Limit on the B0->rho0rho0 Branching Fraction and Implications for the   CKM Angle alpha
  We search for the decay B0 -> rho0 rho0 in a data sample of about 227 million Upsilon(4S)-> BBbar decays collected with the BABAR detector at the PEP-II asymmetric-energy e+e- collider at SLAC. We find no significant signal and set an upper limit of 1.1*10^-6 at 90% CL on the branching fraction. As a result, the uncertainty due to penguin contributions on the CKM unitarity angle alpha measured in B -> rho rho decays is 11 degrees at 68% CL. 
The BABAR Collaboration, B. Aubert et al