Electromagnetic excitations of baryon resonances

Abstract

I will discuss recent results on the electromagnetic excitation of non-strange baryon resonances obtained with the CEBAF Large Acceptance Spectrometer (CLAS). Preliminary data on the N Δ transition multipoles at small and at large photon virtualities are presented, showing the importance of the nucleon pion cloud. In the second nucleon resonance region, new results on the transition amplitudes for the Roper P 11 ( 1440 ) , the S 11 ( 1535 ) , and the D 13 ( 1520 ) states are presented. The transverse Roper photocoupling amplitude shows a strong Q 2 dependence that is best described in terms of large meson cloud contributions. The helicity amplitudes for the S 11 ( 1535 ) are extracted from both single pion production as well as eta production, giving consistent results. In contrast to the Roper resonance, a nearly Q 2 -independent behavior is observed at small Q 2 values supporting the interpretation of this state as a nearly pure three-quark resonance. The D 13 ( 1520 ) amplitudes show the rapid change of the helicity structure predicted by non-relativistic quark models. Searches for new baryon states in two-pion photoproduction and electroproduction show consistently a need for a new state near 1.72 GeV with J P = 3 2 + . Finally, the search for the Θ + ( 1540 ) and other pentaquark baryon states with CLAS is presented.