Investigation of CP violation in $\mathrm{B^0} \rightarrow \mathrm{J}/\psi \mathrm{K^0_S}$decays at LEP

Abstract.

An investigation of CP violation was performed using a total of 24 candidates for $\mathrm{B^0} \rightarrow \mathrm{J}/\psi \mathrm{K^0_S}$decay, with a purity of about 60%. These events were selected from 4.4 million hadronic $\rm Z^0$decays recorded by the OPAL detector at LEP. An analysis procedure, involving techniques to reconstruct the proper decay times and tag the produced b-flavours, $\mathrm{B^0}$or $\bar{\rm B}^0$, has been developed to allow a first direct study of the time dependent CP asymmetry that, in the Standard Model, is $\sin 2\beta$. The result is \[ \sin 2\beta = 3.2 _{-2.0}^{+1.8} \pm 0.5 \;, \]where the first error is statistical and the second systematic. This result is used to determine probabilities for different values of $\sin 2\beta$in the physical region from $-1$to +1.