Light new physics and the $\tau$ lepton dipole moments

TL;DR

This paper analyzes how light new physics particles, especially tauphilic gauge bosons, can affect tau lepton dipole moments and how to interpret experimental asymmetry measurements in this context.

Contribution

It provides a comprehensive analysis of light new physics contributions to tau dipole moments, including interpretation of asymmetry measurements and decoupling from EFT limits.

Findings

Light spin-0 and spin-1 bosons can significantly influence tau dipole moments.

Decoupling to EFT occurs when NP scale is large compared to experimental energy.

Tauphilic gauge vector boson at Belle II offers a specific test case.

Abstract

Testing New-Physics (NP) scenarios that couple predominantly to the third generation is notoriously difficult experimentally, as exemplified by comparing limits for the $\tau$ lepton dipole moments to those of electrons and muons. In this case, extracting limits from processes such as $e^+e^-\to\tau^+\tau^-$ often relies on effective-field-theory (EFT) arguments, which allow for model-independent statements, but only apply if the NP scale is sufficiently large compared to the center-of-mass energy. In this work we offer a comprehensive analysis of light NP contributions to the $\tau$ dipole moments, providing a detailed account of the interpretation of asymmetry measurements in $e^+e^-\to\tau^+\tau^-$ that are tailored towards the extraction of dipole moments, for the test cases of new light spin-$0$ and spin-$1$ bosons. Moreover, we study the decoupling to the EFT limit in these scenarios and discuss the complementarity to constraints from other related processes, such as production in $e^+e^-$ reactions. While covering a wide range of light NP scenarios, as specific case study we present a detailed discussion of a tauphilic gauge vector boson at Belle II.