We report the first observation of the charmless vector-vector decay process $B^{\mp }\to {\rho }^{\mp }{\rho }^0$. The measurement uses a $78\mathrm{f}{\mathrm{b}}^{\mathrm{-}\mathrm{1}}$ data sample collected with the Belle detector at the KEKB asymmetric $e^{+}{e}^{-}$ collider operating at the ${\rm Y}(4S)$ resonance. We obtain a branching fraction of $\mathcal{B}(B^{\mp }\to {\rho }^{\mp }{\rho }^0)=[31.7\pm 7.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}{)}_{-6.7}^{+3.8}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})]\times {10}^{-6}$. An analysis of the $\rho$ helicity-angle distributions gives a longitudinal polarization fraction of ${\Gamma }_L/\Gamma =0.95\pm 0.11(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.02(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$. We also measure the direct-$CP$-violating asymmetry ${\mathcal{A}}_{CP}(B^{\mp }\to {\rho }^{\mp }{\rho }^0)=0.00\pm 0.22(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.03(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$.

• Received 3 June 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.221801