Proton-helium-3 elastic scattering at low energies and the A(y) puzzle

Angular distributions of cross sections σ(&thetas;) and proton analyzing powers A_y have been measured for proton-3He elastic scattering at several energies at and below 5 MeV. A supersonic gas jet target has been refurbished, improved, and installed allowing these low energy cross section measurements to be performed. The σ(&thetas;) distributions have been measured at E_p = 0.99, 1.59, 2.24, 3.11, and 4.02 MeV. Full angular distributions of A_y have been measured at E_p = 1.60, 2.25, 3.13, and 4.05 MeV. These data have been compared with recent theoretical calculations made using realistic two- and three-nucleon interactions. Using the Argonne v₁₈ nucleon-nucleon potential and the Urbana IX three-nucleon interaction, these calculations show discrepancies from the measured σ(&thetas;) distributions on the ≈5% level, which increase at backward angles. When the measured Ay distributions are compared with the theoretical calculations, much larger discrepancies of approximately 40% are seen at the maximum of each distribution. The addition of the Urbana IX three-nucleon interaction has virtually no effect on this difference. This is analogous to the existing “A_y Puzzle” known for the past 20 years in nucleon-deuteron elastic scattering. Single energy phase-shift analyses have been performed on these new data. They show that increases ($⪅$20%) relative to the theoretical predictions in key phase shifts and mixing parameters greatly improve the description of the new data.