  We calculate the s-wave pion-pion scattering length in the isospin I=2
channel in lattice QCD for pion masses ranging from 270 Mev to 485 Mev using
two flavors of maximally twisted mass fermions at a lattice spacing of 0.086
fm. Additionally, we check for lattice artifacts with one calculation at a
finer lattice spacing of 0.067 fm. We use chiral perturbation theory at
next-to-leading order to extrapolate our results. At the physical pion mass, we
find m_pi a_pipi(I=2)=-0.04385(28)(38) for the scattering length, where the
first error is statistical and the second is our estimate of several systematic
effects.
