An analysis of the impact of LHC Run I proton-lead data on nuclear parton densities

TL;DR

This paper evaluates how proton-lead collision data from LHC Run I influences nuclear parton distribution functions, highlighting the data's constraining power and current limitations in systematic uncertainties.

Contribution

It provides a comprehensive analysis of multiple observables' impact on nuclear PDFs using Bayesian reweighting, a first in including diverse measurements in such studies.

Findings

Good agreement between data and nPDFs, but limited by systematic uncertainties.

Some observables show tension with specific PDF choices.

Highlights need for better proton-proton references in future analyses.

Abstract

We report on an analysis of the impact of available experimental data on hard processes in proton-lead collisions during Run I at the Large Hadron Collider on nuclear modifications of parton distribution functions. Our analysis is restricted to the EPS09 and DSSZ global fits. The measurements that we consider comprise production of massive gauge bosons, jets, charged hadrons and pions. This is the first time a study of nuclear PDFs includes this number of different observables. The goal of the paper is twofold: i) checking the description of the data by nPDFs, as well as the relevance of these nuclear effects, in a quantitative manner; ii) testing the constraining power of these data in eventual global fits, for which we use the Bayesian reweighting technique. We find an overall good, even too good, description of the data, indicating that more constraining power would require a better control over the systematic uncertainties and/or the proper proton-proton reference from LHC Run II. Some of the observables, however, show sizable tension with specific choices of proton and nuclear PDFs. We also comment on the corresponding improvements on the theoretical treatment.