Proton polarization from π⁺ absorption in ⁴He

The polarization of protons resulting from π⁺ absorption in ⁴He was measured at bombarding energies of 120 MeV and 250 MeV. Events arising from absorption in a quasi-deuteron were analysed by using kinematical constraints. The apparatus was tested by measuring the polarization of protons resulting from π⁺d→→pp. Differences observed between polarization measured for pion absorption in the deuteron and in quasi-deuterons inside ³He and ⁴He suggest that the density of the absorbing nucleon pair affects this observable. There is however a large discrepancy between the experimental results and theoretical predictions.