Color Glass Condensate at the LHC: hadron multiplicities in $pp$, $pA$ and $AA$ collisions

TL;DR

This paper develops predictions for hadron multiplicities at LHC energies within the Color Glass Condensate framework, emphasizing parton saturation via the saturation scale $Q_s$ as the controlling parameter. It combines a Glauber-based collision geometry with CGC-inspired gluon production, including running-coupling effects, to yield rapidity and centrality-dependent multiplicities for $pp$, $pA$, and $AA$ collisions. The authors fix the nonperturbative inputs using RHIC data and GW/BK/JIMWLK-inspired reasoning, and they provide quantified uncertainties, notably a relatively modest energy growth of multiplicities compared to many alternative models. Their results offer concrete, testable predictions for LHC measurements that can validate or constrain the CGC picture and $Q_s$ evolution, while highlighting larger uncertainties in proton-proton scenarios.

Abstract

We make quantitative predictions for the rapidity and centrality dependencies of hadron multiplicities in $AA$, $pA$ and $pp$ collisions at the LHC energies basing on the ideas of parton saturation in the Color Glass Condensate.

Figures (8)

• Figure 1: The CGC approach for nucleus - nucleus collision with the saturation of parton density.
• Figure 2: Rapidity dependence $dN/d\eta$ of charged hadron multiplicities in proton - proton (antiproton) collisions as a function of the pseudorapidity at different energies. The data are taken from Ref. RTABL.
• Figure 3: Energy dependence of charged hadron multiplicity $dN/d\eta$ at $\eta =0$ in proton - proton (antiproton) collisions and of charged hadron multiplicities per participant pair $(2/N_{part})\,dN/d\eta$ at $\eta =0$ for central nucleus-nucleus collisions. The vertical dotted lines mark the LHC energies for nucleus-nucleus collisions ($W = 5500\,GeV$) and for proton-proton collisions ($W= 14000\,GeV$). collisions. The experimental data are from Ref. RTABLREF1.
• Figure 4: Energy dependence of total multiplicity in proton - proton (antiproton) collisions. The vertical dotted line marks the LHC energies for proton-proton collisions ($W= 14000\,GeV$). collisions. The experimental data are taken from Ref. REF2.
• Figure 5: Rapidity dependence of $d N/d \eta$ lead-lead collisions at the LHC energy at different centrality cuts. The solid lines corresponds to the prediction using Eq. (\ref{['QS']}) for the energy dependence of the saturation scale while the dotted lines show the predictions for Eq. (\ref{['QSRUNF']}) for running QCD coupling. The shadowed area shows the prediction for the minimal bias event.