The status of conventional baryon flux-tubes and hybrid baryons is reviewed. Recent surprises are that a model prediction indicates that hybrid baryons are very weakly produced in glue-rich Psi decays, and an analysis of electro-production data concludes that the Roper resonance is not a hybrid baryon. The baryon decay flux-tube overlap has been calculated in the flux-tube model, and is discussed here. The behavior of the overlap follows naive expectations.
We use the method of QCD sum rules to investigate a possible hybrid baryon with the quantum numbers of the $\Lambda$. Using a current composed of uds quarks in a color octet and a gluon, a strange hybrid, the $\Lambda_H$ is found about 500 MeV above the $\Lambda$, and we identify it as the $\Lambda(1600)$. Using our sigma/glueball model we predict a large branching fraction for the $\Lambda_H \to \Lambda + \sigma(\pi\pi$ resonance), and the experimental search for this decay mode could provide a test of the hybrid nature of the $/Lambda(1600)$.
The gamma n -> K+ K- n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54 +- 0.01 GeV with a width smaller than 25 MeV and a Gaussian significance of 4.6 sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic 5-quark state (uudd{s_bar}) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an anti-decuplet of baryons predicted by the chiral soliton model.
We present the first study of the gluonic excitation in the three-quark (3Q) system in SU(3) lattice QCD with $\beta$=5.8 and $16^3 \times 32 $ at the quenched level. For the spatially-fixed 3Q system, we measure the gluonic excited-state potential, which corresponds to the flux-tube vibrational energy in the flux-tube picture. The lowest gluonic-excitation energy in the 3Q system is found to be about 1GeV in the hadronic scale. This large gluonic-excitation energy is expected to bring about the success of the simple quark model without gluonic modes.
Conventional and hybrid light quark baryons are constructed in the non-relativistic flux-tube model of Isgur and Paton, which is motivated by lattice QCD. The motion of the flux tube with the three quark positions fixed, except for center of mass corrections, is discussed. It is shown that the problem can be reduced to the independent motion of the junction and the strings connecting the junction to the quarks. The important role played by quark-exchange symmetry in constraining the flavor structure of (hybrid) baryons is emphasized. The flavor, quark spin S, total spin J, and parity P of the seven low-lying hybrid baryons are found to be N2(1/2)+, N2(3/2)+, Delta4(1/2)+, Delta4(3/2)+, and Delta4(5/2)+, where the notation is flavor[2S+1](J)P, and the N2(1/2)+ and N2(3/2)+ states are doublets. The motion of the three quarks in an adiabatic potential derived from the flux-tube dynamics is considered. A mass of 1870 +/- 100 MeV for the lightest nucleon hybrids is found by employing a variational method.
We review the status of hybrid baryons. The only known way to study hybrids rigorously is via excited adiabatic potentials. Hybrids can be modelled by both the bag and flux-tube models. The low-lying hybrid baryon is N 1/2^+ with a mass of 1.5-1.8 GeV. Hybrid baryons can be produced in the glue-rich processes of diffractive gamma N and pi N production, Psi decays and p pbar annihilation.
The relativistic five-quark equations are found in the framework of the dispersion relation technique. The Behavior of the low-energy five-particle amplitude is determined by its leading singularities in the pair invariant masses. The solutions of these equations using the method based on the extraction leading singularities of the amplitudes are obtained. The mass spectra of nucleon and delta-isobar hybrid baryons are calculated. The calculations of hybrid baryon amplitudes estimate the contributions of four subamplitudes. The main contributions to the hybrid baryon amplitude are determined by the subamplitudes, which include the excited gluon states.
We study the possibility that four iso-singlet baryons $\Lambda_s(1405)$ $J^P=1/2^{-}$, $\Lambda_s(1520)$ $J^P=3/2^{-}$, $\Lambda_c(2593)$ $J^P=1/2^{-}$ and $\Lambda_c(2625)$ $J^P=3/2^{-}$ are hybrids: three quark one gluon states (udsg). We calculate the mass separations of the candidates, using a degeneracy-lifting hyperfine interaction from an effective single colored gluon exchange between the constituents. The correct ordering of masses is obtained (contrary to the case for the conventional interpretation as 3 quarks with L=1) and the splittings are plausible. The parity of these states is not measured, only assumed to be negative. In the hybrid picture, the lightest states are parity even and the parity odd counterparts lie about 300 MeV higher. Thus the hybrid ansatz predicts that either the parity of the $\Lambda(1405)$ etc is positive, or that there are undiscovered positive parity states about 300 MeV lower. We also remark that in this picture, the H-dibaryon mass may be around 1.5 GeV.
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.
Invited contribution to ``Nuclear and Particle Physics with CEBAF at Jefferson Lab'', 3-10 November 1998, Dubrovnik, Croatia. To be published in Fizika (1999). Work done in collaboration with Simon Capstick
Hybrid baryon states are described in quark potential models as having explicit excitation of the gluon degrees of freedom. Such states are described in a model motivated by the strong coupling limit of Hamiltonian lattice gauge theory, where three flux tubes meeting at a junction play the role of the glue. The adiabatic approximation for the quark motion is used, and the flux tubes and junction are modeled by beads which are attracted to each other and the quarks by a linear potential, and vibrate in various string modes. Quantum numbers and estimates of the energies of the lightest hybrid baryons are provided.
We predict an exotic Z^+ baryon (having spin 1/2, isospin 0 and strangeness +1) with a relatively low mass of about 1530 MeV and total width of less than 15 MeV. It seems that this region of masses has avoided thorough searches in the past.
Hybrid baryons in the bag model
Hybrid baryons in QCD sum rules
Hybrid baryons in large N_c