Proton-Deuteron Elastic Scattering and Three-Nucleon Forces

The spin observables Ay, iT11, and Cy,y in proton-deuteron elastic scattering were measured over the angular range 65° < ϴcm < 115° at a proton energy Tlab = 197 MeV. These data were acquired with a polarized proton beam and an internal vector-polarized deuteron gas target at the Indiana University Cyclotron Facility. The target was polarized by spin exchange with optically-pumped potassium; this experiment was the first to use this novel technique for polarizing hydrogen and deuterium. The data have been compared to existing Faddeev calculations using modern two-nucleon potentials which accurately reproduce the available two-nucleon scattering data. The calculations were also performed with the addition of the Tucson-Melbourne three-nucleon potential to identify kinematic regimes where the three-nucleon force makes a significant contribution. One such regime is in the vicinity of the minimum in the differential cross section for nucleon-deuteron elastic scattering at nucleon energies Tlab >~ 100 MeV, which includes the region covered by this experiment. The failure of calculations using only two-nucleon potentials to reproduce the results of this experiment is interpreted as evidence that some type of three-nucleon force is needed to explain the data