We present a study of the branching fraction of the decay B^0->rho0rho0 and
the fraction of longitudinally polarized rho0 mesons in this decay. The results
are obtained from the final data sample containing 772 million BBbar pairs
collected at the Y(4S) resonance with the Belle detector at the KEKB
asymmetric-energy e+e- collider. We find 166 +- 59 B^0 -> rho0 rho0 events
(including systematic uncertainties), corresponding to a branching fraction of
B(B^0->rho0rho0) = (1.02 +- 0.30 (stat) +- 0.15 (syst)) x 10^{-6} with a
significance of 3.4 standard deviations and a longitudinal polarization
fraction fL = 0.21^{+0.18}_{-0.22} (stat) +- 0.15 (syst). We use the
longitudinal polarization fraction to determine the Cabibbo-Kobayashi-Maskawa
matrix angle phi_2 = (84.9 +- 13.5) degrees through an isospin analysis in the
B->rhorho system. We furthermore find 149 +- 49 B^0->f0rho0 events,
corresponding to B(B^0->f0rho0) x B(f0->pi+pi-) = (0.78 +- 0.22 (stat) +- 0.11
(syst)) x 10^{-6}, with a significance of 3.1 standard deviations. We find no
other significant contribution with the same final state, and set upper limits
at 90% confidence level on the (product) branching fractions,
B(B^0->pi+pi-pi+pi-)rho0pi+pi-)<12.0 x 10^{-6},
B(B^0->f0pi+pi-) x B(f0->pi+pi-) f0f0) x
B(f0->pi+pi-)^{2} < 0.2 x 10^{-6}.Comment: 21 pages, 20 figures, conference paper for the 2012th CKM workshop,
submitted to PR
