CP violation in B(s) → ϕK decays: Standard Model benchmarks and isospin-breaking New Physics

A bstract The penguin loop-suppressed B → ϕK decays are highly sensitive to contributions of hypothetical heavy new particles. Particularly interesting probes for testing the Standard Model and revealing such phenomena are provided by CP violation in the $${B}_{d}^{0}\to \phi {K}_{S}$$ decay. Standard-Model estimates for the corresponding CP-violating observables are theoretically limited by doubly Cabibbo-suppressed penguin contributions. We study these effects using a factorization approach, and provide predictions for CP asymmetries, to be contrasted with future measurements. To gain additional insight into these hadronic effects, we propose the $${B}_{s}^{0}\to \phi {K}_{S}$$ decay as a new channel. We predict the observables for this decay for which currently no measurements exist. By comparing $${B}_{d}^{0}\to \phi {K}_{S}$$ and B + → ϕK + , we further derive state-of-the-art constraints on isospin observables within the Standard Model. The same framework enables probing of possible New-Physics contributions, including general effects and those with non-trivial isospin structure. Interesting prospects arise for the high-precision era of flavour physics ahead.