We measured the differential cross section and the spin correlation parameter Cyy of the p→+3He→ elastic backward scattering at 200, 300, and 400 MeV at θ=180° in the center-of-mass frame to study the mechanism of the reaction and to examine the validity of the He3 wave functions based on two different realistic two-body forces. This is the first measurement of the spin correlation parameter Cyy of the p→+3He→ EBS at intermediate energies. The experimental results were compared with few-body calculations, including three reaction mechanisms: two-nucleon-pair exchange, pion exchange, and direct pp scattering. It was found that few-body calculations describe the differential cross-section data reasonably well. The spin correlation parameter Cyy shows clear evidence for the two-nucleon-pair exchange processes in the reaction, demonstrating that the spin observables are helpful for deeper understanding of the reaction mechanism.
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