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arXiv · 2204.13478

Observation of the $γγ\toττ$ process in Pb+Pb collisions and constraints on the $τ$-lepton anomalous magnetic moment with the ATLAS detector

ATLAS Collaboration
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Abstract

This Letter reports the observation of $τ$-lepton pair production in ultraperipheral lead-lead collisions, $\text{Pb+Pb} \to \text{Pb}(γγ\to ττ)\text{Pb}$, 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 $\sqrt{s_{_\text{NN}}} = 5.02$ 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 $γγ\to ττ$ 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 ($γγ\to μμ$) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for $a_τ$ is $-0.057 < a_τ< 0.024$.

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ATLAS Collaboration. 2023-10-17. Observation of the $γγ\toττ$ process in Pb+Pb collisions and constraints on the $τ$-lepton anomalous magnetic moment with the ATLAS detector. https://doi.org/10.1103/physrevlett.131.151802

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