Observation of $\tau$ lepton pair production in ultraperipheral lead-lead collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV

TL;DR

This paper reports the first observation of photon-photon production of tau lepton pairs in ultraperipheral lead-lead collisions at 5.02 TeV, measuring the cross section and estimating the tau's anomalous magnetic moment.

Contribution

First experimental observation of gamma-gamma to tau+ tau- production in ultraperipheral heavy-ion collisions at LHC energies.

Findings

Measured cross section of 4.8 ± 0.6 (stat) ± 0.5 (syst) μb

Results agree with leading-order QED predictions

Estimated tau anomalous magnetic moment a_τ ≈ 0.001 with large uncertainties

Abstract

We present an observation of photon-photon production of $\tau$ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404 $\mu$b$^{-1}$ collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The $\gamma\gamma$ $\to$ $\tau^+\tau^-$ process is observed for $\tau\tau$ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is $\sigma(\gamma\gamma$ $\to$ $\tau^+\tau^-)$ = 4.8 $\pm$ 0.6 (stat) $\pm$ 0.5 (syst) $\mu$b, in agreement with leading-order QED predictions. Using $\sigma(\gamma\gamma$ $\to$ $\tau^+\tau^-)$, we estimate a model-dependent value of the anomalous magnetic moment of the $\tau$ lepton of $a_\tau$ = 0.001 $^{+0.055}_{-0.089}$.