Measurements of the light-by-light scattering and the Breit–Wheeler processes, and searches for axion-like particles in ultraperipheral PbPb collisions at √s_NN = 5.02 TeV

Indian Institute of Technology, Madras

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Published online: 5 November 2025

Abstract

Measurements of the light-by-light scattering (LbL, γγ → γγ) and the Breit–Wheeler (B–W, γγ → e⁺e⁻) processes are reported in ultraperipheral collisions at 5.02 TeV using the 2018 CMS lead-lead data sample of 1.65 nb⁻¹. Events with a pair of exclusively produced photons or electrons are selected, each with transverse energy E_T^γ,e > 2 GeV, pseudorapidity |η^γ,e| < 2.2, pair invariant mass m^γγ,ee > 5 GeV, pair transverse momentum p_T^γγ,ee < 1 GeV, and pair azimuthal acoplanarity A_ϕ < 0.01. The measured B–W fiducial cross section, σ_fid(γγ → e⁺e⁻) = 271.5 ± 1.9 (stat) ± 18.3 (syst) μb, as well as the differential distributions for various kinematic observables, are in agreement with standard model (SM) predictions. The observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The fiducial LbL scattering cross section, σ_fid(γγ → γγ) = 107 ± 33 (stat) ± 20 (syst) nb, is consistent with SM predictions. Limits on the production of axion-like particles coupling to photons are set over the mass range m_a = 5–100 GeV, including the most stringent limits in 5–10 GeV.