Negative pion absorption at rest in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$

The pnn and dn final states after the absorption of stopped pions in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been measured in a kinematically complete experiment covering the whole phase space. Proton-neutron, deuteron-neutron, and neutron-neutron coincidences have been detected with a charged-particle hodoscope and large-area time-of-flight counters. For the first time, triple coincidences between the pionic K x rays and two particles have been measured thus selecting absorption from the atomic 1s state. It has been found that the absorption is dominated by the two-nucleon mechanism (2NA) on isoscalar nucleon pairs as indicted by the measured ratio of 6.3±1.1 for back-to-back correlated nn and pn pairs. A substantial amount of the three-body final state pnn is affected by the final-state interaction (FSI) or leads to the two-particle final state dn. Ratios of 3.1±0.4 for 2NA to FSI and of 4.2±0.6 for the pnn to the dn final state are measured. No evidence has been found for three-nucleon absorption with an upper limit of %/${\mathrm{\pi }}_{\mathrm{stop}}^{\mathrm{-}}$.