Probing the Color Glass Condensate in $pp$ collisions at forward rapidities and very low transverse momenta

TL;DR

This paper demonstrates that the Color Glass Condensate (CGC) framework successfully describes forward neutral pion production at very low transverse momentum in proton-proton collisions at LHC energies, indicating non-linear QCD effects.

Contribution

It applies the CGC formalism to forward $pp$ collisions at LHC energies, showing its effectiveness in describing low-$p_T$ hadron production and highlighting the role of the saturation scale.

Findings

CGC formalism fits LHCf data well

Evidence of non-linear QCD effects at high energies

Saturation scale acts as the hard momentum scale in the process

Abstract

The description of the hadron production at very forward rapidities and low transverse momentum is usually made using phenomenological models based on nonperturbative physics. However, at high energies and large rapidities the wave function of one of the projectiles is probed at very small Bjorken $x$, being characterized by a large number of gluons. In this kinematical regime, a new state of matter - the Color Glass Condensate (CGC) - is expected to be formed. One the main characteristics of such system is the presence of a new dynamical momentum scale, the saturation scale $Q_s$, which can assume values very larger than the QCD confinement scale $\Lambda_{QCD}$ and give the scale of the running coupling constant. In this paper we assume that in particular kinematical region probed by LHC forward (LHCf) experiment the saturation scale can be considered the hard momentum scale present in the process and calculate the forward neutral pion production at very low-$p_T$ using a perturbative approach. We demonstrate that the CGC formalism is able to successfully describe the LHCf data, which can be considered as a compelling indication of the presence of non-linear QCD effects at LHC energies.