The validity of charge symmetry for parton distributions

Abstract

Recent measurements of the Gottfried Sum Rule have focused attention on the possibility of substantial breaking of flavor symmetry in sea quark distributions of the proton. This has been confirmed by by pp and pD Drell-Yan processes measured at FNAL. The theoretical models used to infer flavor symmetry breaking rely on the assumption that parton distributions are charge symmetric; it is conceivable that current tests of flavor symmetry could be affected by substantial charge symmetry violation. Since all phenomenological parton distributions assume the validity of charge symmetry, in this paper we examine the possibility that charge symmetry is violated [CSV]. We first list definitions for structure functions which do not make the usual assumption that parton distributions obey charge symmetry. We then give some simple model estimates of CSV for both valence and sea quark distributions. Next, we list a set of relations which must hold if charge symmetry is valid, and we review the current experimental limits on charge symmetry violation in parton distributions. We then propose a series of possible experimental tests of charge symmetry. The proposed experiments could either detect charge symmetry violation in parton distributions, or they could provide more stringent upper limits on CSV. We discuss CSV contributions to sum rules, and we propose new sum rules which could differentiate between flavor symmetry, and charge symmetry, violation in nuclear systems.