Probing the Repulsive Core of the Nucleon-Nucleon Interaction via the He-4(e,e'pN) Triple-Coincidence Reaction

We studied simultaneously the He-4(e,e'p), He-4(e,e'pp), and He-4(e,e'pn) reactions at Q(2) 2(GeV/c)(2) and x(B) > 1, for an (e,e'p) missing-momentum range of 400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual A = 2 system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum, in a region where the nucleon-nucleon (NN) force is expected to change from predominantly tensor to repulsive. The abundance of neutron- proton pairs is reduced as the nucleon momentum increases beyond similar to 500 MeV/c. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum. Our data are compared with calculations of two-nucleon momentum distributions in He-4 and discussed in the context of probing the elusive repulsive component of the NN force.