The 3he(n⃗,p)3h Parity-Conserving Asymmetry

Recently, the n^3He collaboration reported a measurement of theparity-violating (PV) proton directional asymmetry A_PV = (1.55±0.97 (st at)± 0.24 (sys))× 10^-8 in the capturereaction of ^3He(n⃗,p)^3H at meV incident neutronenergies. The result increased the limited inventory of precisely measured andcalculable PV observables in few-body systems required to further understandthe structure of hadronic weak interaction. In this letter, we report theexperimental and theoretical investigation of a parity conserving (PC)asymmetry A_PC in the same reaction (the first ever measured PCobservable at meV neutron energies). As a result of S- and P-wave mixing in thereaction, the A_PC is inversely proportional to the neutronwavelength 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 ofthis reaction by solving the four-body scattering problem within thehyperspherical harmonic method. We find that in the ^3He(n⃗,p)^3H reaction, A_PC depends critically on the energy andwidth of the close 0^- resonant state of ^4He, resulting in a largesensitivity to the spin-orbit components of the nucleon-nucleon force and evento the three-nucleon force. The analysis of the accurately measured A_PC and A_PV using the same few-body theoretical models givesessential information needed to interpret the PV asymmetry in the^3He(n⃗, p)^3H reaction.