Measurement of the $\gamma\gamma \to \tau \tau$ cross section and constraints on the anomalous magnetic moment of the $\tau$ lepton in ultraperipheral PbPb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV

Abstract

The production of $\tau$ lepton pairs via photon-photon fusion, $\gamma\gamma \to \tau \tau$, is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The dataset, collected by the CMS experiment in 2018, corresponds to an integrated luminosity of 1.70 nb$^{-1}$. Four different $\tau^+\tau^-$ decay final states are analyzed. A simultaneous likelihood fit to the measured lepton transverse momentum ($p_\mathrm{T}$) distributions, which incorporates information from both spectral shape and normalization, is used to constrain the anomalous magnetic moment of the $\tau$ lepton, $a_\tau$, and to extract the cross section of the process. The measured 95% CL interval for $a_\tau$ is $-$0.039 $\lt$ $a_\tau$ $\lt$ 0.032. The fiducial cross section, $\sigma_{\gamma\gamma \to \tau \tau}^\text{fid}$ = 555$^{+60}_{-14}$ $\mu$b for tau leptons with $p_\mathrm{T}^\tau$ $\gt$ 1 GeV and pseudorapidity $\lvert\eta^\tau\rvert$ $\lt$ 3, is the most precise measurement for this process at the LHC to date and is in agreement with next-to-leading-order quantum electrodynamics predictions.