Observation of the $\gamma \gamma \to \tau \tau $ Process in $\mathrm{Pb}+\mathrm{Pb}$ 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 $\mathrm{Pb}+\mathrm{Pb}\to \mathrm{Pb}\left(\gamma \gamma \to \tau \tau \right)\mathrm{Pb}$ and constraints on the $\tau $-lepton anomalous magnetic moment ${a}_{\tau }$. The dataset corresponds to an integrated luminosity of $1.44\text{}\text{}{\mathrm{nb}}^{-1}$ of LHC $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=5.02\text{}\text{}\mathrm{TeV}$ 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 $\mathrm{Pb}+\mathrm{Pb}$ collisions with a significance exceeding 5 standard deviations and a signal strength of ${\mu }_{\tau \tau }=1.0{3}_{-0.05}^{+0.06}$ 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 $-0.057<{a}_{\tau }<0.024$.