Observation of the $\gamma \gamma \to \tau \tau$ process in Pb+Pb collisions and constraints on the $\tau$-lepton anomalous magnetic moment with the ATLAS detector

This Letter reports the observation of $\tau$-lepton pair production in ultraperipheral lead-lead collisions, $$\text{Pb+Pb}$ \to $\text{Pb}$(\gamma\gamma \to \tau\tau)$\text{Pb}$$,andconstraintsonthe$\tau$-lepton anomalous magnetic moment, $a_{\tau }$. The dataset corresponds to an integrated luminosity of 1.44 nb${}^{-1}$ of LHC Pb+Pb collisions at $\sqrt{s_{{}_{\text{NN}}}}=5.02$ TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a $\tau$-lepton decay, an electron or charged-particle track(s) from the other $\tau$-lepton decay, little additional central-detector activity, and no forward neutrons. The $\gamma \gamma \to \tau \tau$ process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations, and a signal strength of ${\mu }_{\tau \tau }=1.03_{-0.05}^{+0.06}$ assuming the Standard Model value for $a_{\tau }$. To measure $a_{\tau }$, a template fit to the muon transverse-momentum distribution from $\tau$-lepton candidates is performed, using a dimuon ($\gamma \gamma \to \mu \mu$) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for $a_{\tau }$ is $-0.057<a_{\tau }<0.024$.