Parton Charge Symmetry Research Articles

Charge symmetry at the partonic level

This review article discusses the experimental and theoretical status of partonic charge symmetry. It is shown how the partonic content of various structure functions gets redefined when the assumption of charge symmetry is relaxed. We review various theoretical and phenomenological models for charge symmetry violation in parton distribution functions. We summarize the current experimental upper limits on charge symmetry violation in parton distributions. A series of experiments are presented, which might reveal partonic charge symmetry violation, or alternatively might lower the current upper limits on parton charge symmetry violation.

Parton charge symmetry violation: Electromagnetic effects and W production asymmetries

Recent phenomenological work has examined two different ways of including charge symmetry violation in parton distribution functions. First, a global phenomenological fit to high energy data has included charge symmetry breaking terms, leading to limits on the magnitude of parton charge symmetry breaking. In a second approach, two groups have included the coupling of partons to photons in the QCD evolution equations. One possible experiment that could search for isospin violation in parton distributions is a measurement of the asymmetry in W production at a collider. In this work we include both of the postulated sources of parton charge symmetry violation. We show that, given charge symmetry violation of a magnitude consistent with existing high energy data, the expected W production asymmetries would be quite small, generally less than one percent.

Implications of current constraints on parton charge symmetry

For the first time, charge symmetry breaking terms in parton distribution functions have been included in a global fit to high energy data. We review the results obtained for both valence and sea quark charge symmetry violation and compare these results with the most stringent experimental upper limits on charge symmetry violation for parton distribution functions, as well as with theoretical estimates of charge symmetry violation. The limits allowed in the global fit would tolerate a rather large violation of charge symmetry. We discuss the implications of this for various observables, including extraction of the Weinberg angle in neutrino DIS and the Gottfried and Adler sum rules.

Current status of parton charge symmetry

We review constraints on parton charge symmetry from various experiments. Recently charge symmetry violation (CSV) has been included in a global fit to high energy data. We show that CSV compatible with all high energy data would be able to remove completely the NuTeV anomaly.

Charge symmetry violating contributions to neutrino reactions

The NuTeV group has measured charged and neutral current reactions for neutrinos on iron targets. Ratios of these cross sections provide an independent measurement of the Weinberg angle. The NuTeV value for sin2θW is three standard deviations larger than the value measured in other electroweak processes. By reviewing theoretical estimates of parton charge symmetry violation (CSV), we study CSV contributions to the NuTeV measurement. We conclude that charge symmetry violating effects should remove roughly 30% of the discrepancy between the NuTeV result and other determinations of sin2θW.

Strange Quark Distribution and Parton Charge Symmetry Violation in a Semi-Inclusive Process

Strange Quark Distribution and Parton Charge Symmetry Violation in a Semi-Inclusive Process

Evidence for charge symmetry violation in parton distributions

By comparing structure functions measured in neutrino and charged lepton deep inelastic scattering, one can test the validity of parton charge symmetry. New experiments allow us to make such tests, which set rather tight upper limits on parton charge symmetry violation [CSV] for intermediate Bjorken $x$, but which appear to show sizable CSV effects at small $x$. We show that neither nuclear shadowing nor contributions from strange and antistrange quark distributions can account for the experimentally observed difference between the two structure functions. We are therefore forced to consider the possibility of a large CSV effect in the nucleon sea quark distributions. We discuss the consequences of this effect for other observables, and we propose an experiment which could detect a large CSV component in the nucleon sea.

Evidence for Substantial Charge Symmetry Violation in Parton Distributions

In principle one can test the validity of charge symmetry for parton distributions by comparing structure functions measured in neutrino and charged lepton deep inelastic scattering. New experiments make such tests possible; they provide rather tight upper limits on parton charge symmetry violation [CSV] for intermediate Bjorken x, but appear to show evidence for CSV effects at small x. We examine two effects which might account for this experimental discrepancy: nuclear shadowing corrections for neutrinos, and strange quark contributions s(x) unequal to sbar(x). We show that neither of these two corrections removes the experimental discrepancy between the structure functions. We are therefore forced to consider the possibility of a surprisingly large CSV effect in the nucleon sea quark distributions.

Testing parton charge symmetry at HERA

There are strong theoretical indications that the minority valence quark distributions in the nucleon may break charge symmetry by as much as 3-5%. We show that a comparison of $e^{\pm} D$ deep inelastic scattering through the charged current, for example at HERA, could provide a direct test of this effect. This measurement is also be sensitive to an intrinsic component of the strange quark sea which leads to $s \neq \bar{s}$.

Validity of flavor symmetry and charge symmetry for parton distributions

Measurements of the Gottfried Sum Rule suggest significant violation of flavor symmetry in the proton sea. This has recently been confirmed in measurements of pp and pD Drell-Yan processes by the E866 experiment. We review the question of flavor symmetry violation, and possible theoretical explanations of this effect. The interpretation of large flavor symmetry violation rests on the assumption of parton charge symmetry. Our model calculations suggest charge symmetry violation [CSV] for parton valence distributions of a few percent. We review present experimental limits on CSV, and we suggest new experiments which might detect parton CSV.