a Measurement of the Three-Body Photodisintegration of HELIUM-3 and its Relation to Three-Body Forces.

The three-body photodisintegration of ^3He has been measured at energies below the pion production threshold using a continuous bremsstrahlung gamma-ray beam at the Saskatchewan Accelerator Laboratory. Coincident proton pairs emitted on opposite sides of the beam were detected using four sets of plastic scintillator telescopes arranged symmetrically about the beam at 81^ circ and 91^circ. Differential cross sections were extracted, as a function of the undetected neutron momentum, from kinematic regions where three-body forces effects are expected to be maximized and two-body mechanisms suppressed. These measured cross sections show an enhancement over calculations using one-body and two -body photo-absorption mechanisms only. The inclusion of the two-pion-exchange three-body absorption mechanism is needed to adequately describe the data. This constitutes direct evidence for the existence of three-body forces in ^3He, since the two-pion-exchange mechanism is directly related to three-body forces through gauge invariance. Cross sections differential only in the solid angles of the two detected protons were also measured as a function of incident gamma-ray energy. The angular and energy dependence of these cross sections were compared to those expected from 3N phase-space considerations alone. The measured angular dependence is indicative of a non-phase -space distribution, with an enhancement in the collinear geometry. The measured energy distribution followed the general trend of phase-space predictions, but show a departure from phase-space in the 150-200 MeV energy range.