# What does the A2 Collaboration do?

## Meson Physics

The A2 Collaboration has endeavored to improve the understanding of the structure of mesons and nucleons by studying the photoproduction of the former off of the latter. These studies were performed in the A2 hall at the Mainz Microtron (MAMI), where tagged photons, unpolarized or either linearly or circularly polarized, up to energies of 1.6 GeV impinge upon proton or deuteron targets, also either unpolarized or polarized, or a range of other targets. Beyond looking at just the cross section, various polarization observables are accessible with the inclusion of these polarized initial states. The resulting photoproduced mesons are detected in the nearly 4$$\pi$$ steradian system composed of the Crystal Ball and TAPS detectors. The combination of the Bremsstrahlung distribution of photon energies from the selectable initial electron beam energy has provided the ability to investigate the excitation spectrum of the nucleon down to the thresholds of the $$\eta$$ and $$\pi$$ mesons, among others.

## Compton Scattering

What happens when a proton is subjected to an electric field? The simple answer from first-year physics is that it would move. The less simple answer is that it would also deform due to the internal structure of the proton. Such is also the case in a magnetic field, and similarly so for the neutron, which obviously would have otherwise remained at rest. These responses to electric and magnetic fields are described by parameters of the nucleon called polarizabilities. The A2 collaboration at the Mainz Microtron has undertaken a multi-experiment project to extract these polarizabilities. This is done by taking polarized photons, provided from tagged Bremsstrahlung, and Compton scattering them off of either polarized or unpolarized protons. The measurements of various observables from these experiments are then compared with theoretical values from dispersion and chiral perturbation calculations. Values for the proton polarizabilities, some of which had not been experimentally determined previously, have been extracted, and plans to improve these values and continue the program on the neutron are underway.