Quark Models of Baryon Masses and Decays

The description of baryon resonance masses and decays in the constituent quark model is outlined, with emphasis on the potential-model approach combined with the 3P0 pair-creation model of strong decays. This approach allows the estimation of branching fractions for baryon states missing in pion nucleon elastic scattering analyses but expected to be present in electromagnetic production. Prospects for the discovery of hybrid baryons are discussed. A subjective list of the most important corrections required to this approach is presented.Comment: 10 pages, 5 figures. Invited contribution to the COSY Workshop on Baryon Excitations, Juelich, 2-3 May 200

Quark Models of Baryon Masses and Decays

The application of quark models to the spectra and strong and electromagnetic couplings of baryons is reviewed. This review focuses on calculations which attempt a global description of the masses and decay properties of baryons, although recent developments in applying large N_c QCD and lattice QCD to the baryon spectrum are described. After outlining the conventional one-gluon-exchange picture, models which consider extensions to this approach are contrasted with dynamical quark models based on Goldstone-boson exchange and an algebraic collective-excitation approach. The spectra and electromagnetic and strong couplings that result from these models are compared with the quantities extracted from the data and each other, and the impact of various model assumptions on these properties is emphasized. Prospects for the resolution of the important issues raised by these comparisons are discussed.Comment: 91 page review article; 25 figures, 9 tables; submitted to Progress in Particle and Nuclear Physic

Interpretation of the Theta+ as an isotensor pentaquark with weakly decaying partners

The Theta+(1540), recently observed at LEPS, DIANA and CLAS, is hypothesized to be an isotensor resonance. This implies the existence of a multiplet where the Theta++, Theta+ and Theta0 have isospin-violating strong decays, and the Theta+++ and Theta- have weak decays and so are long-lived. Production mechanisms for these states are discussed. The J^P assignment of the Theta is most likely 1/2^- or 3/2^-.Comment: references added, section on pentaquark model revised, 11 pages, LaTeX, accepted for publication in Physics Letters

Semileptonic Decays of Heavy Omega Baryons in a Quark Model

The semileptonic decays of $\Omega_c$ and $\Omega_b$ are treated in the framework of a constituent quark model developed in a previous paper on the semileptonic decays of heavy $\Lambda$ baryons. Analytic results for the form factors for the decays to ground states and a number of excited states are evaluated. For $\Omega_b$ to $\Omega_c$ the form factors obtained are shown to satisfy the relations predicted at leading order in the heavy-quark effective theory at the non-recoil point. A modified fit of nonrelativistic and semirelativistic Hamiltonians generates configuration-mixed baryon wave functions from the known masses and the measured \lcle rate, with wave functions expanded in both harmonic oscillator and Sturmian bases. Decay rates of \ob to pairs of ground and excited \omc states related by heavy-quark symmetry calculated using these configuration-mixed wave functions are in the ratios expected from heavy-quark effective theory, to a good approximation. Our predictions for the semileptonic elastic branching fraction of $\Omega_Q$ vary minimally within the models we use. We obtain an average value of (84$\pm$ 2%) for the fraction of $\Omega_c\to \Xi^{(*)}$ decays to ground states, and 91% for the fraction of $\Omega_c\to \Omega^{(*)}$ decays to the ground state $\Omega$. The elastic fraction of \ob \to \omc ranges from about 50% calculated with the two harmonic-oscillator models, to about 67% calculated with the two Sturmian models.Comment: 52 pages, 8 figure

Kutovi miješanja u barionima S11 i D13

The nature of the short-range interactions between quarks in baryons is of fundamental importance to our understanding of the nature of nuclear matter in the soft-QCD regime. Models of the structure of the negative-parity excited baryons in the S11 and D13 partial waves in Nπ scattering differ in their predictions for the amount by which states of different quark spin but the same total spin are mixed. It may be possible to distinguish among these models by an accurate determination of the mixing angles in these states. A study of the dependence on these mixing angles of their decay branching fractions to Nπ, Nη, and ΛK, using a constituent quark model and the 3P0 decay model, allows conclusions to be drawn about the values of the mixing angles.Priroda kratkodosežnih međudjelovanja među kvarkovima u barionima ima osnovnu važnost za razumijevanje značajki nuklearne tvari u uvjetima mekog QCD. Modeli strukture uzbuđenih bariona negativne parnosti s parcijalnim valovima S11 i D13 u raspršenju Nπ razlikuju se prema svojim predviđanjima ovisno o miješaju stanja s istim ukupnim spinom ali različitih kvarkovskih spinova. Možda će se ti modeli moći razlikovati točnim određivanjem kutova miješanja u tim stanjima. Proučavanje ovisnosti kutova miješanja preko omjera grananja u raspadima Nπ, Nη i ΛK, primjenom modela sastavnih kvarkova i modela raspada 3P0, omogućuje izvođenje zaključaka o iznosima kutova miješanja