Improved Measurement of CP Asymmetries in Bz to (ccbar) K^((*)0) Decays

We present an updated measurements of time-dependent CP asymmetries in fully reconstructed neutral B decays to several CP eigenstates containing a charmonium meson. The measurements use a data sample of (347.5 +/- 3.8) x 10^{6} $\FourS \to B\Bbar$ decays collected with the babar detector at the PEP-II B factory between 1999 and 2006. We determine sin2beta = 0.710 +/- 0.034(stat) +/- 0.019(syst) and |\lambda| = 0.932 +/- 0.026(stat) +/- 0.017(syst). Both of these results are preliminary.