Abstract
Recent experiments using the Crystal Ball/TAPS setup at the MAMI accelerator in Mainz, Germany continue to study the properties and the excitation spectrum of the nucleon with meson photoproduction. Electromagnetic excitations of the proton and neutron are essential for understanding their isospin decomposition. The electromagnetic coupling of photons to protons is different than that of neutrons in certain states. Cross-section data alone is not sufficient for separating resonances, whereas polarization observables play a crucial role being essential in disentangling the contributing resonant and non-resonant amplitudes. Preliminary results of the polarization observable E of double π production off an unpolarized (LD2) and polarized (dButanol) target are shown with comparison to predictions of recent analyses.
Highlights
Quark models describe the behavior of quarks in nucleons at medium energies
Most results still arise from experiments on the proton, which does not allow for as much information regarding the isospin structure of the electromagnetic transitions
Polarization observables can provide understanding of these overlapping resonances by discovering more information about the complex helicity amplitudes, which describe the interaction between photon beams and nucleons
Summary
Quark models describe the behavior of quarks in nucleons at medium energies. Meson photoproduction allows to investigate the excited states (resonances) of nucleons. Advances in experiments on the neutron can provide an integral piece to the understanding of the nucleon spectrum. Polarization observables can provide understanding of these overlapping resonances by discovering more information about the complex helicity amplitudes, which describe the interaction between photon beams and nucleons. These amplitudes are fully determined when a complete set of measurements is performed and give rise to the cross section, complemented by polarization observables including beam, target, and recoil asymmetries. The relationship between the cross section and polarization observable E can be written in terms of the helicity of beam and target as follows: Eversion. Where σ1/2 is the cross section of events with anti-parallel beam and target polarizations with total spin 1/2 and σ3/2 is the cross section of events with parallel beam with total spin of 3/2
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