Observation of e+e- --> rho+rho- near sqrt(s)=10.58 GeV

We report the first observation of $e^+e^- \to \rho^+\rho^-$, in a data sample of 379 $fb^{-1}$ collected with the BABAR detector at the {PEP-II} $e^+e^-$ storage ring at center-of-mass energies near $\sqrt{s} = 10.58 GeV. We measure a cross section of $\sigma({e^+e^-\} \to \rho^+ \rho^-) = {19.5 \pm 1.6(stat) \pm 3.2(syst) fb}$. Assuming production through single-photon annihilation, there are three independent helicity amplitudes. We measure the ratios of their squared moduli to be $|{F}_{00}|^2 : |{F}_{10}|^2 : |{F}_{11}|^2 = 0.51 \pm 0.14(stat) \pm 0.07(syst) : 0.10 \pm 0.04(stat) \pm 0.01(syst) : 0.04 \pm 0.03(stat) \pm 0.01(syst)$. The $|{F}_{00}|^2$ result is inconsistent with the prediction of 1.0 made by QCD models with a significance of 3.1 standard deviations including systematic uncertainties.