David London

CP violation in the B system [1] has been established by the recent measurements of sin 2β, with a world average of sin 2β = 0.78 ± 0.08 [2]. The main B decay used to probe the weak phase β is the so-called " gold-plated " mode B 0 d (t) → J/ψK S. In order to extract weak-phase information cleanly, i.e. with no hadronic uncertainties, a given B decay must be dominated by a single weak decay amplitude. However, even within the SM, B 0 d → J/ψK S receives contributions from two weak amplitudes: the tree amplitude and the b → s penguin amplitude. Nevertheless, this decay mode is very clean for the following two reasons. First, the cc quark pair must be produced in a color-singlet state, requiring three gluons in the penguin amplitude. Consequently, the penguin contribution is expected to be considerably smaller than the tree contribution. Second, in the Wolfenstein parameterization [3] the CKM matrix elements involved in the b → s penguin amplitude (V * tb V ts) and in the tree amplitude (V * cb V cs) are both real. Thus, the weak phases of these two amplitudes are the same, so that effectively only a single weak amplitude contributes to B 0 d → J/ψK S. The extraction of the CP phase β from this decay mode is therefore extremely clean. The decay B 0 d (t) → J/ψK * is also a clean mode, for exactly the same reasons as above. The complication, in comparison to B 0 d (t) → ψK S , is that the final state now consists of two vector particles, so that the CP-even and CP-odd components must be distinguished by performing an angular analysis [1]. Each component can then be treated separately, and β can be obtained cleanly. In the presence of new physics, the extraction of the weak phase β may not be clean. If the new physics contributes only to B 0 d –B 0 d mixing, the measurement of β