Single Inclusive Hadron Production at RHIC and the LHC from the Color Glass Condensate

TL;DR

This paper tests the Color Glass Condensate framework with running-coupling BK evolution to describe forward-rapidity hadron production at RHIC, using a hybrid formalism and unintegrated gluon distributions derived from rcBK with MV initial conditions. It achieves good agreement with RHIC data for charged hadrons without K-factors and with a modest K-factor for pions, supporting CGC as the relevant small-$x$ dynamics. The authors then extrapolate to LHC energies to predict forward production in p+Pb and to estimate the initial-state contribution to suppression in Pb+Pb, highlighting a sizable role for initial-state effects in nuclear modification factors. Overall, the work provides a CGC-based, testable framework for forward particle production and offers concrete expectations for LHC phenomenology, while noting the need to incorporate centrality dependence and final-state interactions for a full Pb+Pb comparison.

Abstract

Using the unintegrated gluon distribution obtained from numerical simulations of the Balitsky-Kovchegov equation with running coupling, we obtain a very good description of RHIC data on single inclusive hadron production at forward rapidities in both p+p and d+Au collisions. No K-factors are needed for charged hadrons, whereas for pion production a rapidity independent K-factor of order 1/3 is needed. Extrapolating to LHC energies, we calculate nuclear modification factors for light hadrons in p+Pb collision, as well as the contribution of initial state effects to the suppression of the nuclear modification factor in Pb+Pb collisions.

Figures (4)

• Figure 1: Negatively charged hadron and $\pi^0$ yields in proton-proton (at pseudo-rapidities (2.2, 3.2) and (3.3, 3.8 and 4)) and deuteron-gold (at pseudo-rapidities (2.2, 3.2) and 4) collisions at $\sqrt{s_{NN}}=200$ GeV. Data by the BRAHMS and STAR collaborations.
• Figure 2: Nuclear modification factors for $\pi^0$ production in p+Pb collisions, $R_{pPb}^{\pi^0}$, for collision energies $\sqrt{s_{NN}}=8.8$ (left) and 6.2 TeV (right) and for rapidities $y_h=2$, 4, and 6. For comparison, the red dashed line corresponds to the same quantity calculated in the k$_t$-factorization scheme.
• Figure 3: Nuclear modification factors for $h^\pm$ production in p+Pb collisions, $R_{pPb}^{h^\pm}$, for collision energies $\sqrt{s_{NN}}=8.8$ (left) and 6.2 TeV (right) and for rapidities $y_h=2$, 4, and 6. For comparison, the red dashed line corresponds to the same quantity calculated in the k$_t$-factorization scheme.
• Figure 4: Gluon level predictions from k$_t$ factorization for Pb+Pb collisions for rapidities $y=0,4$. Solid lines correspond to an initial gluon saturation scale $Q_{s0}^{gluon\,2}=1$ GeV$^2$, and the dashed ones to $Q_{s0}^{gluon\,2}=0.8$ GeV$^2$.