He3(n⃗,p)H3 parity-conserving asymmetry

Recently, the n 3 He Collaboration reported a measurement of the parity-violating (PV) proton directional asymmetry A PV = [1.55 +/- 0.97 (stat) +/- 0.24 (sys)] x 10-8 in the capture reaction of 3 He( n , p ) 3 H 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 A PC 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 A PC is inversely proportional to the neutron wavelength lambda . The experimental value is ( lambda x A PC ) equivalent to beta = [-1.97 +/- 0.28 (stat) +/- 0.12 (sys)] x 10-6 angstrom. 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 3 He( n , p ) 3 H reaction, A PC depends critically on the energy and width of the close 0- resonant state of 4 He, 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 A PC and A PV using the same few-body theoretical models gives essential information needed to interpret the PV asymmetry in the 3 He( n , p ) 3 H reaction.