Upper limits for bound states and resonance behavior in the trineutron system

Abstract The trineutron mass spectrum was observed in the stopped pion radiative capture reaction 3H( π − , γ )3n by measuring the photon energy with a high-resolution spectrometer. The present experiment improves upon an earlier experiment by a factor of 27 in statistics and 2.2 in resolution. An improved upper limit (95 % confidence level) on the branching ratio (bound state)/(total radiative capture rate) is found to be 4 × 10 −3 . No resonant structure is observed beyond that predicted by the pairwise interaction of the outgoing nucleons. The upper limit on the branching ratio to resonances with Γ ≲ 5 MeV is 2 × 10 −2 .