Research with real photons at the MAMI 1.6 GeV electron accelerator

SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

^a Corresponding author: douglas.macgregor@glasgow.ac.uk

Published online: 6 May 2014

Abstract

The A2-CB Collaboration at Mainz is studying the structure of hadrons by meson photoproduction using unpolarised, linearly polarised and circularly polarised photons with energies up to 1.6 GeV. Photons are energy-tagged using the Glasgow-Mainz tagged photon spectrometer and a new high-energy end-point tagger which allows η’ reactions to be studied. Reaction products are detected in a ~4π detector consisting of the Crystal Ball detector and TAPS forward wall. Transverse or longitudinally polarised proton targets are available and new techniques have been developed to measure the polarisation of recoiling protons. These facilities have allowed an extensive programme of double-polarisation meson-photoproduction experiments to be carried out to search for so-called “missing baryon resonances” on proton and deuteron targets. Searches have also been carried out to investigate narrow resonances in the η-photoproduction channel at invariant masses around 1680 MeV. Coherent π⁰ production measurements have been used to estimate the neutron skin thickness in ²⁰⁸Pb. This paper presents selected highlights from the A2-CB collaboration research programme at MAMI.