Investigation of CP violation in B^0 -> J/psi K^0_S decays at LEP

TL;DR

This paper reports on the first direct measurement of the CP violation parameter sin(2beta) in B^0 decays at LEP, using a novel analysis of decay times and flavor tagging.

Contribution

It introduces an analysis method for reconstructing decay times and tagging B-flavors to measure time-dependent CP asymmetry at LEP.

Findings

Measured sin(2beta) = 3.2_{-2.0}^{+1.8} +/- 0.5

Provided probabilities for sin(2beta) values in [-1, 1]

Demonstrated feasibility of direct CP violation measurement at LEP

Abstract

An investigation of CP violation was performed using a total of 24 candidates for B^0 -> J/psi K^0_S decay, with a purity of about 60%. These events were selected from 4.4 million hadronic 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, B^0 or B^0(bar), has been developed to allow a first direct study of the time dependent CP asymmetry that, in the Standard Model, is sin(2beta). The result is sin(2beta) = 3.2_{-2.0}^{+1.8} +/- 0.5, where the first error is statistical and the second systematic. This result is used to determine probabilities for different values of sin(2beta) in the physical region from -1 to +1.