Pion absorption at rest in 4He

The emission of nn-, np-, nd- and nt-pairs following π^--capture in gaseous ⁴He has been studied in a coincidence experiment. Quasifree 2N-absorption is the dominant absorption mechanism with a ratio of r = 4.0 _-1.5^+0.8 for absorption on a nucleon pair with isospin I = 0 and I = 1. Also collinear processes involving more than two nucleons have been observed, including NN final-state interaction with small relative momenta as well as emission of deuterons and tritons. Upper limits are given for phase-space-distributed, i.e. non-collinear 3N- and nnd-emission.