BR(Bs to mu+ mu-) as an electroweak precision test

TL;DR

This paper uses an effective-theory approach to show that measurements of the branching ratio of Bs to mu+ mu- provide powerful constraints on new physics models affecting Z-boson couplings, often surpassing traditional Z peak observables.

Contribution

It demonstrates that BR(Bs to mu+ mu-) can serve as a competitive or superior electroweak precision test for new physics models with modified Z couplings, under specific flavor assumptions.

Findings

BR(Bs to mu+ mu-) bounds are comparable to Z peak constraints in minimal flavor violation.

In models with partial compositeness, these bounds are significantly more stringent.

Effective-theory analysis links rare B decay measurements to electroweak precision constraints.

Abstract

Using an effective-theory approach, we analyze the impact of BR(Bs to mu+ mu-) in constraining new-physics models that predict modifications of the Z-boson couplings to down-type quarks. Under motivated assumptions about the flavor structure of the effective theory, we show that the bounds presently derived from BR(Bs to mu+ mu-) on the effective Z-boson couplings are comparable (in the case of minimal flavor violation) or significantly more stringent (in the case of generic partial compositeness) with respect to those derived from observables at the Z peak.