Hard probes in heavy ion collisions at the LHC: PDFs, shadowing and $pA$ collisions

TL;DR

The paper addresses how to use pA collisions at the LHC to validate factorization in nuclear environments, quantify nuclear parton distributions, and explore potential novel QCD phenomena such as shadowing and gluon saturation. It discusses the division of probes into hard and semi-hard, the importance of nuclear PDFs (nPDFs) for predicting cross sections, and the role of pA data as clean references for AA studies. It outlines the experimental program, including machine parameters, expected event rates, and the need to measure universal nuclear effects versus process-dependent power corrections. It argues that the pA program calibrates heavy-ion measurements, constraints nPDFs, and probes nonlinear QCD dynamics, thereby advancing QCD in high-density regimes.

Abstract

This manuscript is the outcome of the subgroup ``PDFs, shadowing and $pA$ collisions'' from the CERN workshop ``Hard Probes in Heavy Ion Collisions at the LHC''. In addition to the experimental parameters for $pA$ collisions at the LHC, the issues discussed are factorization in nuclear collisions, nuclear parton distributions (nPDFs), hard probes as the benchmark tests of factorization in $pA$ collisions at the LHC, and semi-hard probes as observables with potentially large nuclear effects. Also, novel QCD phenomena in $pA$ collisions at the LHC are considered. The importance of the $pA$ program at the LHC is emphasized.