A lower bound on sin2β from minimal flavour violation

We point out that there exists an absolute lower bound on sin2β in all models with minimal flavour violation (MFV), that do not have any new operators beyond those present in the Standard Model and in which all flavour changing transitions are governed by the CKM matrix with no new phases beyond the KM phase. This bound depends only on |Vcb|, |Vub/Vcb| and the hadronic parameters B̂K, FBdB̂d and ξ relevant for the CP violating parameter ε and the B0d,s–B̄ 0d,s mixings. Performing a simple scanning over the present ranges of these parameters we find sin2β⩾0.34. We illustrate how this bound could become stronger when our knowledge of the parameters in question improves and when the upper bound on the B0s–B̄ 0s mixing ((ΔM)s) will be experimentally known. Provided the future accurate measurements of sin2β through the CP asymmetry in Bd0(B̄d 0)→ψKS will confirm the low values recently reported by BaBar and Belle, there is a likely possibility that this class of models will be excluded. This would firmly imply the necessity of new CP violating phases and/or new effective operators in the weak effective Hamiltonians for K0–K̄ 0 and B0d,s–B̄ 0d,s mixings. We also point out that within the MFV models there exists also an absolute lower bound on the angle γ. We find sinγ⩾0.24. This lower bound could become stronger in the future.