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Description
The B0_s meson (bbar_s) is a rare and beautiful laboratory for the study of CP violation, which manifests itself, as in the neutral kaon system, through the presence of CP-forbidden transitions, and, as in the B0 (bbar_d) system, through time dependent CP asymmetries. In the decay B0_s -> J/psi phi, these two features are inseparable, and can be attributed to a single parameter, beta_s, an angle of the ``squashed'' (bs) unitarity triangle.
The measurement of beta_s is of fundamental interest because the standard model prediction, lying close to zero, is largely free from theoretical uncertaintly. New sources of CP violation, entering through quantum loop corrections that include new unknown particles, will generally cause the measured value to deviate from its standard model prediction.
The measurement of beta_s at CDF and D0 finally became possible after the observation of B0_s-Bbar0_s oscillations in 2006. Both experiments now find experimental constraints which are only marginally in agreement with standard model predictions. CDF's first measurement based on 1.35~fb-1 is consistent with the standard model at the 15% confidence level, while D0's, based on 2.8~fb-1, is consistent at the 6.6% level. A combination of the two analyses finds a standard model consistency of 3.1%, corresponding to 2.2 Gaussian standard deviations. In this talk I will describe the phenomenology of the B0_s -> J/psi phi decay and review the recent experimental measurements and their interpretation.
