The 3He(n⃗,p)3H parity-conserving asymmetry

Recently, the n^3He collaboration reported a measurement of the parity-violating (PV) proton directional asymmetry A_PV = (1.55± 0.97 (st at)± 0.24 (sys))× 10^-8 in the capture reaction of ^3He(n⃗,p)^3H 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 λ. The experimental value is (λ× A_PC)≡β= (-1.97 ± 0.28 (stat)± 0.12 (sys)) × 10^-6 Amstrongs. 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 ^3He(n⃗,p)^3H reaction, A_PC depends critically on the energy and width of the close 0^- resonant state of ^4He, 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 ^3He(n⃗, p)^3H reaction.