Study ofB→πℓνandB→ρℓνdecays and determination of|Vub|

We present an analysis of exclusive charmless semileptonic $B$-meson decays based on $83\times {10}^6$ $B\overline{B}$ pairs recorded with the BABAR detector at the ${\rm Y}(4S)$ resonance. Using isospin symmetry, we measure branching fractions $\mathcal{B}(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.38\pm 0.10\pm 0.16\pm 0.08)\times {10}^{-4}$ and $\mathcal{B}(B^0\to {\rho }^{-}{\ell }^{+}\nu )=(2.14\pm 0.21\pm 0.48\pm 0.28)\times {10}^{-4}$, where the errors are statistical, experimental systematic, and due to form-factor shape uncertainties. We compare the measured distribution in $q^2$, the momentum-transfer squared, with theoretical predictions for the form factors from lattice QCD and light-cone sum rules, and extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|V_{ub}|=(3.82\pm 0.14\pm 0.22\pm {0.11}_{-0.52}^{+0.88})\times {10}^{-3}$ from $B\to \pi \ell \nu$, where the fourth error reflects the uncertainty of the form-factor normalization.