Measurement of the branching fraction and CP asymmetry in B0 → π0π0 decays, and an improved constraint on ϕ2

Abstract

We measure the branching fraction andCPviolation asymmetry in the decayB0→π0π0, usinga data sample of 752×106BBpairs collected at the Υ(4S) resonance with the Belle detector atthe KEKBe+e−collider. The obtained branching fraction and directCPasymmetry areB(B→π0π0) = [1.31±0.19 (stat.)±0.19 (syst.)]×10−6andACP= +0.14±0.36 (stat.)±0.10 (syst.),respectively. The signal significance, including the systematic uncertainty, is 6.4 standard deviations.We combine these results with Belle’s earlier measurements ofB0→π+π−andB±→π±π0toexclude theCP-violating parameterφ2from the range 15.5◦< φ2<75.0◦at 95% confidence level.