Indirect CP violation in the B-system

We show that, contrary to the flavour mixing amplitude q/p, both Re(ε) and Im(ε) are observable quantities, where ε is the phase-convention-independent CP mixing. We consider semileptonic Bd decays from a CP tag and build appropriate time-dependent asymmetries to separate out Re(ε) and Im(ε). “Indirect" CP violation would have in Im(ε)/(1+|ε|2) its most prominent manifestation in the B-system, with expected values in the standard model ranging from −0.37 to −0.18. This quantity is controlled by a new observable phase: the relative one between the CP-violating and CP-conserving parts of the effective hamiltonian. For time-integrated rates we point out a ΔΓ→ΣΓ transmutation which operates in the perturbative CP mixing.