Abstract
This paper reports the observation of τ-lepton pair production in ultraperipheral lead-lead collisions, Pb(γ)Pb(γ) → ττ and constraints on the τ-lepton anomalous magnetic moment, aτ, measured by the ATLAS experiment. The dataset corresponds to an integrated luminosity of 1.44 nb−1 of LHC Pb+Pb collisions at $ \sqrt{S_{NN}}=5.02 {\rm TeV} $. Selected events contain one muon from a τ-lepton decay, an electron or charged-particle track(s) from the other τ-lepton decay, little additional central-detector activity, and no forward neutrons. The γγ → ττ process is observed in Pb+Pb collisions with a signal strength of µττ= 1.03$ _{-0.05}^{+0.06} $. To measure aτ, 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, 0.024).
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