Fig. 3. Missing-mass spectra.

Left: missing-mass spectrum of the four-neutron system extracted from the ${}^8\mathrm{He}\left(\mathrm{p},{\mathrm{p}}^4\mathrm{He}\right)$ reaction. The different curves represent a Breit–Wigner resonance (pink), a non-resonant continuum (dashed blue), the background from two-step processes (green) and the total sum (solid blue). Right: missing-mass spectrum of the two-neutron system extracted from the ${}^6\mathrm{He}\left(\mathrm{p},{\mathrm{p}}^4\mathrm{He}\right)$ reaction. The blue curve represents the theoretical calculation¹⁷ convoluted with the experimental acceptance and resolution, and the green curve represents the background from the two-step reaction.

Source data