⁷H is the nucleus with, by far, the most unbalanced neutron to proton ratio. The first experimental indication of ⁷H being a resonant state came in 2003 from RIKEN (2003Ko11). This study argued that it is unlikely that ⁷H exists as a bound state, but a resonant state near the ³H+4n threshold with J^π=1/2⁺ seems likely. It was assumed that such a state would likely decay either into five outgoing particles (³H+4n) or two particles (³H+⁴n) if the tetraneutron exists.

Most recent observations (2020Be01, 2021Mu04, 2021Hu28, 2022Ca10, and 2023Ni06) indicate that ⁷H ground state is a low-lying, narrow (due to neutron pairing) resonance 1.3 MeV 4 above the ³H+4n mass with a width of Γ<300 keV (2020Be01), and with J^π=1/2⁺ measured in (2022Ca10). Such a state would be consistent with an extended 4-neutron halo interacting with a ³H core, which would decay by emission of a tetraneutron. The decay of the four-neutron-unbound ground state of ⁷H via direct emission of a tetraneutron has not yet been experimentally observed. However, the ongoing analysis of (2021Hu28) seems to be suggestive of this mode of decay.

As for the excited states of ⁷H, the first one is observed |J4 MeV above the ground state with a plausible J^π=(5/2⁺) assignment. This state is expected (2021Mu04) to decay via ⁵H_g.s.+2n, where ⁵H_g.s., in turn, decays to ³H+2n. So, the decay may be sequential. The first excited state may be part of a doublet containing another state at higher energy with J^π=(3/2⁺). α candidate state for the latter was reported in (2021Mu04) but its existence is uncertain. An even higher energy excited state was observed in (2021Mu04) at 9.7 MeV, whose structure may be indicative of the p+6n configuration.

Theory: Numerous investigations have been carried out to study the ⁷H_g.s. properties. These are summarized below.

1985Po10: An early shell model calculation obtained J^π=1/2⁺ for the ⁷H ground state using two different models.

2000Fi22: Using resonating-group method, the wave function of ⁷H as a cluster system of ³H+n+n+n+n was calculated and analyzed hyperharmonically.

2002Ti05: Calculations using the 7-body hyperspherical harmonics functions with no core shell model predicted a ⁷H binding energy of -7.61 MeV, estimated by exponential extrapolation. This estimation was about 300 keV lower than that for ⁵H (2001Ko52), which would agree with the hypothesis of (2001Ko52) that ⁷H may exist as a low lying resonance with the only decay channel being ⁷H |) ³H+n+n+n+n. Later, (2004Ti02) performed the same kind of calculations after improving a Casimir operator such that the hyperharmonics had well defined symmetry when constructed within the shell model approach. This work deduced the ⁷H resonance |J3 MeV above the ³H+4n threshold. This theoretical result also favored a sequential decay of ⁷H into ³H+n+n+n+n.

2004Ao05: α coupled channels calculation treated ⁷H as a combination of both a triton plus four neutrons and as a proton plus three dineutrons. The calculated ground state binding energy is about 1.5 MeV, which is about 7 MeV above the ³H+4n threshold.

2009Ao03: This calculation used the Antisymmetrised Molecular Dynamics with generator coordinate and stochastic variational methods that included basis states with a triton and two dineutrons as well as basis states with a triton and 4 neutrons. This study obtained a ⁷H ground state with a binding energy of 2.8 MeV, which is about 4.2 MeV above the ³H+4n threshold. This work describes the ground state of ⁷H as a ³H+²n+{+2n. These two pairs of neutrons act as two bosons bound together by their interaction with the ³H core in a di-neutron condensate.

2011Gr13: Simultaneous four neutron emission by ⁷H is discussed in this work. They demonstrate, by using simplified 3-body and 5-body Hamiltonians, that few body dynamics of 2n and 4n emissions result in collective barriers that rise quickly with increasing the number of emitted particles. This translates into longer lifetimes being expected for nuclei which decay via 4n than those that decay via the emission of 2n. This work considered the ⁷H_g.s. as a true 4n emitter and estimated that the ground state of ⁷H has a narrow width of Γ|<1 keV.

2019Sh36: Simultaneous non-sequential 4n emission is considered in a phenomenological five-body (core+4n) decay. This theoretical work assumes that the internal structure of the ground state of ⁷H is dominated by a 0p_3/2⁴ configuration. The decay of ⁷H may cause a mixing of configurations such as 0s_1/2²0p_3/2² due to Pauli focusing effect. This would result in correlations in energy, angular distribution, and phase space, which could be used as observable fingerprints of a simultaneous non-sequential 4n decay and to understand the decay dynamics.

2021Li62: The energies and neutron-emission widths of the unbound hydrogen isotopes were computed using the no core Gamow shell model. The ground state of ⁷H was considered as a rigid ³H core and 4 valence neutrons (coupled to J=0), which immediately gives J^π(⁷H_g.s.)=1/2⁺. The many body basis of the Gamow shell model for the ⁷H_g.s. was generated from natural orbitals. The resonance energy of ⁷H was deduced. The results vary between 1-3 MeV with an uncertainty of 400-600 keV, depending on the different phenomenological NN interactions used. These results are more or less in agreement with the previous experimental results. α width of Γ≈0.1 MeV was deduced for the ⁷H_g.s., and it was recommended that the ground state of ⁷H is a very narrow resonance due to the 0p_3/2 being a closed sub neutron shell in ⁷H.

2022Hi06: The ground state of ⁷H was considered as a five-body consisting of a solid ³H core interacting with 4 valence neutrons. The properties of the ⁷H_g.s. were computed in the 5-body cluster approximation (³H-n-n-n-n), which is considered to be the dominant decay channel for a low energy resonant state. α n-³H local interaction was constructed without any tensor component and adjusted in order to reproduce the n-³H phase shifts. These were calculated by solving the ab-initio four-nucleon scattering problem. The Gaussian Expansion Method was used to solve the five-body Schr~odinger equation for the ³H-n-n-n-n system. The Stabilization Method was used to estimate the complex energies of the ⁷H resonant state. As a result, instead of a narrow ⁷H resonant state in the vicinity of the ³H+4n threshold, a resonance was found at 9.5 MeV with a width of Γ=3.5 MeV. This result is in agreement with that of (2004Ao05), but it is in sharp contrast with the result of (2021Li62). The authors of (2022Hi06) argue that the Gamow shell model used in (2021Li62) underestimates the width. The results of (2022Hi06) are also inconsistent with the recent experimental results for the ⁷H_g.s. (2003Ko11, 2007Ca28, 2010Ni10, 2020Be01). Thus, it was mentioned in (2022Hi06) that the deduced wide resonance at 9.5 MeV may be linked to the experimental results of (2020Be01), where a resonance was found at E=6.5 MeV 5 with a width of Γ=2.0 MeV 5. It should be noted that the 6.5 MeV state measured in (2020Be01) is an unresolved doublet consisting of the first excited state and a candidate for the second excited state of ⁷H.

In the following reactions, excitation and resonance energies in ⁷H are given relative to the ³H+4n threshold.