The Validity of Charge Symmetry for Parton Distributions

Recent measurements of the Gottfried sum rule have focused attention on the possibility of substantial flavor symmetry breaking in the proton sea. This is confirmed by pp and pD Drell-Yan processes measured at FNAL. Theoretical models used to `prove' flavor symmetry breaking rely on the assumption of charge symmetric parton distributions. Substantial charge symmetry violation [CSV] could affect current tests of flavor symmetry. In this review we examine the possibility of CSV for parton distributions. We first give definitions for structure functions without making the usual assumption of charge symmetry. Next we make estimates of CSV for both valence and sea quark distributions. We list a set of relations which must hold if charge symmetry is valid, and review current experimental limits on CSV. We propose a series of experimental tests of charge symmetry. These tests could either detect CSV, or they could provide stronger upper limits on CSV effects. We discuss CSV contributions to sum rules, and we propose sum rules which could differentiate between flavor symmetry, and charge symmetry violation in nuclear systems.