  We present results for the isospin-0 $\pi\pi$ s-wave scattering length
calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge
configurations. We use three $N_f = 2$ ensembles with unitary (valence) pion
mass at its physical value (250$\sim$MeV), at 240$\sim$MeV (320$\sim$MeV) and
at 330$\sim$MeV (400$\sim$MeV), respectively. By using the stochastic Laplacian
Heaviside quark smearing method, all quark propagation diagrams contributing to
the isospin-0 $\pi\pi$ correlation function are computed with sufficient
precision. The chiral extrapolation is performed to obtain the scattering
length at the physical pion mass. Our result $M_\pi a^\mathrm{I=0}_0 =
0.198(9)(6)$ agrees reasonably well with various experimental measurements and
theoretical predictions. Since we only use one lattice spacing, certain
systematics uncertainties, especially those arising from unitary breaking, are
not controlled in our result.
