Electroweak probes in heavy-ion collisions with ATLAS

Abstract

Electroweak bosons produced in lead-lead (Pb+Pb) collisions are an excellent tool to constrain initial-state effects which affect the rates of hard-scattering processes in nucleus-nucleus interactions. The production yields of massive electroweak bosons, observed via their leptonic decay channels, offer a high-precision test of the binary collision scaling expected in Pb+Pb and a way to quantify nuclear modifications of the parton distribution functions (PDFs). The large samples of Pb+Pb data at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV collected by the ATLAS experiment in 2015, and the corresponding high-statistics $pp$ data at the same collision energy used as a baseline, allow for a detailed experimental study of these phenomena and comparisons to predictions from a variety of theoretical calculations. This report presents the latest ATLAS results on electroweak boson production, including updated results on $Z$ boson production and high-precision $W^\pm$ boson results in Pb+Pb collisions. Inclusive production of prompt photons in proton-lead ($p$+Pb) collisions at $\sqrt{s_\mathrm{NN}} = 8.16$ TeV is also covered. Various predictions of nuclear modifications to PDFs are discussed. A new measurement of light-by-light scattering in ultra-peripheral Pb+Pb collision data recorded in 2015 and 2018 at $\sqrt{s_{\text{NN}}} = 5.02$ TeV with an integrated luminosity of 2.2 nb$^{-1}$ is also presented. Integrated and differential fiducial cross sections are measured. The invariant mass distribution of the diphoton system is used to extract limits on axion-like particles decaying to a two-photon system. This results in the most stringent limits to date over the diphoton mass range of 6-100~GeV.