Measurement of the time-dependent CP asymmetry in the B-0 ->phi K-0 decay

We present a measurement of the time-dependent C P asymmetry for the neutral B -meson decay B 0 → ϕ K 0 . We use a sample of approximately 114 × 10 6 B -meson pairs taken at the Υ ( 4 S ) resonance with the BABAR detector at the PEP-II B -meson factory at SLAC. We reconstruct the C P eigenstates ϕ K 0 S and ϕ K 0 L , where ϕ → K + K − , K 0 S → π + π − , and K 0 L is observed via its hadronic interactions. The other B meson in the event is tagged as either a B 0 or ¯¯¯ B 0 from its decay products. The values of the C P -violation parameters are S ϕ K = 0.47 ± 0.34 ( s t a t ) + 0.08 − 0.06 ( s y s t ) and C ϕ K = 0.01 ± 0.33 ( s t a t ) ± 0.10 ( s y s t ) .