Phenomenology with a non-zero Bₛ decay width difference

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The experimentally established non-zero decay width difference of the Bₛ meson system gives us access to a mass eigenstate rate asymmetry for each Bₛ transition. This observable is not only the key ingredient in converting between differing definitions of a Bₛ 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 Bₛ mixing. We then shift our focus to the rare decay Bₛ 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 Bₛ to mu+ mu- can discriminate between.