Fermi Constant from Muon Decay Versus Electroweak Fits and Cabibbo-Kobayashi-Maskawa Unitarity.

Andreas Crivellin,Claudio Andrea Manzari,Martin Hoferichter

doi:10.1103/physrevlett.127.071801

Abstract

The Fermi constant G_{F} is extremely well measured through the muon lifetime, defining one of the key fundamental parameters in the standard model (SM). Therefore, to search for physics beyond the SM (BSM) via G_{F}, the constraining power is determined by the precision of the second-best independent determination of G_{F}. The best alternative extractions of G_{F} proceed either via the global electroweak (EW) fit or from superallowed β decays in combination with the Cabibbo angle measured in kaon, τ, or D decays. Both variants display some tension with G_{F} from muon decay, albeit in opposite directions, reflecting the known tensions within the EW fit and hints for the apparent violation of Cabibbo-Kobayashi-Maskawa unitarity, respectively. We investigate how BSM physics could bring the three determinations of G_{F} into agreement using SM effective field theory and comment on future perspectives.

Highlights

The Fermi constant GF is extremely well measured through the muon lifetime, defining one of the key fundamental parameters in the standard model (SM)
To search for physics beyond the SM (BSM) via GF, the constraining power is determined by the precision of the second-best independent determination of GF
We investigate how BSM physics could bring the three determinations of GF into agreement using SM effective field theory and comment on future perspectives

Summary

Introduction

The Fermi constant GF is extremely well measured through the muon lifetime, defining one of the key fundamental parameters in the standard model (SM). One can only conclude the presence or absence of BSM effects by comparing GμF to another independent determination. Since the different GF measurements turn out to display some disagreement beyond their quoted uncertainties, the second part of this Letter is devoted to a systematic analysis of possible BSM contributions in SM effective field theory (SMEFT) [6,7] to see which scenarios could account for these tensions without being excluded by other constraints.

Results

Conclusion