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

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.