Current-mass ratios of the light quarks.
We investigate the current-mass ratios of the light quarks by fitting the squared meson masses to second order in chiral-symmetry breaking, determining corrections to Weinberg's first-order values: m/sub u//m/sub d/ = 0.56, m/sub s//m/sub d/ = 20.1. We find that to this order, m/sub s//m/sub d/ is a known function of m/sub u//m/sub d/. The values of the quark-mass ratios can be constrained by limiting the size of second-order corrections to the squares of meson masses. We find that for specific values of presently unmeasured phenomenological parameters one can have a massless u quark. In that case 30% of the squares of meson masses arise from operators second order in chiral-symmetry breaking.