Precise determination of theI=2ππscattering length from mixed-action lattice QCD

The $I=2$ $\pi \pi$ scattering length is calculated in fully dynamical lattice QCD with domain-wall valence quarks on the asqtad-improved coarse MILC configurations (with fourth-rooted staggered sea quarks) at four light-quark masses. Two- and three-flavor mixed-action chiral perturbation theory at next-to-leading order is used to perform the chiral and continuum extrapolations. At the physical charged pion mass, we find $m_{\pi }{a}_{\pi \pi }^{I=2}=-0.04330\pm 0.00042$, where the error bar combines the statistical and systematic uncertainties in quadrature.