UC Irvine

The determination of the S-wave ππ phase shifts δ0I at low energy from the analysis of πN→(2π)N is examined critically from the standpoint of the one-pion-exchange model with absorptive corrections. It is found that: (1) The value of δ0I depends strongly on the P-wave phase shifts, which cannot be unambiguously determined, at mππ<600 MeV, by using a Breit-Wigner formula. (2) The ratio of the production density matrix elements ρ (with the ππ elastic scattering amplitudes factored out) depends strongly on mππ for mππ<600 MeV. (3) The (F-B)(F+B) asymmetry shows a sizeable dependence on the momentum transfer t to the nucleon. It is concluded that more accurate data at low mππ are required in order to determine δ0I for mππ<600 MeV. Tables of the ρ(mππ,A t) calculated from the absorption model for an incident-pion laboratory kinetic energy of 4 BeV are included. These could be directly applied to the data to obtain the low-energy ππ phase shifts. © 1968 The American Physical Society.