Intermediate energy pion production
An experimental study of intermediate energy pion production has been conducted at TRIUMF with polarized protons. Angular distributions of differential cross sections and polarization analysing powers are presented for (p/π) reactions on ¹H with 305 to 425 MeV protons, on ²H with 305 to 400 MeV protons, and on ⁹Be and ¹²C with 200 MeV protons. In addition the inclusive ¹²C(p,π⁺)X reaction has been studied for protons with 330 to 425 MeV energy. Theoretical model calculations of the ¹H (p,π⁺)²H
... of the ¹H (p,π⁺)²H reaction involve relatively few transition amplitudes in the 'threshold' kinematic region (where only s-and p-wave pions contribute). Prior to this experiment the threshold regicn was thought to extend to 140 MeV above threshold (289 MeV). The new data show that the threshold description is only valid up to 320 MeV. A second important observation cf this experiment is the similarity of the analysing power for ¹H (p,π⁺) pn to that for ¹H (p,π⁺)²H for pions with center of mass energies 10 to 20 MeV below the two body reaction. The similarity is also observed in the ²H (p,π⁺) X reaction, where the inclusive reaction analysing powers were slightly more positive than for ²H (p,π⁺)³H. In the 200 MeV nuclear (p,π) reactions studied, the shape of the angular distributions of cross section agree well with earlier 185 MeV results. The angular distributions of analysing power in the exclusive nuclear (p,π) reactions, never previously measured, show a remarkably large magnitude in the forward direction and enable a thorough testing of current theoretical models. These results are an indication of the explicit role of the two nucleon mechanism in nuclear pion production. Finally the results for the inclusive reaction, ¹²C (p,π⁺)X have been compared with a semiclassical model. The model results are reasonable, however at these low energies uncertainties in the input NN→NNπ distributions lead to an underestimate of the cross section for low energy pions.