Candidate Resonant Tetraneutron State Populated by the ${}^4\mathrm{He}({}^8\mathrm{He},{}^8\mathrm{Be})$ Reaction

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

Figure 1

Left: schematic of the experimental setup downstream of BigRIPS-F6. Right: momentum correlation between the ${}^8\mathrm{He}$ beam ($P_{{}^8\mathrm{He}}$) at F6 and the ${}^8\mathrm{Be}$ ejectile ($P_{{}^8\mathrm{Be}}$) at S2 for candidate events. $P_{{}^8\mathrm{He}}=P_{{}^8\mathrm{Be}}=0\%$ corresponds to the central position of the focal plane. The shaded diagonal line shows the energy threshold of four-neutron decay. The diagonal axis is $T_{{}^8\mathrm{He}}-T_{{}^8\mathrm{Be}}+Q$, where $T_{{}^8\mathrm{He}}$ and $T_{{}^8\mathrm{Be}}$ denote the kinetic energies of ${}^8\mathrm{He}$ and ${}^8\mathrm{Be}$, respectively, and $Q$ is the $Q$ value of the ${}^4\mathrm{He}({}^8\mathrm{He},{}^8\mathrm{Be})4n$ reaction. This axis corresponds to the sum of the missing mass and the recoil energy of the tetraneutron. The recoil energy for each event ranges from 0 to 3 MeV depending on each scattering angle, which is another degree of freedom from the magnitudes of the momenta.

Figure 2

A scatter plot of the missing mass of the tetraneutron vs the relative energy between two $\alpha$ particles, together with the projected histogram for $E_{\alpha \alpha }$. The solid (red) and dashed (blue) curves in (a) represent the response functions for ${}^8\mathrm{Be}(0^{+})$ and ${}^8\mathrm{Be}(2^{+})$, respectively. The magnitudes of the response functions are described in the text.

Figure 3

(a) Missing-mass spectrum of the tetraneutron system. The solid (red) curve represents the sum of the direct decay of correlated two-neutron pairs and the estimated background. The dashed (blue) curve represents the estimated background multiplied by a factor of 10. The schematic of the decay process is discussed in the text. (b) Evaluation of the goodness of fit for each bin using the likelihood ratio test. The $s_i$ were defined in Eq. (3).