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

Abstract

The anomalous magnetic moment of the tau lepton aτ=(gτ-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 δaτ and electric dipole moments δdτ. 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 δaτ,δdτ. Assuming 10% systematic uncertainties, the current 2 nb-1 lead-lead dataset could already provide constraints of -0.0080 <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.