Measurement of the CP-Violating Asymmetry Amplitude sin2beta

TL;DR

This paper reports a precise measurement of the CP-violating asymmetry parameter sin2beta in neutral B meson decays, confirming the Standard Model prediction of CP violation in the B system.

Contribution

First measurement of sin2beta using a large data sample from the BABAR experiment, providing key evidence for CP violation in B meson decays.

Findings

Measured sin2beta = 0.741 +/- 0.067 (stat) +/- 0.034 (syst)

Found |lambda| = 0.948 +/- 0.051 (stat) +/- 0.030 (syst)

Results are consistent with Standard Model predictions

Abstract

We present results on time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurements use a data sample of about 88 million Y(4S) --> B Bbar decays collected between 1999 and 2002 with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We study events in which one neutral B meson is fully reconstructed in a final state containing a charmonium meson and the other B meson is determined to be either a B0 or B0bar 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 = 0.741 +/- 0.067 (stat) +/- 0.034 (syst) and |lambda| = 0.948 +/- 0.051 (stat) +/- 0.030 (syst). The magnitude of lambda is consistent with unity, in agreement with the Standard Model expectation of no direct CP violation in these modes.