Abstract
My lectures here will expound on the simple quark model of baryons to understand the variety of baryon resonances that are encountered experimentally. We would mainly concentrate on the excited states of the nucleon (N) and the delta (Δ), that are “seen” in πN scattering, and in the pion photoproduction and electroproduction experiments. Resonances are associated with peaks in cross sections, and are manifest, for example, in a plot of the total πN cross section, σT as a function of the pion laboratory momentum or the CM energy squared, s. In Fig. 1, σT(π+p), which is entirely in the isospin I = 3/2 channel, is plotted. It has a huge bump corresponding to the Δ(1232), and some minor peaks where many other overlapping Δ- resonances contribute. In Fig. 2, σT(π-р) is plotted, and here both the I = 1/2 and I = 3/2 channels contribute in the ratio 2:1. This plot has more structure, and some of the N-resonances can be identified. It is clear from these plots, however, that other experiments and more careful analysis of the data are necessary to isolate the characteristics of these excited states.
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Bhaduri, R.K. (1986). Spectroscopy of N-Δ Excited States and the Quark-Model. In: Tomusiak, E.L., Caplan, H.S., Dressler, E.T. (eds) New Vistas in Electro-Nuclear Physics. NATO ASI Series, vol 142. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-5200-6_4
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