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 $<$ $a_{\tau }$ $<$ 0.032. The fiducial cross section, ${\sigma }_{\gamma \gamma \to \tau \tau }^{\text{fid}}$ = 555${}_{-14}^{+60}$ $\mu$b for tau leptons with $p_{\mathrm{T}}^{\tau }$ $>$ 1 GeV and pseudorapidity $|{\eta }^{\tau }|$ $<$ 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.

Submitted to Physics Letters B. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-24-011 (CMS Public Pages)