New physics and tau $g-2$ using LHC heavy ion collisions

Abstract

The anomalous magnetic moment of the tau lepton $a_{\tau }=(g_{\tau }-2)/2$ strikingly evades measurement, but is highly sensitive to new physics such as compositeness or supersymmetry. We propose using ultraperipheral heavy ion collisions at the LHC to probe modified magnetic $\delta a_{\tau }$ and electric dipole moments $\delta d_{\tau }$. We introduce a suite of one electron/muon plus track(s) analyses, leveraging the exceptionally clean photon fusion $\gamma \gamma \to \tau \tau$ events to reconstruct both leptonic and hadronic tau decays sensitive to $\delta a_{\tau },\delta d_{\tau }$. Assuming 10% systematic uncertainties, the current 2 nb${}^{-1}$ lead-lead dataset could already provide constraints of $-0.0080<a_{\tau }<0.0046$ at 68% CL. This surpasses 15 year old lepton collider precision by a factor of three while opening novel avenues to new physics.

Comments: 5 pages, 3 figures + bibliography and appendices