Phenomenology with a non-zero B_s decay width difference

TL;DR

This paper explores how the non-zero decay width difference in the B_s meson system enables new measurements and observables, such as effective lifetimes and asymmetries, to probe and constrain New Physics beyond the Standard Model.

Contribution

It introduces the use of decay width difference-induced asymmetries and effective lifetimes as novel tools for constraining B_s mixing and New Physics in B_s to mu+ mu- decays.

Findings

Effective lifetimes can constrain B_s mixing parameters.

Asymmetries significantly affect the Standard Model branching ratio.

New observables can discriminate between BSM models.

Abstract

The experimentally established non-zero decay width difference of the B_s meson system gives us access to a mass eigenstate rate asymmetry for each B_s transition. This observable is not only the key ingredient in converting between differing definitions of a B_s branching ratio, but can also be a sensitive probe of New Physics that does not require flavour tagging. We discuss how a pair of effective lifetimes for CP even and odd final states, which probe this asymmetry, can constrain the parameters of B_s mixing. We then shift our focus to the rare decay B_s to mu+ mu-, for which the Standard Model branching ratio prediction receives a sizable correction due to a maximal asymmetry. We present how this asymmetry, which can be extracted from an untagged time-dependent measurement, serves as a new observable, complementary to the branching ratio, for constraining New Physics. Further, we analyse types of models beyond the Standard Model that this pair of observables for B_s to mu+ mu- can discriminate between.