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

The anomalous magnetic moment of the tau lepton a_τ= (g_τ-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 δa_τ and electric dipole moments δd_τ. We design a suite of analyses with signatures comprising one electron/muon plus track (s), leveraging the exceptionally clean photon fusion γγ→ττ events to reconstruct both leptonic and hadronic tau decays sensitive to δa_τ, δd_τ. Assuming 10% systematic uncertainties, the current 2 nb^-1 lead-lead dataset could already provide constraints of -0. 0080<a_τ<0. 0046 at 68% C. L. This surpasses 15-year-old lepton collider precision by a factor of 3 while opening novel avenues to new physics.