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

## TL;DR

This paper proposes using ultraperipheral heavy ion collisions at the LHC to measure the tau lepton's magnetic and electric dipole moments, achieving constraints surpassing previous collider results and opening new physics exploration avenues.

## Contribution

It introduces a novel method utilizing photon fusion events in heavy ion collisions to probe tau lepton dipole moments with improved sensitivity.

## Key findings

- Current dataset could constrain $a_\tau$ to -0.0080 to 0.0046 at 68% CL.
- Method surpasses 15-year-old lepton collider precision by a factor of three.
- Potential to explore new physics through tau dipole moment measurements.

## 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.