8.3 Multi-Neutron Resonances
385
8.3
Multi-Neutron Resonances
Whether the four-neutron system exists or not is a long-standing question which
rests on the hypothesis that such a system could be formed as a result of a subtle
interplay between the many-body components of the nuclear interaction, the Pauli
principle and the coupling to the neutron continuum. Tremendous efforts have been
made during the last few decades to understand few-nucleon systems, in particular
few-neutron systems [115–117]. Earlier experiments failed to find positive evidence
for the existence of multi-neutron systems (see a discussion in Ref. [115]). In 2002,
Marqués et al. reported a possible observation of a tetraneutron in a breakup reaction
for the 14 Be → 10 Be+4n channel [117]. After that experiment, several theoretical
attempts were performed to examine the possible existence of bound tetraneutron,
but all calculations failed to reproduce experimental data [118, 119].
The experimental hints of the tetraneutron resonance provided by the recent
measurement [116] exacerbate the need for reliable nuclear interactions and ab
initio methods able to cope with couplings to the continuum. According to those
experimental results, if such a state exists it would have an energy E = 0.83 ± 0.65
(stat) ± 1.25 (syst) MeV above the tetraneutron threshold and a maximal width
Γ = 2.6 MeV. This large width is unlikely to correspond to a nuclear state [120].
In Ref. [121], the possibility to form the tetraneutron resonance by adding a
phenomenological T = 3/2 three-body force to a realistic two-body interaction was
investigated. In this study, the continuum was included and it was shown that the
unrealistic modifications of the nuclear interaction would be necessary to obtain the
tetraneutron. On the contrary, ab initio calculation in the harmonic oscillator basis
[122] using the two-body JISP16 interaction [123] obtained the energy and the width
of the tetraneutron (E = 0.8 MeV and Γ = 1.4 MeV) from extrapolation into the
continuum of the phase shifts of artificially bound tetraneutron, close to the values
published in Ref. [116]. However, couplings to the continuum at low energy below
and above the particle-emission thresholds may strongly affect the configuration
mixing and make such extrapolations misleading for broad resonances. Results of
Ref. [121] based on the uniform complex scaling method [124,125] are not affected
by this issue and suggest that if the tetraneutron system exists its energy and width
should be larger than in Ref. [122]. Below, one will investigate the conditions of
existence of the tetraneutron system using different ab initio methods and various
two-body chiral interactions while including the continuum coupling.
The quantum Monte Carlo framework has been used to calculate multi-neutron
systems based on local chiral interactions via an extrapolation of bound states,
obtained with a confining auxiliary potential, to the physical domain of unbound
multi-neutron systems. These calculations predicted that the trineutron is located at
1.11 MeV, below the energy of tetraneutron, at 2.12 MeV [126,127]. Calculations in
this approach suggested that the existence of multi-neutron resonances can only be
obtained by strongly modifying standard nuclear forces, which is incompatible with
the current understanding of nucleon–nucleon interactions [121].
385
8.3
Multi-Neutron Resonances
Whether the four-neutron system exists or not is a long-standing question which
rests on the hypothesis that such a system could be formed as a result of a subtle
interplay between the many-body components of the nuclear interaction, the Pauli
principle and the coupling to the neutron continuum. Tremendous efforts have been
made during the last few decades to understand few-nucleon systems, in particular
few-neutron systems [115–117]. Earlier experiments failed to find positive evidence
for the existence of multi-neutron systems (see a discussion in Ref. [115]). In 2002,
Marqués et al. reported a possible observation of a tetraneutron in a breakup reaction
for the 14 Be → 10 Be+4n channel [117]. After that experiment, several theoretical
attempts were performed to examine the possible existence of bound tetraneutron,
but all calculations failed to reproduce experimental data [118, 119].
The experimental hints of the tetraneutron resonance provided by the recent
measurement [116] exacerbate the need for reliable nuclear interactions and ab
initio methods able to cope with couplings to the continuum. According to those
experimental results, if such a state exists it would have an energy E = 0.83 ± 0.65
(stat) ± 1.25 (syst) MeV above the tetraneutron threshold and a maximal width
Γ = 2.6 MeV. This large width is unlikely to correspond to a nuclear state [120].
In Ref. [121], the possibility to form the tetraneutron resonance by adding a
phenomenological T = 3/2 three-body force to a realistic two-body interaction was
investigated. In this study, the continuum was included and it was shown that the
unrealistic modifications of the nuclear interaction would be necessary to obtain the
tetraneutron. On the contrary, ab initio calculation in the harmonic oscillator basis
[122] using the two-body JISP16 interaction [123] obtained the energy and the width
of the tetraneutron (E = 0.8 MeV and Γ = 1.4 MeV) from extrapolation into the
continuum of the phase shifts of artificially bound tetraneutron, close to the values
published in Ref. [116]. However, couplings to the continuum at low energy below
and above the particle-emission thresholds may strongly affect the configuration
mixing and make such extrapolations misleading for broad resonances. Results of
Ref. [121] based on the uniform complex scaling method [124,125] are not affected
by this issue and suggest that if the tetraneutron system exists its energy and width
should be larger than in Ref. [122]. Below, one will investigate the conditions of
existence of the tetraneutron system using different ab initio methods and various
two-body chiral interactions while including the continuum coupling.
The quantum Monte Carlo framework has been used to calculate multi-neutron
systems based on local chiral interactions via an extrapolation of bound states,
obtained with a confining auxiliary potential, to the physical domain of unbound
multi-neutron systems. These calculations predicted that the trineutron is located at
1.11 MeV, below the energy of tetraneutron, at 2.12 MeV [126,127]. Calculations in
this approach suggested that the existence of multi-neutron resonances can only be
obtained by strongly modifying standard nuclear forces, which is incompatible with
the current understanding of nucleon–nucleon interactions [121].