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Charged particle multiplicities in pA interactions at the LHC from the Color Glass Condensate

Amir H. Rezaeian

TL;DR

This work uses the Color Glass Condensate framework to predict charged-hadron pseudorapidity distributions in minimum-bias pA collisions at the LHC via $k_T$-factorization, tying the unintegrated gluon densities to impact-parameter dependent dipole amplitudes. By comparing the b-CGC and MCrcBK saturation models and incorporating a mini-jet mass regularization along with Local Parton-Hadron Duality, the authors produce quantitative predictions and assess model differences. A key feature is the inclusion of the MLLA gluon-decay cascade, which boosts multiplicities by ~20–25% in high-density regimes, and the resulting forward-backward asymmetry in p+Pb due to asymmetric saturation scales. The predicted midrapidity multiplicities and the distinct distributions offered by different saturation pictures enable a decisive experimental test of small-$x$ dynamics at the LHC.

Abstract

The forthcoming LHC measurement of hadron multiplicity in proton-nucleus collisions is a crucial test of the k_t factorization and gluon saturation based models. Here, we provide quantitative predictions for the pseudorapidity distribution of charged particles produced in minimum bias proton-nucleus collisions at the LHC based on the idea of gluon saturation in the color-glass condensate framework. Our formulation gives good descriptions of the LHC and RHIC data for the charged-hadron multiplicities in both proton-proton and nucleus-nucleus collisions, and also the deep inelastic scattering at HERA at small Bjorken-x.

Charged particle multiplicities in pA interactions at the LHC from the Color Glass Condensate

TL;DR

This work uses the Color Glass Condensate framework to predict charged-hadron pseudorapidity distributions in minimum-bias pA collisions at the LHC via -factorization, tying the unintegrated gluon densities to impact-parameter dependent dipole amplitudes. By comparing the b-CGC and MCrcBK saturation models and incorporating a mini-jet mass regularization along with Local Parton-Hadron Duality, the authors produce quantitative predictions and assess model differences. A key feature is the inclusion of the MLLA gluon-decay cascade, which boosts multiplicities by ~20–25% in high-density regimes, and the resulting forward-backward asymmetry in p+Pb due to asymmetric saturation scales. The predicted midrapidity multiplicities and the distinct distributions offered by different saturation pictures enable a decisive experimental test of small- dynamics at the LHC.

Abstract

The forthcoming LHC measurement of hadron multiplicity in proton-nucleus collisions is a crucial test of the k_t factorization and gluon saturation based models. Here, we provide quantitative predictions for the pseudorapidity distribution of charged particles produced in minimum bias proton-nucleus collisions at the LHC based on the idea of gluon saturation in the color-glass condensate framework. Our formulation gives good descriptions of the LHC and RHIC data for the charged-hadron multiplicities in both proton-proton and nucleus-nucleus collisions, and also the deep inelastic scattering at HERA at small Bjorken-x.

Paper Structure

This paper contains 3 sections, 3 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Main formulation
  3. DISCUSSION AND PREDICTIONS

Figures (2)

  • Figure 1: Pseudo-rapidity distribution of charged particles production in p+p (right panel) and A+A (left panel) collisions at various energies. Right: the theoretical curves from top to bottom are at $\sqrt{s}=14, 7$ and $4.4$ TeV. The band in the right panel indicates about $2\%$ theoretical error. The total theoretical uncertainties is less than $6\%$ mainly due to our freedom in fixing the overall normalization and mini-jet mass with the experimental data at lower energy, see Ref. ap. The experimental data are from CMSApb1ppd.
  • Figure 2: Pseudo-rapidity distribution of the charged particles production in p+A minimum (Mini) bias collisions at the LHC $\sqrt{s}=4.4$ TeV. The theoretical curves labeled by CGC-R1 and MCrcBK JA are based on the leading log $k_t$-factorization formalism but two different saturation models. The curve labeled by CGC-R2 is based on the modified $k_t$-factorization formalism incorporating the gluon-decay cascade effects from the Modified Leading Logarithmic Approximation scheme, see the text for the details. The experimental data are from pa-rhic.