Improved measurement of the CP-violating asymmetry amplitude sin2beta

Abstract

We present updated results on time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurements use a data sample of about 62 million Upsilon(4S) --> B anti-B decays collected between 1999 and 2001 by the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. In this sample we study events in which one neutral B meson is fully reconstructed in a final state containing a charmonium meson and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2beta, is derived from the decay time distributions in such events. We measure sin2beta = 0.75 +/- 0.09 (stat) +/- 0.04 (syst) and |λ| = 0.92 +/- 0.06 (stat) +/- 0.02 (syst). The latter is consistent with the Standard Model expectation of no direct CP violation. These results are preliminary.