The π+π+π+π+ scattering length from maximally twisted mass lattice QCD

We calculate the s-wave pion–pion scattering length in the isospin I=2I=2 channel in lattice QCD for pion masses ranging from 270 MeV270 MeV to 485 MeV485 MeV using two flavors of maximally twisted mass fermions at a lattice spacing of 0.086 fm0.086 fm. Additionally, we check for lattice artifacts with one calculation at a finer lattice spacing of 0.067 fm0.067 fm. We use chiral perturbation theory at next-to-leading order to extrapolate our results. At the physical pion mass, we find mπaππI=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.