Baryon Spectroscopy with Polarization Observables from CLAS

Abstract

The spectrum of nucleon excitations is dominated by broad and overlapping resonances. Polarization observables in photoproduction reactions are key in the study of these excitations. They give indispensable constraints to partial-wave analyses and help clarify the spectrum. A series of polarized photoproduction experiments have been performed at the Thomas Jefferson National Accelerator Facility with the CEBAF Large Acceptance Spectrometer (CLAS). These measurements include data with linearly and circularly polarized tagged-photon beams, longitudinally and transversely polarized proton and deuterium targets, and recoil polarizations through the observation of the weak decay of hyperons. An overview of these studies and recent results will be given.The nucleon is a color-neutral object which consists of color-charged quarks and gluons. Quantum chromodynamics (QCD) is the fundamental theory of the strong interaction between quarks and gluons. Valuable information about QCD can be learned from nuclear spectroscopy; e.g. information about the internal degrees of freedom in a nucleon. Experiments, especially pion-nucleon scattering, have confirmed low-lying excited states predicted by quark models with three independent quark degrees of freedom. These models, however, predict an overabundance of higher-lying excited states compared to what has been observed until now [1]. Also, recent lattice QCD calculations [2] find a large number of not-yet-discovered nucleon resonances. To clarify the nucleon resonance spectrum is an important task in the study of QCD.Nucleon resonances are short-lived, and the identification of resonances in partial-wave analyses of experimental data is complicated by their large width and overlap. Experimental cross sections are insufficient, and polarization observables are crucial to constrain these analyses. A complete set of certain polarization observables is necessary to unambiguously determine the amplitudes of the reaction. In the photoproduction of pseudoscalar mesons, a formally complete experiment requires at each energy and angle the measurement of at least eight carefully chosen observables [3]. In the photoproduction of two mesons, even more observables are needed [4]. It is also important to include in the analysis data from a variety of excitation and decay channels, as some of the missing states may couple only weakly to, e.g., the πN channel.