A compendium of cold-nuclear matter baseline predictions in light-ion collisions

Abstract

The recent light-ion collision programme at RHIC and the LHC provides a unique opportunity to investigate the onset of quark-gluon plasma formation and parton energy loss in small systems. A quantitative interpretation of emerging jet quenching measurements requires precise control over cold nuclear matter (CNM) effects, which modify hard-process cross sections independently of any hot-medium dynamics. In this work, we present a comprehensive set of perturbative QCD baseline calculations for nuclear modification factors ($R_{AA}$) in proton-oxygen (pO), oxygen-oxygen (OO) and neon-neon (NeNe) collisions at LHC energies. The study includes charged hadron, neutral pion, prompt photon, and electroweak-boson production computed at next-to-leading order using a broad set of recent nuclear parton distribution functions (nPDFs). We demonstrate that CNM effects alone can induce sizeable suppressions in light-ion systems, with large associated nPDF uncertainties that currently limit the quantitative extraction of parton energy loss. To address this limitation, we explore a range of multi-cross-section ratios in which CNM effects and their uncertainties largely cancel. In particular, ratios of neutral pion $R_{OO}$ to prompt photon $R_{OO}$ or charged hadron $R_{OO}$ to $R_{pO}^2$ provide theoretically robust observables with substantially reduced nPDF uncertainties, thereby enhancing sensitivity to possible energy-loss signatures.

Figures (16)

• Figure 1: $x$ dependence of nPDFs for different partons for oxygen nucleus at $Q^2=100GeV^2$ with respect to the corresponding free-proton baseline. The bands denote the 68% nPDF confidence interval.
• Figure 2: (Left) Modification of the gluon nPDF with respect to the corresponding free-proton baseline at $x=10^{-3}$ and $Q^2=100GeV^2$ as a function of mass number $A$. Bands show 68% confidence intervals. (Right) Relative nPDF uncertainty as a function of mass number $A$.
• Figure 3: Nuclear modification factors $R_{\rm pO}$ (left), $R_{\rm OO}$ (middle) and $R_{\rm NeNe}$ (right) of charged hadron production. The shown uncertainty bands denote the uncertainty of the nPDF at 68% confidence level.
• Figure 4: Cross-section ratio of charged hadron production in pp collisions at various centre-of-mass energies with respect to the cross section at $\sqrt{s_{\rm NN}}=9.62TeV$. Scale uncertainties are denoted as shaded bands.
• Figure 5: Mass number ($A$) dependence of the nuclear modification factor $R_{\rm AA}$ of charged hadron production at $p_{\rm T}=6GeV/c$. The uncertainty band denotes the nPDF uncertainties at 68% confidence level.