Exploring Bs→Ds(⁎)±K∓ decays in the presence of a sizable width difference ΔΓs

The Bs→Ds(⁎)±K∓ decays allow a theoretically clean determination of ϕs+γ, where ϕs is the Bs0-B¯s0 mixing phase and γ the usual angle of the unitarity triangle. A sizable Bs decay width difference ΔΓs was recently established, which leads to subtleties in analyses of the Bs→Ds(⁎)±K∓ branching ratios but also offers new “untagged” observables, which do not require a distinction between initially present Bs0 or B¯s0 mesons. We clarify these effects and address recent measurements of the ratio of the Bs→Ds±K∓, Bs→Ds±π∓ branching ratios. In anticipation of future LHCb analyses, we apply the SU(3) flavour symmetry of strong interactions to convert the B-factory data for Bd→D(⁎)±π∓, Bd→Ds±π∓ decays into predictions of the Bs→Ds(⁎)±K∓ observables, and discuss strategies for the extraction of ϕs+γ, with a special focus on untagged observables and the resolution of discrete ambiguities. Using our theoretical predictions as a guideline, we make simulations to estimate experimental sensitivities, and extrapolate to the end of the planned LHCb upgrade. We find that the interplay between the untagged observables, which are accessible thanks to the sizable ΔΓs, and the mixing-induced CP asymmetries, which require tagging, will play the key role for the experimental determination of ϕs+γ.