Authors: Mark Sikora, Derek Glazier, Dan Watts
Title: Recoil Nucleon Polarimetry in Meson Photoproduction Reactions at MAMI
Keywords: recoil polarimetry nucleon excitation spectrum
Abstract:
Accurately establishing the spectrum and properties of the excited states of the nucleon has proven to be an elusive goal despite decades of study. Many states have poorly established properties, and many resonances predicted by theoretical models do not reveal themselves in the current experimental data.
Most modern attempts to address this situation utilise meson photoproduction from the nucleon. In the case of pseudoscalar meson production, the reaction can be described theoretically by 4 complex helicity amplitudes which include all the information on the process, including the underlying excitation spectrum. This leads to 16 experimental observables: the differential cross section, 3 single polarisation observables (beam, target, recoil) and 12 double polarisation observables corresponding to simultaneous measurements of beam-target, beam-recoil, and target-recoil polarisation. To fully constrain the helicity amplitudes, 8 suitably chosen observables must be measured, including double polarisation observables. This has led to major programmes of measurements exploiting both polarised photon beam and spin polarised nucleon targets at the major electromagnetic beam facilities such as ELSA, JLAB and MAMI. However, the goal of a complete measurement cannot be achieved without measurements including recoil nucleon polarisation. With this aim, the Edinburgh group has developed a novel design for a large acceptance recoil nucleon polarimeter for use in meson photoproduction reactions with the Glasgow/Mainz polarised photon beam and the Crystal Ball at MAMI. Preliminary measurements of double-polarisation observables in neutral pion photoproduction will be presented and compared with the various current partial wave analyses of the reaction. |