Hidenori Fukaya (Osaka University)
In lattice QCD simulations, there is a strong tendency to over-estimate the form factors (and therefore, under-estimate the related charge radii) of pions and other hadrons. In fact, the three major systematic in lattice QCD: finite lattice spacing a, lack of chiral symmetry, and finite volume all could contribute to the over-estimation of the form factors. This can be easily understood within the chiral effective theory, as a distortion of the chiral logarithm: ln M_pi^2 -> ln (M_pi^2+delta m^2), where M_pi denotes the pion mass, and delta m is the unphysical mass scale induced by finite a, and/or explicit chiral symmetry breaking, and/or finite lattice size. In this talk, we would like to focus on the finite volume effects using chiral perturbation theory (Ref. 1) and discuss how to avoid such an over-estimation of the (pion) form factors. We also present our recent lattice results with dynamical overlap (Ref. 2 and see attached Figure) and M"obius domain-wall fermions (Ref. 3). References 1. H. Fukaya and T. Suzuki, Phys. Rev. D 90, no. 11, 114508 (2014) [arXiv:1409.0327]. 2. JLQCD collaboration, Phys. Rev. D 90, no. 3, 034506 (2014) [arXiv:1405.4077]. 3. JLQCD collaboration, work in progress.