The description of the electromagnetic properties of baryons with explicit reference to the quark degrees of freedom is reviewed. After a short discussion of the available phenomenological information, such as magnetic moments, form factors and photoexcitation amplitudes, the constituent quark model in the potential approach is introduced and the consequent description of the baryon spectrum is briefly discussed. The model is then systematically used as a theoretical framework for the interpretation of the experimental baryon data, including photon-proton scattering and pion photoproduction, and for the discussion of recent studies of the electromagnetic properties of few-nucleon systems. In particular, the reformulation of meson exchange currents in terms of quark degrees of freedom is reported and the results of various quark approaches to the form factors are presented. Finally, some recent improvements and future developments are briefly discussed.

http://iopscience.iop.org/article/10.1088/0034-4885/54/3/003/pdf

Electromagnetic excitations in the constituent quark model

3He and 3H electromagnetic form factors

Microscopic nuclear structure with sub-nucleonic degrees of freedom

Enhancement in axial-charge matrix elements from meson-exchange currents

The Potential Like Electromagnetic Meson Exchange Currents in the {OBE} Approximation

The problem studied in the paper concerns the structure of the isovector e.m. MEC operators of the A1−, ϱ- and π ranges in the interval of intermediate energies. The two main dynamic principles which we invoke in our considerations are the current conservation and the gauge chiral invariance. Respecting them consistently allows us to describe correctly the interaction of NA1 ϱπ system with the external e.m. field. Our main results are as follows:

(i)

We verified that our Lagrangian approach is consistent with the prediction of the low energy theorem at threshold.




(ii)

We showed explicitly the continuity equation which the longitudinal parts of our currents obey.




(iii)

We proved the equivalence relation for the MEC operator of the pion range and demonstrated the existence of the seagull current in the MEC operator built up using PS πN coupling. This new term influences strongly the exchange charge density.

E. Truhlík

Papers

Isovector electromagnetic exchange currents in the chiral approach