

It introduces a thermal emission component into the KLN model, linking the temperature to the saturation scale, and emphasizes the necessity of this term for matching experimental data.

## Key findings

- Thermal radiation term is crucial for accurate $p_T$ spectra.
- Thermal temperature $T_{th}$ is proportional to the saturation scale $Q_s$.
- Model fails without thermal term, regardless of hadronization details.

## Abstract

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 $T_{\rm th}$ turns out to be proportional to the saturation scale, $T_{\rm th} = 1.8/2\pi \,Q_s$. The formalism for the calculation of the transverse momenta spectra in CGC/saturation approach is developed, in which two stages of the process are seen: creation of the colour glass condensate, and hadronization of the gluon jets. Our calculations are based on the observation that even for small values of $p_T$, the main contribution in the integration over the dipole sizes stems from the kinematic region in vicinity of the saturation momentum, where theoretically, we know the scattering amplitude. Non-perturbativ corrections need to be included in the model of hadronization. This model incorporates the decay of a gluon jet with effective mass $m^2_{\rm eff} = 2 Q_s \mu_{\rm soft}$ where $\mu_{\rm soft}$ denotes the soft scale, with the fragmentation functions at all values of the transverse momenta. We use the KLN 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 colour glass condensate. Second, even changing the model for the hadronization, without a thermal radiation term, we fail to describe the $p_T$ spectrum. Consequently, we conjecture that theexistence of the thermal radiation term is independent of the model of confinement.