T20 problem and spin observables in pion-deuteron scattering

There has been a long-standing problem in the coupled πNN-NN theory concerning its apparent failure in reproducing the tensor polarization T20 in the elastic πd scattering at medium energies. A mechanism recently proposed by Jennings to solve this problem is examined. It is then found that it comes from the positive-energy pole in one of the nucleon propagators in the exchange impulse diagram within the covariant picture which is usually overlooked when devising the spectator on-mass-shell approximation, and that it does not lead to the claimed near cancellation of the Pauli exchange contribution, but is important to be included. The inclusion of this contribution is shown to considerably improve in reproducing the pion-deuteron spin observables including T20, particularly above the delta resonance energy