Electroweak probes in heavy-ion collisions with ATLAS

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 $Z$ 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. Also a new measurement of light-by-light scattering in ultra-peripheral Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV with an integrated luminosity of 2.2~nb$^{-1}$ data collected in 2015 and 2018 is 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.