Gabriel Karl

Hyperfine Interactions in Negative Parity Baryons

The hyperfine interaction between quarks suggested in chromodynamics can, without free parameters, explain the size and pattern of the splittings and the mixing angles observed experimentally in nonstrange, negative parity baryons.

SINGLE PION PHOTOPRODUCTION IN THE QUARK MODEL.

Abstract We present the results of a detailed quark model calculation of the resonance contributions to the total, and backward differential, cross sections for single-pion photoproduction off nucleons. All prominent nucleonic resonances below 2 GeV are considered and assigned to the usual quark configurations with L P =0 + , 1 − , 2 + . The model is found to be in reasonable agreement with experiment and to predict that the main features of photoproduction off neutrons should be qualitatively different from those off protons. An experimental test is proposed for the presence of the ‘spin-orbit’ radiative coupling in the quark electromagnetic interaction. Finally, the same formalism is used to discuss the radiative decays of the strange Y o ∗ (1520) and Y o ∗ (1405) resonances.

Symmetry breaking in baryons

Abstract Assuming universal confining forces between quarks we discuss a new symmetry breaking mechanism specific to baryons: a pair of orbital states which are degenerate when all three quarks have the same mass splits up when the quarks have different masses. This effect is operative in all low mass negative parity hyperons. The observed inversion of Λ5 2 − (1830) and Σ5 2 − (1765) relative to the ground states Λ1 2 + (1115) and Σ1 2 + (1190) is one consequence of this mechanism. The zero-order strange eigenstates of the confining hamiltonian (without hyperfine interactions) are similar to the states in the approximate mixing schemes of Petersen and Rosner and of Faiman.

Hadron spectroscopy and quarks

Our understanding of the ultimate structure of matter has advanced greatly in the last few years. It was nearly twenty years ago that Murray Gell‐Mann and George Zweig gave us the key to much of this understanding with their revolutionary proposals that protons, neutrons and all other strongly interacting particles—the hadrons—are made of quarks, a theretofore unobserved kind of particle. Over the last ten years, this proposal has become firmly established even though we have not observed free quarks directly. More recent research has found that forces between quarks are extremely simple and universal—the same for all types of quarks—and that these forces explain why free quarks cannot be seen. In this article we will look at hadron spectroscopy, which has been one of the main venues for this great progress.

Quantization of chiral solitons for three flavors and the large-N limit

Abstract The quantization of a skyrmion in a theory with three flavors is discussed. The conventional choice of octets does not maintain correspondence with the large-N limit of quark model. When the skyrmion is quantized so as to maintain this correspondence in the spectrum of states, magnetic moments computed in the skyrmion model and the large-N quark model also agree (in the limit of flavor SU3 symmetry). When flavor SU3 is broken sum rules are found in the quark model valid for any number of colors which connect magnetic moments of spin 1 2 baryons.

Quark spin coupling in baryons reexamined

A direct connection can be made between mixing angles in negative parity baryons and the spin coupling of constituent quarks. The mixing angles do not depend on spectral data. These angles are recalculated for gluon exchange and pion exchange between quarks. For pion exchange the results of Glozman and Riska are corrected. The experimental data on mixing are very similar to those derived from gluon exchange but substantially different from the values obtained for pion exchange.

3S1−3D1 mixing and E1 radiative transitions in charmonium

AbstractThe radiative transitions ψ′→γ+χJ and χJ→γ+ψ are studied in the quark model with the ψ and ψ′ interpreted conventionally as3S1

EARLY HISTORY OF QCD AND QUARKS

c\bar c