Observation of the decay $B^0\to {\rho }^{+}{\rho }^{-}$ and measurement of the branching fraction and polarization

We have observed the rare decay $B^0\to {\rho }^{+}{\rho }^{-}$ in a sample of 89 million $B\overline{B}$ pairs recorded with the $\mathit{BABAR}$ detector. The number of observed events is ${88}_{-21}^{+23\mathrm{}}\pm 9,$ with a significance of 5.1 standard deviations with systematic uncertainties included. The branching fraction and the longitudinal polarization are measured to be $\mathcal{B}{(B}^0\to {\rho }^{+}{\rho }^{-}{)=(25\mathrm{}}_{-6-6}^{+7+5\mathrm{}})\times {10}^{-6}$ and ${\Gamma }_L/\Gamma {=0.98\mathrm{}}_{-0.08}^{+0.02\mathrm{}}\pm 0.03,$ respectively.