Anomalous electromagnetic moments of $\tau$ lepton in $\gamma \gamma \to {\tau }^{+}{\tau }^{-}$ reaction in Pb+Pb collisions at the LHC

Abstract

We discuss the sensitivity of the $\gamma \gamma \to {\tau }^{+}{\tau }^{-}$ process in ultraperipheral Pb+Pb collisions on the anomalous magnetic ($a_{\tau }$) and electric ($d_{\tau }$) moments of $\tau$ lepton at LHC energies. We derive the corresponding cross sections by folding the elementary cross section with the heavy-ion photon fluxes and considering semi-leptonic decays of both $\tau$ leptons in the fiducial volume of ATLAS and CMS detectors. We present predictions for total and differential cross sections, and for the ratios to $\gamma \gamma \to e^{+}{e}^{-}({\mu }^{+}{\mu }^{-}$) process. These ratios allow to cancel theoretical and experimental uncertainties when performing precision measurement of $a_{\tau }$ at the LHC. The expected limits on $a_{\tau }$ with existing Pb+Pb dataset are found to be better by a factor of two comparing to current best experimental limits and can be further improved by another factor of two at High Luminosity LHC.

Comments: 18 pages, 10 figures, the second version contains a study of the spin correlation effect