Observation of B0→K*0¯¯¯¯K*0 and Search for B0→K*0K*0

We report the observation of the b → d penguin-dominated decay B 0 → K * 0 ¯¯¯¯ K * 0 with a sample of 383.2 ± 4.2 million B ¯¯¯ B pairs collected with the BABAR detector at the PEP-II asymmetric-energy e + e − collider at the Stanford Linear Accelerator Center. The measured branching fraction is B ( B 0 → K * 0 ¯¯¯¯ K * 0 ) = [ 1.28 + 0.35 − 0.30 ± 0.11 ] × 10 − 6 and the fraction of longitudinal polarization is f L ( B 0 → K * 0 ¯¯¯¯ K * 0 ) = 0.80 + 0.10 − 0.12 ± 0.06 . The first error quoted is statistical and the second systematic. We also obtain an upper limit at the 90% confidence level on the branching fraction for B ( B 0 → K * 0 K * 0 ) < 0.41 × 10 − 6 .