CMS-PAS-HIN-21-009

CMS-PAS-HIN-21-009
Observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions
CMS Collaboration
March 2022

Abstract: The first observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions, a pure quantum electrodynamics (QED) process, is presented. The measurement is based on a data sample collected by the CMS experiment at a per nucleon center-of-mass energy of 5.02 TeV, and corresponding to an integrated luminosity of 404 $\mu$b$^{-1}$. The photon-induced $\gamma\gamma\to\tau^{+}\tau^{-}$ production is observed with a statistical significance of at least five standard deviations for $\tau^{+}\tau^{-}$ events with a muon and three charged hadrons in the final state. The cross section is measured in a fiducial phase space region, and is found to be $\sigma(\gamma\gamma\to\tau^{+}\tau^{-}) = $ 4.8 $\pm$ 0.6 (stat) $\pm$ 0.5 (syst) $\mu$b, in agreement with leading-order QED predictions. The measurement, produced with a fraction of the expected integrated luminosity of the LHC program, establishes the potential for a substantially more precise determination of the anomalous magnetic moment of the $\tau$ lepton, which is currently poorly constrained.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 131 (2023) 151803. The superseded preliminary plots can be found here.

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