Abstract
The A2 Collaboration at MAMI (Mainz) carried out new measurements of the helicity dependence of the total inclusive photo-absorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron in the photon energy region 200 He targets. Hadronic reaction products were detected by the large acceptance Crystal Ball-TAPS spectrometer, complemented by plastic scintillators and vertex detectors for charged particle tracking and identification.These new, high-quality doubly-polarised pion-photoproduction data sets provide a valuable input to the study of the nucleon structure and excitation spectrum by significantly constraining the electromagnetic multipole evaluation performed by the different available partial wave analysis models. Furthermore, the helicity dependent observables provide the main ingredient for the verification of the well-known Gerasimov-Drell-Hearn (GDH) sum rule, which relates the helicity-dependent photoasborption process to the main static nucleon properties (mass, charge, spin).
Highlights
Thanks to the use of polarised proton, deuteron and 3He targets, the A2 Collaboration can carry out a comprehensive experimental program in order to give a better insight into the GDH sum rule on the proton and on the neutron, and to perform an accurate investigation of the properties of baryonic resonances through experiments devoted to the study of double-polarisation observables
The results are compared to data from the CBELSA/TAPS collaboration ([8] for the G observable and [9] for E) and to different partial wave analyses: the agreement with the existing data and mostly with the different PWA models, especially with the BnGa201402 PWA solution, is shown for both the double polarisation observables
The A2 results are compared to a simple plane-wave impulse approximation (PWIA) model, where the cross sections are evaluated as an incoherent sum of quasi-free single nucleon contributions, determined using the MAID multipole analysis and the momentum distribution of the nucleons inside 3He as parametrised in [19]
Summary
Thanks to the use of polarised proton, deuteron and 3He targets, the A2 Collaboration can carry out a comprehensive experimental program in order to give a better insight into the GDH sum rule on the proton and on the neutron, and to perform an accurate investigation of the properties of baryonic resonances through experiments devoted to the study of double-polarisation observables
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
