Electromagnetic Properties of Light and Heavy Baryons in the Relativistic Quark Model (original) (raw)
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Electromagnetic Properties of Baryons and the Quark Model
Progress of Theoretical Physics, 1966
I I 1243. The electromagnetic properties of baryons are investigated from the viewpoint of the Sakata Model, in which quarks are chosen as the three fundamental particles. Since vector mesons couple to the baryon indirectly through the constituent quarks, the electromagnetic form factor of the baryon has a further contribution coming from the baryon wave function as well as contributions from p, w and ifJ. This allows us to get rid of the difficulty encountered in the usual vector meson model in fitting the data with only the known p, w and r/J. It is also shown that in the "nonet limit" only p and w contribute to the nucleon form factors ; This explains the experimental relations and•
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The hypercentral Constituent Quark Model (hCQM) for the baryon structure is reviewed and its applications are systematically discussed. The model is based on a simple form of the quark potential, which contains a Coulomb-like interaction and a confinement, both expressed in terms of a collective space coordinate, the hyperradius. The model has only three free parameters, determined in order to describe the baryon spectrum. Once the parameters have been fixed, the model, in its non relativistic version, is used to predict various quantities of physical interest, namely the elastic nucleon form factors, the photocouplings and the helicity amplitudes for the electromagnetic excitation of the baryon resonances. In particular, the Q2Q^2Q2 dependence of the helicity amplitude is quite well reproduced, thanks to the Coulomb-like interaction. The model is reformulated in a relativistic version by means of the Point Form hamilton dynamics. While the inclusion of relativity does not alter the r...
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An elementary constituent-quark (CQ) model of mesons was previously presented. In this paper, we continue research into a study of the baryons in the constituent-quark model. Mac Gregor proposed a comprehensive model of elementary particles for which both mesons and baryons shared common mass-band structure in quantized units of m = 70 MeV, B= 140 MeV and X = 420 MeV. A review of the baryon data is under taken for comparison with the CQ model. It is shown in this paper that baryons possess an isospin I related to the mass quantum m = 70 MeV and to the B = 140 MeV quantum (or the mass of the pion). In order to establish a consistency with the quark model of Gell-Mann, we identify the SU(3) baryon decuplet as a standard feature to be maintained with only slight changes to the constituent-quark masses. By insisting on the J = 3/2, P-states of the SU(3) baryon decuplet to be in the same CQ excitation states, we are lead to establish baryon cores in the P-states with J = 1/2. Core corrections to Mac Gregor's CQ model of baryons are presented. Exact shell structure is found among all the baryons regardless of isospin as evidenced in the data from the Particle Data Group listing. New baryons are predicted to exist. The possible existence of magnetic charge in hadronic structure is suggested.
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These lectures review calculations of the electromagnetic properties of one-and twobaryon systems using the nonrelativistic quark model. We start with a short discussion of spontaneous chiral symmetry breaking, which is essential in understanding the transition from QCD to the constituent quark model. We then discuss the chiral version of the nonrelativistic quark model which simulates the symmetries and dynamical content of the underlying field theory in terms of gluon, pion, and sigma exchange between constituent quarks. In the second lecture, the electromagnetic properties of the one-baryon system are investigated in this framework. In particular, we focus on the effect of gluon and pion degrees of freedom in the electromagnetic current operators. These must be included in order for the electromagnetic current to be conserved. We also study the effect of scalar exchange currents connected with the confinement and sigma exchange potentials. By including these two-body exchange currents we go beyond the single-quark impulse approximation which has mainly been used up to now. The consistent treatment of gluon-, pion-, and scalar-exchange currents in the quark potential model is the new point of the present work. We show that exchange currents have a large effect on various electromagnetic properties, such as magnetic moments, charge and magnetic radii of the nucleon and A(1232). We point out that one cannot extract quantitative information about the quark-quark potential from electromagnetic observables without taking two-body exchange currents into account. In the third lecture, we use the quark potential model to study the electromagnetic properties of the deuteron. We emphasize that a quark-model description leads to short-range modifications of the conventional single-nucleon and two-nucleon currents, due to the quark-exchange currents. These appear as a consequence of the Pauli principle at the quark level.
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