U(2)^5 flavor symmetry and lepton universality violation in W -> tau nu_tau

TL;DR

This paper investigates the apparent violation of lepton universality in tau couplings to the W boson using EFT, concluding that standard EFT cannot explain the deviation without new light particles or strongly coupled new physics.

Contribution

It applies U(2)^5 flavor symmetry within EFT to analyze lepton universality violation and derives bounds on new physics scales from tau decay data.

Findings

Leptonic and semileptonic tau decays constrain new physics to 3-4 TeV.

EFT cannot accommodate the observed universality violation.

New physics must involve light particles or strong coupling at the electroweak scale.

Abstract

The seeming violation of universality in the tau lepton coupling to the W boson suggested by LEP II data is studied using an Effective Field Theory (EFT) approach. Within this framework we explore how this feature fits into the current constraints from electroweak precision observables using different assumptions about the flavor structure of New Physics, namely [U(2) x U(1)]^5 and U(2)^5. We show the importance of leptonic and semileptonic tau decay measurements, giving 3-4 TeV bounds on the New Physics effective scale at 90% C.L. We conclude under very general assumptions that it is not possible to accommodate this deviation from universality in the EFT framework, and thus such a signal could only be explained by the introduction of light degrees of freedom or New Physics strongly coupled at the electroweak scale.