Observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions with the CMS experiment and the first limits on $(g-2{)}_{\tau }$ at the LHC

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 $5.02\mathrm{TeV}$, and corresponding to an integrated luminosity of $404\mu {\mathrm{b}}^{-1}$. The $\gamma \gamma \to {\tau }^{+}{\tau }^{-}$ 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 $\sigma (\gamma \gamma \to {\tau }^{+}{\tau }^{-})=4.8\pm 0.6(\mathrm{stat})\pm 0.5(\mathrm{syst})\mu \mathrm{b}$, in agreement with leading-order QED predictions. The measurement, based on a small fraction of the expected integrated luminosity of the LHC nuclear program, establishes the potential for a substantially more precise determination of the anomalous magnetic moment of the $\tau$ lepton, which is currently poorly constrained.