Simon Capstick

Quark models of baryon masses and decays

Abstract 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 appling large Nc 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.

Baryon current matrix elements in a light-front framework

Current matrix elements and observables for electro- and photo-excitation of baryons from the nucleon are studied in a light-front framework. Relativistic effects are estimated by comparison to a nonrelativistic model, where the authors use simple basis states to represent the baryon wavefunctions. Sizeable relativistic effects are found for certain transitions, for example, to radial excitations such as that conventionally used to describe to the Roper resonance. A systematic study shows that the violation of rotational covariance of the baryon transition matrix elements stemming from the use of one-body currents is generally small.

Strange decays of nonstrange baryons

The strong decays of excited nonstrange baryons into the final states {Lambda}K, {Sigma}K, and for the first time into {Lambda}(1405)K, {Lambda}(1520)K, {Sigma}(1385)K, {Lambda}K{sup *}, and {Sigma}K{sup *}, are examined in a relativized quark pair creation model. The wave functions and parameters of the model are fixed by previous calculations of N{pi} and N{pi}{pi}, etc., decays. The results show that it should be possible to discover several new negative parity excited baryons and confirm the discovery of several others by analyzing these final states in kaon production experiments. They also establish clear predictions for the relative strengths of certain states to decay to {Lambda}(1405)K and {Lambda}(1520)K, which can be tested to determine if a three-quark model of the {Lambda}(1405)K is valid. The authors results compare favorably with the results of partial wave analyses of the limited existing data for the {Lambda}K and {Sigma}K channels. They do not find large {Sigma}K decay amplitudes for a substantial group of predicted and weakly established negative-parity states, in contrast to the only previous work to consider decays of these states into the strange final states {Lambda}K and {Sigma}K.

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

The {Theta}{sup +}(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}{sup ++}, {Theta}{sup +} and {Theta}{sup 0} have isospin-violating strong decays, and the {Theta}{sup +++} and {Theta}{sup -} have weak decays and so are long-lived. Production mechanisms for these states are discussed. The J{sup P} assignment of the {Theta} is most likely 1/2{sup -} or 3/2{sup -} or 5/2{sup -}.

The physical meaning of scattering matrix singularities in coupled-channel formalisms⋆

Abstract.The physical meaning of bare and dressed scattering matrix singularities has been investigated. Special attention has been attributed to the role of the well-known invariance of the scattering matrix with respect to the field transformation of the effective Lagrangian. Examples of evaluating bare and dressed quantities in various models are given.

Hybrid and conventional baryons in the flux-tube model

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.

Taking the “naive” and “non-relativistic” out of the quark potential model

Abstract Some recent results are highlighted and synthesized which place the quark potential model on a more solid foundation within the context of quantum chromodynamics.

Constructing hybrid baryons with flux tubes

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. (c) 1999 The American Physical Society.

New baryons in the {Delta}{eta} and {Delta}{omega} channels

The decays of excited nonstrange baryons into the final states {Delta}{eta} and {Delta}{omega} are examined in a relativized quark pair creation model. The wavefunctions and parameters of the model are fixed by previous calculations of N{pi} and N{pi}{pi}, etc., decays through various quasi-two body channels including N{eta} and N{omega}. The results show that the combination of thresholds just below the region of interest and the isospin selectivity of these channels should allow the discovery of several new baryons in such experiments.

Meson-loop contributions to the ρ-ω mass splitting and ρ charge radius

Contributions of two-pseudoscalar and vector-pseudoscalar meson loops to the rho-omega mass splitting are evaluated in a covariant model based on studies of the Schwinger-Dyson equations of QCD. The role and importance of the different time orderings of the meson loops is analyzed and compared with those obtained within time-ordered perturbation theory. It is shown that each meson loop contributes less than 10% of the bare mass, and decreases as the masses of the intermediate mesons increase beyond approximately m_rho/2. A mass splitting of m_omega - m_rho ~= 25 MeV is obtained from the pi pi, K K(bar), omega pi, rho pi, omega eta, rho eta and K* K channels. The model is then used to determine the effect of the two-pion loop on the rho-meson electromagnetic form factor. It is shown that the inclusion of pion loops increases the rho-meson charge radius by 10%.