Radial Excited States of the Nucleon in Quark Models with Dynamical Confinement

The P11(1440) (Roper) resonance 1 is the lowest positive-parity N state, most clearly visible in partial-wave decompositions of πN → πN and πN → ππN scattering, as well as in pion photoproduction. Although this four-star resonance is within the energy reach of many modern research facilities, the experimental analyses so far have not ventured beyond the determination of its mass, width, and electromagnetic couplings . The only experimental information on the helicity amplitudes for Roper excitation with virtual photons comes from a very scarce data set obtained in older electroproduction experiments at DESY and NINA . The electroexcitation of the Roper is also receiving considerable theoretical interest. In the spherically-symmetric SU(6) quark model, the Roper can be understood as a radial excitation (“breathing mode”) of the proton, with one quark occupying the 2s state, yielding a (1s)(2s) configuration. This physical picture implies a sizable Coulomb monopole contribution. Newer developments 5,6 have indicated a possible description of the Roper as a gluonic partner of the proton, represented as a (qg) hybrid baryon. In this approach, the monopole strength is expected to be highly suppressed, in contrast to the concept of “breathing”. These two opposing concepts result in very different predictions for the Q-dependence of the transverse (A1/2) and scalar (S1/2) electroproduction helicity amplitudes, and they both fail to reproduce the electromagnetic couplings in the real-photon limit. The Roper has been investigated as a hybrid baryon by using QCD sum rules , and in terms