A Study of Pion Photoproduction on Helium-4 in the Delta Resonance Region using Polarised and Unpolarised Photons

Abstract

Measurements of the reaction on a 4 He target have been carried out using both unpolarised and polarised photons. This work is the latest in a series of experiments conducted by the Edinburgh University Nuclear Physics group in collaboration with the Universities of Glasgow and Tubingen. The experiment was carried out using the 855 MeV MAMI­B electron accelerator at the Institut fur Kernphysik, Mainz, between March and August of 1996. The 855 MeV electrons from the accelerator were directed either, on to a 4 micron Nickel foil to produce unpolarised photons or on to a 0.1 mm Diamond to produce polarised photons. These photons were then tagged with a resolution of 2 MeV using a spectrometer. The photons then impinged on a 4 He cryotarget. Two sets of detectors were used to detect the reaction products. PiP, a plastic scintillator hodoscope, was used to detect the positive pions; and TOF a time ­of­ flight array, was used to detect the neutrons. Data was analysed for unpolarised photon energy regions centred at E =260,300,340 and 380 MeV and two polarised regions centred at E =258 and 338 MeV. The unpolarised data is presented as double and triple differential cross sec­ tions while the polarised data is presented as photon asymmetries of the corres­ ponding double differential cross section. A comparison is made with Plane Wave Impulse Approximation (PWIA) calculations by Louis Wright, both with and without final state interaction corrections. It is concluded that for the unpolar­ ised data a more sophisticated treatment of the final state interactions is required while for the polarised data reasonable agreement is found between experiment and theory.