David London
نویسنده
چکیده
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 d(t) → J/ψKS. 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 d → J/ψKS 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 ∗ tbVts) and in the tree amplitude (V ∗ cbVcs) 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 d → J/ψKS. The extraction of the CP phase β from this decay mode is therefore extremely clean. The decay B d(t) → J/ψK ∗ is also a clean mode, for exactly the same reasons as above. The complication, in comparison to B d(t) → ψKS, 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 d–B 0 d mixing, the measurement of β Talk given by Rahul Sinha at Flavor Physics and CP Violation (FPCP), Philadelphia, PA, USA, May 2002
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