Speaker
Description
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 collisions at 5.02 TeV using the 2018 CMS lead-lead data sample of $1.65~\mathrm{nb}^{-1}$. Events with a pair of exclusively produced photons or electrons are selected, each with transverse energy $E_\mathrm{T}^{\gamma,\mathrm{e}}>2~\mathrm{GeV}$, pseudorapidity $|\eta^{\gamma,\mathrm{e}}|<2.2$, pair invariant mass $m^{\gamma\gamma,\mathrm{ee}}> 5~\mathrm{GeV}$, pair transverse momentum $p_\mathrm{T}^{\gamma\gamma,\mathrm{ee}}<1~\mathrm{GeV}$, and pair azimuthal acoplanarity $A_\phi<0.01$. The measured B--W fiducial cross section, $\sigma_\text{fid} (\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-)= 271.5 \pm 1.9~\mathrm{(stat)}~\pm 18.3~\mathrm{(syst)}~\mu$b, 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, $\sigma_\text{fid} (\gamma\gamma \to \gamma\gamma)= 107 \pm 33~\mathrm{(stat)}~\pm 20~\mathrm{(syst)}~\mathrm{nb}$, is consistent with SM predictions. 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 5--10 GeV.
| Category | Experiment |
|---|---|
| Collaboration | CMS |
