Evidence for substantial charge symmetry violation in parton distributions

Abstract

Charge symmetry for parton distributions can be tested by comparing structure functions from neutrino and charged lepton deep inelastic scattering. New experiments provide rather tight upper limits on parton charge symmetry violation (CSV) for intermediate x, but suggest CSV effects at small x. Careful study of several corrections fails to remove this low- x discrepancy. We are thus forced to consider surprisingly large CSV effects in nucleon sea distributions.