Neutron-deuteron analyzing power data at E_n = 21 MeV and the energy dependence of the three-nucleon analyzing power puzzle

We present measurements of n-d analyzing power, , at E_n = 21.0 MeV. The experiment produces neutrons via the ²H(, )³He reaction and uses a deuterated liquid-scintillator center detector and six pairs of liquid-scintillator neutron side detectors. Elastic neutron scattering events are identified by using time-of-flight techniques and by setting a gate in the center-detector pulse-height spectrum. Beam polarization is monitored by using a high-pressure helium gas scintillator. The n-d data at 21.0 MeV show a significant discrepancy with the results of rigorous three-body calculations and are consistent with data taken previously by us at 19.0 and 22.5 MeV. We review the overall energy dependence of the three-nucleon analyzing power puzzle in neutron-deuteron elastic scattering, using the best data available. We find that the relative difference between calculations and data is nearly constant at 25% up to E_n = 22.5 MeV.