The weight of experimental evidence reviewed in the previous compilation of Fiarman and Hanna (1975FI08) is strongly against the existence of a bound state of the three-neutron system, and only controversial evidence of 3n resonances was cited. Several experiments carried out more recently have strengthened the evidence against the bound trineutron and have failed to discover resonance structure that cannot be otherwise explained. The most suggestive work is the (π-, π+) double charge-exchange experiment of (1974SP08) (see reaction 2 below), the result of which could be interpreted as indicating a T = 3/2 resonance around 12 MeV excitation. This observation was supported by more recent (π-, π+) measurements reported in (1986ST09).
Some of the contradictory aspects (1975FI08, 1977PH2A) of calculations on bound states of 3n have been clarified. (1979LI19) showed that disagreements between Faddeev calculations (which predicted a bound trineutron) and variational calculations (which did not), arose from the potentials used in the Faddeev calculations. (1978GL07) has calculated the S-matrix pole trajectory in a three-neutron model and concluded that its pattern rules out the possibility of a low-energy resonance. In (1979OF01) a study using the Reid potential found that the interaction potential must be enhanced by ≈ 4 to bind 3n near zero energy, indicating that a low energy resonance is extremely unlikely. The trineutron binding energy was calculated (1980BE22) using an interaction which reproduces 3H, 3He, 4H, 4He, and 4Li ground states. The model suggests that 3n is unbound. A Faddeev calculation (1980SU05) with a realistic nucleon-nucleon interaction in coordinate space indicated that 3n states with 1/2 < J < 7/2 of either parity are unbound for the Reid soft core potential. A method for finding the lower estimates of energies of specific states is proposed by (1981KA39). They investigate four varieties of potentials with the super-soft core and the Reid potential. The method indicates that only T = 1/2 is allowed for A = 3 and forbids states of 3n or 3Li.
Measurements (1976BI05, 1979BI13) for π- momentum p = 200 MeV/c with a high-resolution pair spectrometer gave no evidence for bound 3n, but could not rule out resonance struture in the excitation region from 7 to 16 MeV. (See also the review of (1977BA2A). An improved experiment (1980MI12) gave an upper limit on the branching ratio (bound state width)/(total radiative capture width) of 4 × 10-3 and an upper limit on the branching ratio to resonances with width < 5 MeV was determined to be 2 × 10-2. These results along with other radiative pion capture experiments are reviewed in (1982GM02).
In recent work by (1985WE04), the 1s π- absorption width for 3H(π-, γ)3n is predicted to be 2.25 ± 0.55 eV on the basis of the isospin dependence of the two-nucleon pion absorption operator.
At Eπ = 140 MeV and π+ detection angle between 20° and 40° (1974SP08) measured an enhancement in the π+ distribution near threshold which can be interpreted either as a T = 3/2 three-nucleon resonance or a resonance of the nucleons in the 3He nucleus. They obtain an upper limit of 0.12 μb/sr for the production cross section of a bound state of 3n. A theoretical study by (1984JI01) using the method of hyperspherical functions in the coordinate representation showed that the resonance-like behavior of the differential cross section observed in the experiment is reproduced by the inclusion of the final state interaction, and therefore it is unlikely that it proves the formation of a 3n resonance state. More recently a measurement of the differential cross section σ(Eπ-, Eπ+, θ) for the 3He(π-, π+) reaction at 140, 200, and 295 MeV was reported in (1986ST09). The missing-mass plot showed a strong enhancement resembling a broad 3-nucleon resonance.
Pion absorption studies have been carried out by several groups. Proton spectra were measured at Eπ = 60, 100, 200 MeV and θ = 45° and 90° (1977JA15); at Eπ = 100, 160, 220 MeV and θ between 30° and 150° (1981MC09); at Eπ = 50 - 300 MeV and θ = 20° (1981KA41); at Eπ = 400, 475 MeV and θ = 30° (1981KA43); at Eπ = 50, 100, 150, 200, 250 MeV and θ = 20° and 40° (1983KA14). The breakup of 4He by π- was studied by (1981RAZV, 1981RAZZ). Proton energy spectra from the capture at rest of π- by 4He were measured (1981CE01). No evidence was reported for 3n bound states in any of the pion absorption experiments.
A search for 4He recoils corresponding to 3n formation was carried out (1977BA47) with 7Li loaded emulsions. An upper limit for the branching ratio of 3n formation to all other channels was found to be < 1.2 × 10-3 at the 90% confidence level.
Particle identification spectra were obtained (1974CE06) at a 7Li bombarding energy of 79.6 MeV. The 11C energy distribution was well fit by four-body phase-space predictions. An upper limit of 70 nb/sr for 3n bound state production was obtained. No resonant structure was evident.