Evidence for B0→ρ0ρ0 Decays and Implications for the Cabibbo-Kobayashi-Maskawa Angle α

Abstract

We search for the decays B 0 → ρ 0 ρ 0 , B 0 → ρ 0 f 0 ( 980 ) , and B 0 → f 0 ( 980 ) f 0 ( 980 ) in a sample of about 384 × 10 6 Υ ( 4 S ) → B ¯¯¯ B decays collected with the BABAR detector at the PEP-II asymmetric-energy e + e − collider at Stanford Linear Accelerator Center. We find evidence for B 0 → ρ 0 ρ 0 with 3.5 σ significance and measure the branching fraction B = ( 1.07 ± 0.33 ± 0.19 ) × 10 − 6 and longitudinal polarization fraction f L = 0.87 ± 0.13 ± 0.04 , where the first uncertainty is statistical, and the second is systematic. The uncertainty on the Cabibbo-Kobayashi-Maskawa quark-mixing matrix unitarity angle α due to penguin contributions in B → ρ ρ decays is 18° at the 1 σ level. We also set upper limits on the B 0 → ρ 0 f 0 ( 980 ) and B 0 → f 0 ( 980 ) f 0 ( 980 ) decay rates.