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

Abstract

We measure the branching fraction and CP violation asymmetry in the decay B0 → π0π0, using a data sample of 752 × 106 BB¯ pairs collected at the ϒð4SÞ resonance with the Belle detector at the KEKB eþe− collider. The obtained branching fraction and direct CP asymmetry are BðB → π0π0Þ ¼ ½1.31 0.19ðstatÞ 0.19ðsystÞ × 10−6 and ACP ¼ þ0.14 0.36ðstatÞ 0.10ðsystÞ, respectively. The signal significance, including the systematic uncertainty, is 6.4 standard deviations. We combine these results with Belle’s earlier measurements of B0 → πþπ− and B → ππ0 to exclude the CP-violating parameter ϕ2 from the range 15.5° < ϕ2 < 75.0° at 95% confidence level.