He 3 (n,p) H 3 parity-conserving asymmetry

Recently, the nHe3 Collaboration reported a measurement of the parity-violating (PV) proton directional asymmetry APV=[1.55±0.97(stat)±0.24(sys)]×10-8 in the capture reaction of He3(n,p)H3 at meV incident neutron energies. The result increased the limited inventory of precisely measured and calculable PV observables in few-body systems required to further understand the structure of hadronic weak interaction. In this Letter, we report the experimental and theoretical investigation of a parity conserving (PC) asymmetry APC in the same reaction (the first ever measured PC observable at meV neutron energies). As a result of S- and P-wave mixing in the reaction, the APC is inversely proportional to the neutron wavelength λ. The experimental value is (λ×APC)β=[-1.97±0.28(stat)±0.12(sys)]×10-6Å. We present results for a theoretical analysis of this reaction by solving the four-body scattering problem within the hyperspherical harmonic method. We find that in the He3(n,p)H3 reaction, APC depends critically on the energy and width of the close 0- resonant state of He4, resulting in a large sensitivity to the spin-orbit components of the nucleon-nucleon force and even to the three-nucleon force. The analysis of the accurately measured APC and APV using the same few-body theoretical models gives essential information needed to interpret the PV asymmetry in the He3(n,p)H3 reaction.