The physical program of the A1 collaboration with its 50 members from 20 countries is focused on hadron structure. The spectrometer setup is well suited to explore fundamental properties of the nucleon, for example, charge distributions by elastic form factor measurements and generalized polarizabilities by virtual Compton scatterig. Meson production gives access to the resonance structure of the nucleon and meson properties.
The largest experimental hall of the MAMI accelerator complex houses three high-resolution, focussing magnetic spectrometers operated by the A1 Collaboration. The high momentum resolution (Δ p/p < 10-4) together with the large acceptance in solid angle (up to 28 msr) and in momentum (up to 25%) makes this setup ideal for electron scattering in coincidence with hadron detection. One of the spectrometers can be tilted up to an out-of-plane angle of 10°, allowing for out-of-plane kinematics. A proton recoil polarimeter gives, in combination with the polarized MAMI beam and a polarized helium-3 gas target, access to a broad variety of spin observables. A fourth spectrometer (KAOS/A1, covering high momenta with a moderate path length for the detection of kaons, is currently in the commissioning phase.
The main physics goals are:
- Form factors in elastic electron scattering belong to the most fundamental observables characteristic for nuclear and sub-nuclear systems. They are directly related to transverse spatial densities of charge and magnetization. At MAMI elastic electron-nucleon scattering is studied at small momentum transfer Q² < 2 GeV²/c² with a very high precision.
- In radiative inelastic electron scattering, where an additional low energy photon is emitted (virtual Compton-scattering), the response of nucleons to quasi-static electromagnetic fields can be studied. This response is described in terms of polarizabilities and their spatial distribution.
- Inelastic electron scattering in coincidence with mesons (pions, etas, kaons) provides information about the excitation spectrum of protons and neutrons. Form factors for the transition of a nucleon to specific excited states can be studied with high precision.
- The structure and the wave functions of nuclei and hypernuclei, where one proton or neutron has been replaced by a heavier lambda or sigma baryon, are studied in electron scattering off nuclei in coincidence with knocked-out nucleons or produced mesons.