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We have observed the rare decay B0 => rho+ rho- in a sample
of 89 million BB pairs recorded with the BABAR detector.
The number of observed events is 88 +23-21 +-9,
with a significance of 5.1 standard deviations with
systematic uncertainties included. The branching fraction
and the longitudinal polarization are measured to be
BR(B0 => rho+ rho-) = (25 +7-6 +5-6)*10^{-6} and
Gamma_L/Gamma = 0.98 +0.02-0.08 +-0.03, respectively.
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