Light meson form factors at high Q^2 from lattice QCD
Measurements and theoretical calculations of meson form factors are essential for our understanding of internal hadron structure and QCD, the dynamics that bind the quarks in hadrons. The pion form factor has been measured at small momentum transfer (Q^2) by scattering from atomic electrons and from scattering of electrons from the pion cloud around a proton. On the other hand, in the limit of very large (or infinite) Q^2 perturbation theory is applicable. This leaves a gap in the intermediate Q^2 where the form factors are not known.
Jefferson Lab are upgrading their experiment and will measure pion and kaon form factors in this intermediate region, up to Q^2 of 6∼10 GeV^2 for the pion. This is then an ideal opportunity for lattice QCD to make an accurate prediction ahead of the experimental results. Lattice QCD provides the state-of-the-art techniques to calculate form factors, and the challenge here is to control the statistical and systematic uncertainties as errors grow when going to higher Q^2 values. In this talk we report of a calculation that tests the method using an eta_s meson, a 'heavy pion' made of strange quarks, and also present preliminary results for kaon and pion form factors. We use the n_f=2+1+1 ensembles made by the MILC collaboration and Highly Improved Staggered Quarks, which allows us to use high statistics. The HISQ action is also designed to have small dicretisation errors. Using several light quark masses and lattice spacings allows us to control the chiral and continuum extrapolation and keep systematic errors in check.
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