Prospects for Measuring $CP$-Violation in $B_s^0 \rightarrow \phi \mu^+\mu^-$ via Time-Dependent Angular Analysis

Abstract

This work investigates the prospects for performing a time-dependent angular analysis of $B_s^0 \rightarrow \phi \mu^+\mu^-$ decays at hadron colliders, and introduces new optimised angular observables associated with $B_s^0$-mixing in the decay rate. The time-dependent normalised decay rate and corresponding probability density function is presented both for when the flavour of the $B_s^0$ meson at production is tagged and when it is untagged. The normalised angular terms linked to $B_s^0$-mixing in the tagged (untagged) case are denoted $\mathcal{Z}_i$ ($\mathcal{H}_i$), and their optimised counterparts as $Q_i$ ($\mathcal{M}_i$). The expected sensitivities of these observables at the end of Run 3, Run 4, and Run 5 of the LHC are determined using pseudoexperiments generated with a decay-time resolution, background level, and signal yield similar to those reported by the LHCb collaboration. It is found that all observables can be extracted with Run 3 statistics, and that the $\mathcal{H}_i$ and $\mathcal{Z}_i$ observables have similar sensitivity, despite the former being suppressed by mixing terms. Moreover, the angular observables only accessible via flavour tagging, such as the equivalent of $P_5^{\prime}$ in the $B^0_s$ system, are found to exhibit sensitivities comparable to current measurements in $B^0 \rightarrow K^{\ast 0} \mu^+\mu^-$ decays once Run 5 datasets are available. Fits to the observables to extract the Wilson coefficients show a significant increase in precision when either the observables accessible via time-dependent analysis, or flavour tagging, are included. A marked increase in sensitivity to $CP$-violating short-distance effects is observed for a subset of the new optimised $M_i$ and $Q_i$ observables.