Speaker
Description
The A2 Collaboration in the Crystal Ball/TAPS experiment at the MAMI accelerator
facilty in Mainz, Germany has a diversi?ed research program using real photons. The
Crystal Ball/TAPS setup has the ability to provide almost full coverage in solid angle and
is well suited to detect multi particle ?nal states. The experiments use high intensity circu-
larly, linearly, or unpolarized photon beams and unpolarized or polarized targets. To fully
understand the strong interaction in the non-pertubative region, the excitation spectrum
of nucleons is an important tool to exploit. Comparing experimentally observed excited
nucleon states to model predictions or lattice QCD calculations, large discrepancies arise,
speci?cally concering the number of states. The electromagnetic coupling of photons to
protons is di?erent than that of neutrons in certain states. A complete partial wave anal-
ysis (PWA) can assist in yielding more information about any reaction with polarization
observables playing a crucial role, as well as measurements of cross-sections. Spin observ-
ables are essential in disentangling the contributing resonant and non-resonant amplitudes,
whereas cross-section data alone is not su?cient for separating resonances. Recent results,
the current status, and future plans of the A2 Collaboration will be discussed.
