Comparison of the Color Glass Condensate to multiplicity dependence of the average transverse momentum in p+p, p+Pb and Pb+Pb collisions at the LHC

TL;DR

This paper demonstrates that the observed patterns of average transverse momentum versus multiplicity in various LHC collisions can be explained by gluon saturation within the Color Glass Condensate framework, linking initial state effects to final particle production.

Contribution

The study provides a detailed theoretical calculation showing that gluon saturation and geometric scaling explain the multiplicity dependence of <p_T> across different collision systems at the LHC.

Findings

<p_T> versus N_{ch} features are well described by gluon saturation models.

The flatness of <p_T> at high N_{ch} is consistent with initial state effects.

Scaling between p+p and p+Pb supports geometric-scaling and gluon saturation evidence.

Abstract

The first moment <p_T> of the charged-particle transverse momentum spectrum and its correlation with the charged-particle multiplicity N_{ch} provide vital information about the underlying particle production mechanism. The ALICE collaboration recently reported that <p_T> versus N_{ch} in Pb+Pb collisions is smaller than in p+p and p+Pb collisions. Other interesting features of data is rather flatness of <p_T> at high N_{ch} in Pb+Pb and p+Pb collisions in seemingly striking contrast to the case of p+p collisions. With a detailed calculation, we show all these peculiar features in a wide range of energies and system sizes can be well described by the idea of gluon saturation within the Color Glass Condensate framework using the k_T-factorization. This establishes an important fact that the bulk of the produced particles in heavy-ion collisions at the LHC carries signature of the initial stage of collisions. We also show that the recent scaling property seen by the CMS collaboration between the number of tracks in p+p and p+Pb collisions may provide a strong evidence in favor of geometric-scaling phenomenon and gluon saturation, indicating that the underlying dynamics of high multiplicity events in p+p and p+Pb collisions should be similar.