Pion vector form factor from lattice QCD at the physical point

We present an investigation of the electromagnetic pion form factor, FπðQ2Þ, at small values of the four- momentum transfer Q2 (≲0.25 GeV2), based on the gauge configurations generated by the European Twisted Mass Collaboration with Nf = 2 twisted-mass quarks at maximal twist including a clover term. Momentum is injected using nonperiodic boundary conditions and the calculations are carried out at a fixed lattice spacing (a ≃ 0.09 fm) and with pion masses equal to its physical value, 240 MeV and 340 MeV. Our data are successfully analyzed using chiral perturbation theory at next-to-leading order in the light-quark mass. For each pion mass two different lattice volumes are used to take care of finite size effects. Our final result for the squared charge radius is hr2iπ = 0.443ð29Þ fm2, where the error includes several sources of systematic errors except the uncertainty related to discretization effects. The corresponding value of the SU(2) chiral low-energy constant l ̄6 is equal to l ̄6 = 16.2ð1.0Þ