Pion absorption by 3He at the Δ-resonance energy

Abstract

Pion absorption by 3He was studied at Tπ=165 MeV in a kinematically complete experiment. The cross section for absorption on a (pn) pair of nucleons, σpn(π+), was found to be 17.0±2.6 mb; that for absorption on a (pp) pair, σpp(π-), 0.91±0.20 mb. The angular distribution in the πNN center-of-mass system for σpn(π+) resembles that for the π++d→p+p reaction while the angular distribution for σpp(π-) is strongly backward peaked. Evidence that a significant fraction of the absorptions involves all three target nucleons is seen in the angular correlation between the two detected nucleons as well as in the momentum distribution of the unobserved nucleon. For π+ and π- absorption, the three-body cross sections were found to be 9.6±2.1 and 4.2±01.2 mb, respectively. Neither initial- nor final-state interactions appear to be major contributors to the observed three-body absorption, though initial-state interactions may be contributing to the enhancement of the three-body π+ absorption at the Δ resonance.