Speaker
Description
We study the potential of the FCC‑$ee$ to perform time-dependent $CP$ violation measurements in rare decays $B_s^0 \to \phi(\to K^+K^-)\mu^+\mu^-$ at the $Z$-pole, where large statistics can be achieved. These decays serve as sensitive probes for physics beyond the Standard Model (SM). In the SM, $CP$ violation enters only through loop processes and is therefore highly suppressed, so any significant $CP$-violating effect could be a clear indication of New Physics (NP). By examining these rare decay modes, we aim to uncover signatures of complex NP contributions that are not accessible with current facilities. The FCC‑$ee$, operating at the $Z$-pole, benefits from high luminosity, an exceptionally clean experimental environment, and advanced detector capabilities for particle identification and precise vertexing. Our studies, based on the IDEA detector concept, PYTHIA Monte Carlo simulations, and DELPHES reconstruction, demonstrate that a relative precision better than $1\%$ on the branching ratio and an accuracy of $\mathcal{O}(10^{-2})$ on the time-integrated $CP$ asymmetry are achievable. Furthermore, the analysis exhibits promising sensitivity of $\mathcal{O}(10^{-2}-10^{-1})$ in observables $D_f$, $C_f$ and $S_f$, which characterize the time-dependent $CP$ violation. These results underscore the critical role of the FCC‑$ee$, especially at the $Z$-pole, which stands as perhaps the only facility capable of making such precise measurements in this sector, thereby opening a new window into the study of $CP$ violation and the potential for discovering NP.
