Photoreactions on Few-Body Systems

The N-N tensor force comes about largely from one-pion exchange. Most observables sensitive to the tensor interaction (e.g. deuteron quadrupole moment, asymptotic D/S ratio) are essentially determined by the NN coupling constant, and are quite stable. In contrast, the n-p scattering phase , the traditional observable used to constrain the short range part of the tensor interaction and the form factor at the NN vertex, continues to be quite poorly determined. Recent measurements of polarization asymmetries in deuteron photodisintegration from LEGS are quite sensitive to the short range tensor force and have provided new constraints. [Bl91].

The cross section for two-body photodisintegration of the deuteron was measured at several angles for photon energies between 1.6 and 2.8 GeV at SLAC (experiment NE17). [Be95] As can be seen in Figure the data at appear to follow the constituent counting rules over this energy range, while the data at 37 do not. This suggests that region of validity of constitient counting may have a threshold effect with the transverse momentum in the process. Since the transverse momentum ) is maximized at , this threshold might be exceeded for the 90 data but not for the smaller angle data. A CEBAF experiment will extend the data to a photon energy of 4 GeV. This will allow further exploration of the possible threshold effect in and the validity of constituent counting at these energies.

The evidence for three-body forces in nuclei is indirect, at best. Multi- nucleon currents have been studied through the reaction, one of the simplest system in which 3-body correlations can occur and for which exact calculations can be performed. During the last three years, extensive measurements using large solid angle detector arrays have been carried out at SAL, LEGS [Te94][Ru94], Mainz and INS- Tokyo [Em94]. Different regions of phase space emphasize different reaction mechanisms. Final states in which two protons carry a significant fraction of the total energy are dominated by absorption. Polarization observables have provided an important window on the dynamics. In regions of phase space dominated by absorption, asymmetries are very similar to those of deuteron photodisintegration. In contrast, the polarization asymmetries vanish in the regions, since there can be no prefered direction when all three nucleons are involved. Theory has been moderately successful in accounting for the general features of the cross sections, but asymmetries have proved more challenging.[Te94]