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

TL;DR

This study uses Bayesian reweighting to assess how LHC Run I p-Pb data constrain nuclear PDFs (EPS09 and DSSZ) against two proton PDF baselines. It finds generally good agreement but only modest constraining power, with some observables hinting at tensions and others largely uninformative due to systematics and reference uncertainties. The CMS dijet data provide the strongest constraint, but overall a full reanalysis with flexible parametrizations and flavor separation is needed. The work emphasizes the importance of improved correlations and Run II data to fully exploit p-Pb measurements for nuclear parton densities.

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.

Figures (10)

• Figure 1: Forward-to-backward asymmetries based on $W^{\pm}$ measurements by the ALICE collaboration. The upper (lower) graphs correspond to the theoretical calculation with EPS09 (DSSZ) nuclear PDFs. The comparisons with no nuclear effects are included as dashed lines. The rapidity values at the horizontal axes are only indicative as the rapidity bins are different in forward and backward directions (different results are also horizontally displaced for visibility).
• Figure 2: Forward-to-backward asymmetries for $W^{+}$ (upper panels) and $W^{-}$ (lower panels) measured by the CMS collaboration Khachatryan:2015hha, as a function of the charged-lepton pseudorapidity in the laboratory frame. The left-hand (right-hand) graphs correspond to the theoretical calculations with EPS09 (DSSZ) nPDFs. Results with no nuclear effects are included as dashed lines.
• Figure 3: Forward-to-backward asymmetry of $Z$ bosons measured by CMS CMS:2015vqa as a function of the lepton pair rapidity. The left-hand panel (right-hand panel) shows the predictions obtained with EPS09 (DSSZ). Results with no nuclear effects (NNE) are shown as dashed lines.
• Figure 4: As in Figure \ref{['fig:Z_cms']} but for the ATLAS measurement.
• Figure 5: Forward-to-backward ratios based on the ATLAS jet cross-section measurements as a function of jet $p_{\rm T}$. The theoretical predictions and uncertainty bands were computed using the eigenvectors of EPS09 (left) and DSSZ (right). Upper panels: $0.3 < |y*| < 0.8$. Middle panels: $0.8 < |y*| < 1.2$. Lower panels: $1.2 < |y*| < 2.1$.