Properties of light quarks from lattice QCD simulations¹

By numerical study of the simple bound states of light quarks, in particular the π and K mesons, we are able to deduce fundamental quark properties. Using the "improved staggered" discretization of QCD, the MILC Collaboration has performed a series of simulations of these bound states, including the effects of virtual quark-antiquark pairs ("sea" quarks). From these simulations, we have determined the masses of the up, down, and strange quarks. We find that the up quark mass is not zero (at the 10 sigma level), putting to rest a twenty-year-old suggestion that the up quark could be massless. Further, by studying the decays of the π and K mesons, we are able to determine the "CKM matrix element" V_us of the Weak Interactions. The errors on our result for V_us are comparable to the best previous determinations using alternative theoretical approaches, and are likely to be significantly reduced by simulations now in progress.