Electroweak bosons in Pb+Pb and p+Pb collisions from ATLAS

TL;DR

This paper reports measurements of electroweak bosons in Pb+Pb and p+Pb collisions at the LHC, investigating nuclear effects on parton distribution functions and their dependence on collision centrality and rapidity.

Contribution

It provides new differential measurements of electroweak boson production in heavy-ion collisions, exploring nuclear modifications and spatial dependence of nuclear PDFs with improved analysis techniques.

Findings

Electroweak boson yields are independent of centrality in Pb+Pb collisions.

Distributions in p+Pb show asymmetry and suggest nuclear effects are significant at large Bjorken x.

Binary scaling of Z boson yields is confirmed after bias correction.

Abstract

Electroweak boson ($W$, $Z$, $\gamma$) measurements in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV and in $p$+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV are presented with the ATLAS detector at the LHC. In Pb+Pb, electroweak boson yields are shown to be independent of centrality. Differential measurements in absolute pseudorapidity are used to investigate nuclear effects to the free-proton parton distribution function (PDF). The distributions lack the experimental precision to unambiguously identify the presence of nuclear modifications. In $p$+Pb, the $Z$ boson cross section is measured as a function of center-of-mass rapidity $y_{Z}^{*}$ and the momentum fraction of the lead-going parton (Bjorken $x_{Pb}$). The distributions are asymmetric and model predictions underestimate the data at large $x_{Pb}$. The overall shape is best described by including nuclear effects. The differential cross section is also measured in different centrality classes and shows evidence of spatially-dependent nuclear PDFs. The $Z$ boson production yields are measured as a function of the mean number of participants using Glauber and Glauber-Gribov color fluctuation models. Binary scaling of the yields is observed utilizing the standard Glauber model after applying a centrality-bias correction.