Observation ofB∓→ρ∓ρ0Decays

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