Abstract
The double-polarization observable E and the helicity-dependent cross sections σ1/2 and σ3/2 have been measured for the first time for single π0 photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.
Highlights
The excitation spectrum of a composite system reflects the properties of the underlying interaction
A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects
The best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons
Summary
The excitation spectrum of a composite system reflects the properties of the underlying interaction. A study of the properties of nucleon resonances is as important for the understanding of the strong interaction as the interpretation of atomic level schemes was in the development of Quantum Electrodynamics (QED). One difference between QED and quantum chromodynamics (QCD) is that, in the low-energy range of excited nucleon states, QCD cannot be solved in a perturbative way. The interpretation of experimental data commonly relied on phenomenological constituent quark models. Much progress has been made with the numerical methods of lattice gauge calculations. While most published results have been for predictions of ground-state properties, the first unquenched lattice simulations of excited states have recently been reported [1]
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