The Study of Excited Baryon Resonances

Nucleons are complex systems of confined quarks and exhibit characteristic spectra of excited states. Highly excited nucleon states are sensitive to details of quark confinement which is poorly understood within Quantum Chromodynamics (QCD), the fundamental theory of strong interactions. Observing and understanding these higher-mass resonances is crucial, but they are difficult to observe since they are broad and overlapping. Very often, these higher-lying states reveal themselves more clearly through interference with dominant amplitudes. The interference terms can be isolated via polarization observables. At Jefferson Lab, extensive data sets on photo- as well as electro-production of pseudo-scalar mesons (π, 2π, η, K) and vector mesons (ρ, ω, φ) have been accumulated over the last few years using the CLAS spectrometer. The current efforts with CLAS focus on utilizing highly-polarized hydrogen and deuterium targets in combination with polarized photon beams toward a complete measurement of a large number of reaction channels. This contribution discusses recent results on single- and double-pion photoproduction. (author)