Observation of CP violation in the neutral B meson system

This dissertation presents a measurement of time-dependent CP -violating asymmetries in neutral B meson decays collected with the BABAR detector at the PEP-11 asymmetric-energy B Factory at the Stanford Linear Accelerator Center. The data sample consists of about 88 million ϒ(4S) → BB¯ decays collected between 1999 and 2002. We study events in which one neutral B meson decay to the CP-eigenstates $J/yK0S,y2SK0S,cc1K0S$, and $hcK0S$, or to flavor-eigenstates involving D(*)π/ρ/ a1 and $J/y$K*0(K*0 → K+ π−), is fully reconstructed. The flavor of the other neutral B meson is tagged at the time of its decay, mainly using the charge of identified leptons and kaons. The proper time elapsed between the meson decays is determined by measuring the distance between the decay vertices. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2β, is determined from a simultaneous maximum-likelihood fit to the time-difference distribution of the flavor- and CP-eigenstate samples. We measure sin2β = 0.755 ± 0.074 (stat) ±0.030 (syst).