CMS-PAS-HIN-21-015

CMS-PAS-HIN-21-015
Measurements of the light-by-light scattering and the Breit-Wheeler processes, and searches for axion-like particles in ultraperipheral PbPb collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} =$ 5.02 TeV
CMS Collaboration
10 April 2024

Abstract: Measurements of the light-by-light scattering (LbL, $\gamma\gamma\to\gamma\gamma$ ) and the Breit-Wheeler (B-W, $\gamma\gamma\to\mathrm{e}^+\mathrm{e}^-$ ) processes are reported in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV using a data sample corresponding to an integrated luminosity of 1.65 nb $^{-1}$ . Events with a pair of exclusively produced photons or $\mathrm{e}^+\mathrm{e}^-$ are selected, each with transverse energy $E_\mathrm{T}^{\gamma,\mathrm{e}} >$ 2 GeV, pseudorapidity $\|\eta^{\gamma,\mathrm{e}}\| <$ 2.2, pair invariant mass $m^{\gamma\gamma,\mathrm{ee}} >$ 5 GeV, pair transverse momentum $p_\mathrm{T}^{\gamma\gamma,\mathrm{ee}} <$ 1 GeV, and pair azimuthal acoplanarity $A_\phi <$ 0.01. The measured B-W fiducial cross section, $\sigma_\text{fid} (\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-)=$ 271.5 $\pm$ 1.9 (stat) $\pm$ 18.3 (syst) $\mu$ b, as well as the differential distributions for various kinematic observables, are in agreement with the standard model (SM) predictions. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.8 $\pm$ 3.1 events expected for the signal and 12.0 $\pm$ 2.9 for the background. 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, $\sigma_\text{fid} (\gamma\gamma \to \gamma\gamma)=$ 107 $\pm$ 33 (stat) $\pm$ 20 (syst) nb, is consistent with the SM prediction. Limits on the production of axion-like particles coupling to photons are set over the mass range $m_\mathrm{a} =$ 5-100 GeV, including the most stringent limits in the 5-10 GeV mass range.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here.

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