$I=0$ $\pi\pi$ $s$-wave scattering length from lattice QCD

We deliver lattice results for the $I=0$ $\pi\pi$ elastic $s$-wave scattering length calculated with the MILC $N_f=3$ flavors of the Asqtad-improved staggered fermions. The scattering phase shifts are determined by L\"uscher's formula from the energy-eigenvalues of $\pi\pi$ systems at one center of mass frame and four moving frames using the moving wall source technique. Our measurements are good enough to resolve the scattering length $a$ and effective range $r$, moreover, it allows us to roughly estimate the shape parameter $P$. Using our lattice results, the scattering length $a$ and effective range $r$ at the physical point are extrapolated by chiral perturbation theory. Our results are reasonably consistent with the Roy equation determinations and the newer experimental data. Numerical computations are carried out with two MILC fine ($a\approx0.09$~fm, $L^3 \times T = 40^3\times 96$) and one MILC superfine ($a\approx0.06$~fm, $L^3 \times T = 48^3\times 144$) lattice ensembles at three pion masses of $m_\pi\sim247~{\rm MeV}$, $249~{\rm MeV}$, and $314~{\rm MeV}$, respectively.