CERN Accelerating science

ATLAS Note
Report number	ATL-PHYS-PROC-2024-023
Title	Measurement of dilepton production from photon fusion processes in Pb+Pb UPC with the ATLAS detector
Author(s)	Grabowska-Bold, Iwona (AGH University of Krakow (PL)) (+)
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2024
Imprint	26 Mar 2024
Number of pages	mult.
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	ultra-peripheral collisions, photon-induced processes, exclusive production of lepton pairs, tau anomalous magnetic moment, non-UPC dimuons ; HEAVYIONS
Abstract	Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This report presents a series of measurements of dilepton production from photon fusion performed by the ATLAS Collaboration. Recent measurements of exclusive dielectron production in ultra-peripheral collisions (UPC) are presented. These processes provide strong constraints on the nuclear photon flux and its dependence on the impact parameter and photon energy. Comparisons of the measured cross sections to QED predictions from the Starlight and SuperChic models are also presented. Tau-pair production measurements can constrain the tau lepton's anomalous magnetic dipole moment (g-2), and a recent ATLAS measurement using muonic decays of tau leptons in association with electrons and tracks provides one of the most stringent limits available to date. Furthermore, measurements of muon pairs produced via two-photon scattering processes in hadronic (i.e. non-UPC) Pb+Pb collisions are discussed. These non-UPC measurements provide a novel test of strong-field QED and may be a potentially sensitive electromagnetic probe of the quark-gluon plasma. These measurements include the dependence of the cross section and angular correlation on the mean-$p_\mathrm{T}$ of the dimuon pair, the rapidity separation between the muons, and the angle of the pair relative to the second-order event-plane, all measured differentially as a function of the Pb+Pb collision centrality.

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