A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

Candidate Resonant Tetraneutron State Populated by the He 4 (He 8, Be 8) Reaction

Nuclear Data Sheets for A=40☆

https://escholarship.org/content/qt1mg151pc/qt1mg151pc.pdf?t=pgp78k

Nuclear Data Sheets for A=40 ☆

In light of a new experiment which claims a positive identification, we discuss the possible existence of the tetraneutron. We explore a model based on a dineutron–dineutron molecule. We show that this model is not able to explain the tetraneutron as a bound state, in agreement with other theoretical models already discussed in the literature.

/pdf/is-the-tetraneutron-a-bound-dineutron-dineutron-molecule-2wqqux89vo.pdf

Is the tetraneutron a bound dineutron-dineutron molecule?

The long history of the research concerning the possible existence of bound or resonant states in light multineutron systems, essentially $$^3$$
 n and $$^4$$
 n, is reviewed. Both the experimental and the theoretical points of view have been considered, with the aim of showing a clear picture of all the different detection and calculation techniques that have been used, with particular emphasis in the issues that have been found. Finally, some aspects of the present and future research in this field are discussed.

The quest for light multineutron systems

Radiative muon capture (RMC) on hydrogen produces photons with a yield of ∼10−8 per stopped muon. To measure RMC at TRIUMF we have constructed a large-solid-angle photon pair-spectrometer which surrounds the liquid hydrogen target. The spectrometer consists of a cylindrical photon converter and a large-volume cylindrical drift chamber to track the e+e− pairs. It is enclosed in a spectrometer magnet which produces a highly uniform axial magnetic field. The detector subsystems, the hardware trigger and the data acquisition system are described, chamber calibration and tracking techniques are presented, and the spectrometer performance and its Monte Carlo simulation are discussed.

The TRIUMF radiative muon capture facility

A measurement of the photon asymmetry $({\ensuremath{\alpha}}_{\ensuremath{\gamma}})$ in radiative muon capture RMC on ${}^{40}\mathrm{Ca}$ has been carried out at TRIUMF. Negative muons were stopped in a ${}^{40}\mathrm{Ca}$ target and the resulting RMC photons were then converted by a 5-cm thick NaI detector placed immediately in front of our main NaI detector, a cylindrical crystal of diameter 46 cm and depth 51 cm. The data sample consisted of 5200 high-energy $(57\mathrm{MeV}l{E}_{\ensuremath{\gamma}}l95\mathrm{MeV})$ RMC photon candidates. Our measured value for the energy-averaged photon asymmetry is ${\ensuremath{\alpha}}_{\ensuremath{\gamma}}=1.00\ifmmode\pm\else\textpm\fi{}0.23.$ The extracted values for the induced pseudoscalar coupling constant ${(g}_{P})$ based on this photon asymmetry measurement are ${g}_{P}{/g}_{A}l8.0,$ utilizing the impulse approximation IA model, and ${g}_{P}{/g}_{A}l14.5$ in terms of the modified impulse approximation (MIA) model. The two extracted values, although both consistent with the Goldberger-Treiman value of ${g}_{P}{/g}_{A}\ensuremath{\approx}7,$ are considerably different, indicating a significant theoretical model dependency.

Photon asymmetry measurement in radiative muon capture on 40 Ca

Radiative muon capture on carbon, oxygen, and calcium

A search for the production of bound tetraneutrons has been carried out with a time projection chamber using the reaction $^{4}\mathrm{He}$(${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\pi}}^{+}$${)}^{4}$n at =80 MeV and at 130\ifmmode^\circ\else\textdegree\fi{}. No evidence for tetraneutron formation was found, and a 90% confidence level upper limit for the production cross section of d\ensuremath{\sigma}/d\ensuremath{\Omega}\ensuremath{\le}13 nb ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$ was obtained.

Search for the tetraneutron using the reaction 4 He(π - ,π + ) 4 n

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The TRIUMF radiative muon capture facility

Photon asymmetry measurement in radiative muon capture on 40 Ca

Radiative muon capture on carbon, oxygen, and calcium

Search for the tetraneutron using the reaction 4 He(π - ,π + ) 4 n