Flavor-singlet hybrid baryons may already have been discovered

Abstract

The splittings between the spin 1/2 and spin 3/2 iso-singlet baryons Lambda_s(1405) and Lambda_s(1520), and their charmed counterparts Lambda_c(2593) and Lambda_c(2625), have been a theoretical conundrum. Here we investigate the possibility that the QCD binding of color octets comprised of three quarks in a flavor singlet configuration is stronger than previously envisaged, allowing these states to be interpreted as hybrids consisting of three quarks plus a valence gluon (udsg) and (udcg). A fit of their mass separation allows the mass prediction of the strange and charmed flavor octet and decuplet hybrid baryons and the prediction of the mass separation of the beauty hybrids. Such hybrid states come in parity-doubled pairs with the even parity state lighter by about 300 MeV. Existing data accommodates either parity assignment for the observed states and the existence of the required unobserved partners at either higher or lower mass. We discuss difficulties with and strategies for observing the other states under the two cases. A corollary of the strong-binding-in-flavor-singlet-channel hypothesis is that the H-dibaryon may be very long lived or stable with m_H ~< 2 GeV.