Hadron production at the LHC: Any indication of new phenomena

TL;DR

The paper analyzes soft hadron production at the LHC by comparing soft Pomeron models and a gluon-saturation-based CGC description against early inclusive-hadron data. It frames two theoretical strands: soft Pomeron/Reggeon models with Pomeron parameters derived from N=4 SYM (e.g., the Pomeron intercept $\alpha_{IP}(0)$) and a high-density QCD CGC approach with the saturation scale $Q_s(x)$ in $k_T$-factorization. The results indicate that soft Pomeron descriptions do not fully capture all LHC features, while the CGC framework provides an adequate description of inclusive hadron production and yields concrete predictions for heavy-ion collisions at the LHC. If gluon saturation is experimentally confirmed, it would represent a major QCD discovery, with testable implications for $pp$ and $AA$ collisions at LHC energies.

Abstract

We confront soft Pomeron and gluon saturation models with the first LHC data on inclusive hadron production. We claim that while the first type of models are not able to describe some part of the LHC data, the Colour-Glass-Condensate (gluon saturation) approach gives an adequate description of the data. Here, we compare our published predictions with the recently available 7 TeV data. We firmly believe that if further experimental measurements confirm that the gluon saturation works, it will be a major discovery.

Figures (5)

• Figure 1: Hadron multiplicity from the soft Pomeron model GLMM at various energy. Dotted curve shows the prediction of Ref. GLMM divided by $\sigma_{ND}$. The experimental data are from Refs. CMS1AL1CMSparticleb.
• Figure 2: The Mueller diagram for the inclusive production of hadrons in Pomeron approach. The wave line denotes the full Green function of the Pomeron. $\sigma_{tot}$, $\sigma_{el}$, $\sigma_{sd}$ and $\sigma_{dd}$ denote the total, elastic, single and double diffraction cross-section, respectively.
• Figure 3: a) Mini jet production in hadron-hadron collisions in the transverse plane within the $k_T$ factorization scheme. The impact-parameter between two hadrons is $\vec{B}$. b) shows the average impact parameter of the produced mini jet $\langle b^2_{jet} \rangle$ as a function of energy within two rapidity bins. c) The comparison with the experimental data and prediction for $dN_{ch} / d y$. The curves are normalized by data at $\sqrt{s} = 546\,\text{GeV}$LRPP. d) Energy dependence of the charged hadrons multiplicity in the central region of rapidity $\eta=0$ in $pp$ collisions. The theoretical curve (Saturation model LR) is our prediction coming from the saturation model LRPP. The band indicates about $2\%$ theoretical error. The total theoretical uncertainties is less $6\%$ at high energies. We also show the KLN prediction KLN with the same error band as ours. Notice that in c panel we have taken a fixed mini jet mass $m_{jet}=0.4$ GeV for all energies while in $d$ panel uncertainties due to the assumption of a fixed energy-independent mini jet mas was included in the band. The experimental data are from Refs. CMS1AL1CMSparticlebua1. The experimental error bars indicate systematic uncertainties.
• Figure 4: a) The energy dependence of the average transverse momentum of charged hadrons. b,c)The differential yield of charged hadrons. The LHC experimental data are from the CMS collaboration CMS1CMS. d) The average transverse momentum of charged hadrons as a function of the number of charged particles for events with $n_{ch}\geq 1$ within the kinematic range $p_T>500~\text{MeV}$. The experimental data are from ATLAS for $\sqrt{s}=0.9$ TeV and $|\eta|<2.5$ATLAS. The theoretical curves was obtained for $|\eta|=0$ and with the same kinematic constraint $p_T>500~\text{MeV}$ at various energies for two value of $\langle z\rangle =0.48, 0.5$ corresponding to the dashed and the solid lines, respectively. The mini jet mass is taken $m_{jet}=0.4$ GeV in all plots. The normalization is the same as in Fig. \ref{['dndy']}.
• Figure 5: a) Pseudo-rapidity distribution of charged particles produced in Au-Au and Pb-Pb central $0-6\%$ collisions at RHIC and the LHC energies. b) Energy dependence of the charged hadrons multiplicity at midrapidity $\eta=0$ in central collisions in $pp$ and $AA$ collisions. The theoretical curve Saturation model (LR) is our prediction. The band indicates less than $3\%$ theoretical errors. The total theoretical uncertainties is less than $7\%$. The experimental data are from CMS1AL1CMSparticlebua1rhic1. The plots are taken from Ref. LR2.