We consider how to extract the pion form factors in the epsilon regime. Using the correlators with non-zero momenta and taking appropriate ratios of them, we eliminate the dominant finite volume effect from the zero-momentum pion mode. Our preliminar
y lattice result for the pion charge radius is consistent with the experiment.
A simulation of lattice QCD at (or even below) the physical pion mass is feasible on a small lattice size of sim 2 fm. The results are, however, subject to large finite volume effects. In order to precisely understand the chiral behavior in a finite
volume, we develop a new computational scheme to interpolate the conventional epsilon and p regimes within chiral perturbation theory. In this new scheme, we calculate the two-point function in the pseudoscalar channel, which is described by a set of Bessel functions in an infra-red finite way as in the epsilon regime, while chiral logarithmic effects are kept manifest as in the p regime. The new ChPT formula is compared to our 2+1- flavor lattice QCD data near the physical up and down quark mass, mud sim 3 MeV on an L sim 1.8 fm lattice. We extract the pion mass = 99(4) MeV, from which we attempt a chiral interpolation of the observables to the physical point.
