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

TL;DR

This paper provides detailed perturbative QCD baseline predictions for nuclear modification factors in light-ion collisions at LHC energies, highlighting the significant role of cold nuclear matter effects and proposing ratios to reduce uncertainties for better energy-loss detection.

Contribution

It offers comprehensive NLO calculations of $R_{AA}$ in light-ion collisions using recent nPDFs and introduces ratios that mitigate uncertainties to improve hot-medium effect detection.

Findings

CNM effects can cause significant suppression in light-ion systems.

Large nPDF uncertainties limit the interpretation of energy loss.

Ratios like $R_{OO}/R_{pO}^2$ reduce uncertainties and improve sensitivity.

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.