Observation of the $γγ → ττ$ Process in $\mathrm{Pb + Pb}$ Collisions and Constraints on the $\tau$-Lepton Anomalous Magnetic Moment with the ATLAS Detector

This Letter reports the observation of τ-lepton pair production in ultraperipheral lead-lead collisions, Pb+Pb→Pb(γγ→ττ)Pb, and constraints on the τ-lepton anomalous magnetic moment, a_τ. The dataset corresponds to an integrated luminosity of 1.44 nb^-1 of LHC Pb+Pb collisions at $\sqrt{s_{NN}}$ =5.02 TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a τ-lepton decay, an electron or charged-particle track(s) from the other τ-lepton decay, little additional central-detector activity, and no forward neutrons. The γγ→ττ process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations, and a signal strength of μ_ττ=1.03$^{+0.06}_{-0.05}$ assuming the Standard Model value for a_τ. To measure a_τ, a template fit to the muon transverse-momentum distribution from τ-lepton candidates is performed, using a dimuon (γγ→μμ) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for aτ is -0.057 < a_τ < 0.024.. Authors: Aad, G. [Aix-Marseille University, Marseille (France); ATLAS Collaboration. et al.; SLAC] (ORCID:0000000266654934); Abbott, B. (ORCID:0000000258882734); Abbott, D. C. (ORCID:0000000272483203); Abeling, K. (ORCID:0000000210021652); Abidi, S. H. (ORCID:0000000284969294). DOE Contract: AC02-76SF00515. Subjects: 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Particle production; leptons; precision measurements; quantum electrodynamics