Measurements of the light-by-light scattering and the Breit–Wheeler processes, and searches for axion-like particles in ultraperipheral PbPb collisions at √ <i>s</i> _<i>NN</i> = 5.02 TeV

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 &gt; 2 GeV, pseudorapidity |η γ,e | &lt; 2.2, pair invariant mass m γγ,ee &gt; 5 GeV, pair transverse momentum p T γγ,ee &lt; 1 GeV, and pair azimuthal acoplanarity A ϕ &lt; 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.