Two neutron decay from the $2_1^{+}$ state of ${}^6\mathrm{He}$

Decay mode of the $2_1^{+}$ resonant state of ${}^6$He populated by the ${}^6$He breakup reaction by ${}^{12}$C at 240 MeV/nucleon is investigated. The continuum-discretized coupled-channels method is adopted to describe the formation of the $2_1^{+}$ state, whereas its decay is described by the complex-scaled solutions of the Lippmann-Schwinger equation. From analysis of invariant mass spectra with respect to the $\alpha$-$n$ and $n$-$n$ subsystems, coexistence of two decay modes is found. One is the simultaneous decay of two neutrons correlating with each other and the other is the emission of two neutrons to the opposite directions. The latter is found to be free from the final-state interaction and suggests existence of a dineutron in the $2_1^{+}$ state of ${}^6$He.

Figure 1

DDBUX of ${}^6$He by ${}^{12}$C at 240 MeV/nucleon. Panels (a) and (b) represent the cross sections with respect to the subsystem energies of $\alpha$-$n$ and $n$-$n$, respectively (see the text for details).

Figure 2

Invariant mass spectra with respect to ${\varepsilon }_{\alpha \text{−}n}$ (solid line). The dashed and dotted lines show the resonant and nonresonant parts, respectively.

Figure 3

Same as Fig. 2 but for $d\sigma /d{\varepsilon }_{n\text{−}n}$.