Flavor Asymmetry of the Light Quark Sea from Semi-inclusive Deep-Inelastic Scattering

The flavor asymmetry of the light quark sea of the nucleon is determined in the kinematic range $0.02<x<0.3$ and $1<{Q}^{2}<10{\mathrm{GeV}}^{2}$ for the first time from semi-inclusive deep-inelastic scattering. The quantity $[\overline{d}(x)\ensuremath{-}\overline{u}(x)]/[u(x)\ensuremath{-}d(x)]$ is derived from a relationship between the yields of positive and negative pions from unpolarized hydrogen and deuterium targets. The flavor asymmetry $\overline{d}\ensuremath{-}\overline{u}$ is found to be nonzero and $x$ dependent, showing an excess of $\overline{d}$ over $\overline{u}$ quarks in the proton.