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A Roadmap to Control Penguin Effects in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$

Measurements of CP violation in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$ decays play key roles in testing the quark-flavour sector of the Standard Model. The theoretical interpretation of the corresponding observables is limited by uncertainties from doubly Cabibbo-suppressed penguin topologies. With continuously increasing experimental precision, it is mandatory to get a handle on these contributions, which cannot be calculated reliably in QCD. In the case of the measurement of $\sin2β$ from $B^0_d\to J/ψK_{\rm S}^0$, the $U$-spin-related decay $B^0_s\to J/ψK_{\rm S}^0$ offers a tool to control the penguin effects. As the required measurements are not yet available, we use data for decays with similar dynamics and the $SU(3)$ flavour symmetry to constrain the size of the expected penguin corrections. We predict the CP asymmetries of $B^0_s\to J/ψK_{\rm S}^0$ and present a scenario to fully exploit the physics potential of this decay, emphasising also the determination of hadronic parameters and their comparison with theory. In the case of the benchmark mode $B^0_s\to J/ψϕ$ used to determine the $B^0_s$-$\bar B^0_s$ mixing phase $ϕ_s$ the penguin effects can be controlled through $B^0_d\to J/ψρ^0$ and $B^0_s\to J/ψ\bar{K}^{*0}$ decays. The LHCb collaboration has recently presented pioneering results on this topic. We analyse their implications and present a roadmap for controlling the penguin effects.