Delta isobar masses, large N-c relations, and the quark model

Motivated by recent remarks on the Delta(+) mass and comparisons between the quark model and relations based on large-iri, with perturbative flavor breaking, two sets of Delta masses consistent with these constraints are constructed. These two sets, based either on an experimentally determined mass splitting or a quark model of isospin symmetry breaking, are shown to be inconsistent. The model dependence of this inconsistency is examined, and suggestions for improved experiments are made. An explicit quark model calculation and mass relations based on the large-N-c limit with perturbative flavor breaking are compared. The expected level of accuracy of such relations is realized in the quark model, except for mass relations derived by combining others which correspond to different SU(6) representations. It is shown that the Delta(0) and Delta(++) pole masses and Delta(0) - Delta(+) = (Delta(-) - Delta(++))/3 similar or equal to 1.5 MeV are more consistent with model expectations than the analogous Breit-Wigner masses and their splittings. [S0556-2821(99)02301-2].