Hybrid Baryons, a Brief Review

This contribution is a brief review of the status of hybrid baryons, which are hypothetical baryons that incorporate a gluonic excitation. We first summarize the status of hybrid mesons, since this closely related topic has seen considerable recent activity with the identification of two exotic candidates. Next we review theoretical expectations for the masses and quantum numbers of hybrid baryons, which have come from studies of the bag model, QCD sum rules and the flux tube model. Finally hybrid baryon experiment is discussed, including suggestions for experimenters at COSY. 1 Hybrid Mesons Hybrids are hadrons in which the dominant component of the state consists of quarks (antiquarks) and excited glue. This deliberately vague definition is necessary at present because the subject has been studied mainly through various models, and each assumes a particular description of excited glue. Fortunately the models often reach similar conclusions regarding the quantum numbers and approximate masses of these states, so from the experimental viewpoint there are relatively clear predictions regarding how one might find states with excited glue in the spectrum. Hybrid mesons are usually modelled as q ¯ q+excited glue, and this system can have all J P C quantum numbers. This implies that one can most usefully search for hybrids as mesons with the so-called " exotic quantum numbers " J P C = 0 −− , 0 +− , 1 −+ , 2 +− , 3 −+. .. because these are strictly forbidden to the nonrelativistic quark model's q ¯ q states. Thus if one discovers a J P C exotic meson, it is certain that something other than q ¯ q has been found. Whether this is a hybrid or not may be a more difficult question to answer, and depends on a comparison with theoretical predictions for the masses, quantum numbers