Fermi Constants and ``New Physics''

TL;DR

This paper compares various precision measurements of the Fermi constant, confirming the Standard Model's accuracy at 0.1% and setting bounds on new physics scenarios such as heavy neutrinos and extra gauge bosons.

Contribution

It provides a comprehensive comparison of Fermi constant determinations and illustrates their use in constraining new physics models beyond the Standard Model.

Findings

Good agreement with the Standard Model at 0.1% level

Stringent constraints on heavy neutrino mixing and Higgs models

Lower bound of 2.9 TeV on excited W bosons

Abstract

Various precision determinations of the Fermi constant are compared. Included are muon and (leptonic) tau decays as well as indirect prescriptions employing $\alpha, m_Z, m_W, \ssthwmzms, \Gamma(Z\to\ell^+\ell^-)$, and $\Gamma(Z \to \nu\bar \nu)$ as input. Their good agreement tests the standard model at the $\pm 0.1%$ level and provides stringent constraints on new physics. That utility is illustrated for: heavy neutrino mixing, 2 Higgs doublet models, S, T, and U parameters and excited $W^{\ast^\pm}$ bosons (Kaluza-Klein excitations). For the last of those examples, $m_{W^\ast}\gsims 2.9$ TeV is found.