$B\to \pi \mathrm{\ell }\nu$ semileptonic form factors from unquenched lattice QCD and determination of $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$

We compute the $B\to \pi \mathrm{\ell }\nu$ semileptonic form factors and update the determination of the CKM matrix element $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$. We use the MILC asqtad ensembles with $N_f=2+1$ sea quarks at four different lattice spacings in the range $a\approx 0.045$~fm to $0.12$~fm. The lattice form factors are extrapolated to the continuum limit using SU(2) staggered chiral perturbation theory in the hard pion limit, followed by an extrapolation in $q^2$ to the full kinematic range using a functional $z$-parameterization. The extrapolation is combined with the experimental measurements of the partial branching fraction to extract $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$. Our preliminary result is $|V_{ub}|=(3.72\pm 0.14)\times 10^{-3}$, where the error reflects both the lattice and experimental uncertainties, which are now on par with each other.