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 introduce a suite of one electron/muon plus track(s) analyses, leveraging the exceptionally clean photon fusion $γγ\to ττ$ 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% CL. This surpasses 15 year old lepton collider precision by a factor of three while opening novel avenues to new physics.

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