CGC predictions for p+A collisions at the LHC and signature of QCD saturation

TL;DR

The paper addresses particle production in high-energy p+Pb collisions within the color glass condensate framework, focusing on small-x saturation dynamics encoded in the rcBK evolution of the dipole amplitude. It develops predictions for hadron and photon production using $k_T$ factorization and the hybrid formalism, incorporating elastic and inelastic contributions and exploring uncertainties from the initial nuclear saturation scale $Q_{0A}$ and inelastic coupling $ abla ext{α}_s^{in}$. The authors forecast charged-hadron multiplicities, nuclear modification factors $R_{pA}$ for hadrons and photons, and photon–hadron azimuthal correlations across rapidities, noting that constraints on $Q_{0A}$ from data at one rapidity can substantially reduce predictions’ uncertainties. They also discuss the limitations of current higher-order control and the need for more small-x data in nuclei, offering a concrete strategy to test CGC dynamics at the LHC via upcoming measurements.

Abstract

We present various predictions for the upcoming p+Pb collisions at \sqrt{S}=5 TeV within the color glass condensate (CGC) formalism, including single inclusive charged hadron production, single inclusive prompt photon production, direct photon production, charged hadron multiplicity distribution and photon-hadron azimuthal correlations. Using the running-coupling Balitsky-Kovchegov evolution equation for calculating various observables, we show that the main source of uncertainties is due to less constrained initial nuclear saturation scale. This gives rise to rather large theoretical uncertainties for nuclear modification factor R_{pA} at the LHC. Nevertheless, we propose a simple scheme in which one can still test the main dynamics of the CGC/saturation in p+A collisions at the LHC.

Figures (8)

• Figure 1: Pseudorapidity distribution of the charged particles production in p+Pb collisions at the LHC $\sqrt{S}=5.02$ TeV at various centralities, in right panel, from top to down: $0-20\%$, $20-40\%$, minimum-bias, $40-60\%$, $60-80\%$. Left panel: for a better comparison, we show again theoretical curves for more peripheral centrality bins (the theoretical curves in both panels are the same). The theoretical uncertainties of about $5\%$ due to fixing the over-all normalization at RHIC are also shown.
• Figure 2: The nuclear modification factor $R_{pA}^{ch}$ for inclusive charged hadrons $h^{+}+h^{-}$ production in minimum-bias p+Pb collisions at $\sqrt{S}=5$ TeV at different rapidities $\eta=0,2,4,6$ obtained from the hybrid factorization Eq. (\ref{['qa']}) with the solutions of the rcBK with different initial saturation scale for nucleus. The band labeled CGC-rcBK includes uncertainties due to the variation of the initial saturation scale of nucleus and different factorization scale $Q$. At every rapidity, we also show the results by taking $\alpha_s^{in}=0$ (only elastic contribution) and $0.3\div 0.2\le \alpha_s^{in}\le 0.1$. The lines labeled by a number $N$ are the results with a fixed hard-scale $Q=p_T$ and a fixed saturation scale $Q_{0A}^2=N Q_{0p}^2$ with $N=3\div 7$ constrained in Eq. (\ref{['qa']}) and $Q_{0p}^2=0.168\,\text{GeV}^2$.
• Figure 3: The nuclear modification factor $R_{pA}^{\gamma}$ for direct photon production in minimum-bias p+Pb collisions at $\sqrt{S}=5$ TeV at different rapidities $\eta_{\gamma}=0,2,4,6$ obtained from Eq. (\ref{['pho4']}) with the solutions of the rcBK with different initial saturation scale for nucleus. The band labeled CGC-rcBK includes uncertainties due to the variation of the initial saturation scale of nucleus and different factorization scale $Q$. Similar to Fig. \ref{['rp-h']}, the lines labeled with a number $N$ are the results with a fixed hard-scale $Q=p^{\gamma}_T$ and a fixed saturation scale $Q_{0A}^2=N Q_{0p}^2$ with $N=3\div 7$ constrained in Eq. (\ref{['qa']}) and $Q_{0p}^2=0.168\,\text{GeV}^2$.
• Figure 4: The nuclear modification factor $R_{pA}^{\gamma}$ for single inclusive prompt photon production in minimum-bias p+Pb collisions at $\sqrt{S}=5$ TeV at different rapidities $\eta_{\gamma}=0,2$. The descriptions of the band and the solid black lines are the same as in Fig. \ref{['rp-p']}. The dashed red line (labeled CGC-IIM) is calculated from the same master Eq. (\ref{['cs']}) but a different saturation model, namely the so-called Iancu-Itakura-Munier (IIM) saturation model iimme-nosh.
• Figure 5: The nuclear modification factor in minimum-bias p+Pb collisions at the LHC $\sqrt{S}=5$ TeV and $\eta=\eta_\gamma=2$ for inclusive hadron production (right panel) and direct photon production (left panel) with different hard-scale (or factorization scale) $Q$ in Eqs. (\ref{['pho4']},\ref{['final']}). The initial saturation scale for nucleus is fixed for all lines with $N=7$. The bands are the same as in Figs. (\ref{['rp-h']},\ref{['rp-p']}).