Probing the Repulsive Core of the Nucleon-Nucleon Interaction via the He4(e,e′pN) Triple-Coincidence Reaction

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We studied simultaneously the $^{4}\mathrm{He}(e,{e}^{\ensuremath{'}}p)$, $^{4}\mathrm{He}(e,{e}^{\ensuremath{'}}pp)$, and $^{4}\mathrm{He}(e,{e}^{\ensuremath{'}}pn)$ reactions at ${Q}^{2}=2(\mathrm{GeV}/c{)}^{2}$ and ${x}_{B}>1$, for an $(e,{e}^{\ensuremath{'}}p)$ missing-momentum range of 400 to $830\text{ }\text{ }\mathrm{MeV}/c$. 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 $\ensuremath{\sim}500\text{ }\text{ }\mathrm{MeV}/c$. 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 $^{4}\mathrm{He}$ and discussed in the context of probing the elusive repulsive component of the $NN$ force.

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