Abstract
Photon-induced reactions, like meson photoproduction, allow to excite the nucleon, to have access to many different polarisation observables and are an essential tool to disentangle the role of the different electromagnetic multipoles due to the change of sign of some contributions and the presence of interference terms between different multipole amplitudes. In addition, the use of polarised beams and/or targets allow to access additional observables which are fundamental in order to accurately determine the nucleon resonance properties. The A2@MAMI collaboration is carrying out a broad and systematic study on this topics, both on the proton and the neutron. The experiments are performed at the tagged photon beam facility of the MAMI accelerator in Mainz, using circularly and linearly polarised photons on longitudinally polarised proton and deuteron targets, for energies ranging from the pion production threshold up to 1.6 GeV. Hadronic reaction products are then measured with the large acceptance Crystal Ball spectrometer, complemented by charged particle and vertex detectors for tracking and identification. An overview of the results obtained so far for the double polarisation observable E (circularly polarised photon beam on a longitudinally polarised target) on the single π0 photoproduction off the proton and the neutron will be given. Furthermore, new results on the helicity-dependent total and differential cross sections on the deuteron will be presented.
Highlights
The internal structure of the nucleon, its spin structure, has been a central issue for many theoretical models and experiments of nuclear and particle physics since many years
The study of the excited states of the nucleon is of particular interest because they are related to fundamental properties of the strong interaction, just like the excitation spectra of atoms reflect the properties of the electromagnetic interaction
An experimental alternative aimed to a better understanding of the nucleon excitation spectra is represented by photon-induced reactions, like meson photoproduction:
Summary
The internal structure of the nucleon, its spin structure, has been a central issue for many theoretical models and experiments of nuclear and particle physics since many years. The study of the excited states of the nucleon is of particular interest because they are related to fundamental properties of the strong interaction, just like the excitation spectra of atoms reflect the properties of the electromagnetic interaction. In this energy regime, Quantum Chromodynamics (QCD) is non-perturbative; only phenomenological quark models can provide reliable interpretations of the experimental results. Quantum Chromodynamics (QCD) is non-perturbative; only phenomenological quark models can provide reliable interpretations of the experimental results While these models are quite in agreement with the experimental findings in the low mass region of the spectrum, there are still a lot of higher lying states predicted by theories but not yet observed. An experimental alternative aimed to a better understanding of the nucleon excitation spectra is represented by photon-induced reactions, like meson photoproduction:
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