We investigate the dineutron in the $2_1^{+}$ state of ${}^6\mathrm{He}$ via analysis of its decay mode by using the complex scaling method. In this Letter, we propose the cross section for the resonant state to distinguish the resonant contributions from the nonresonant ones. As a result, it is found that the shoulder peak appears in the cross section for the resonant state as a function of ${\varepsilon }_{n\text{−}n}$. Furthermore, we show that the $S=0$ component of the cross section, where $S$ is the total spin of the two valence neutrons, has a peak around the shoulder peak, which comes from the dineutron configuration in the $2_1^{+}$ state. Thus we conclude that the shoulder peak is expected to indicate the existence of the dineutron in the $2_1^{+}$ state.

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• Breakup reactions
• Direct reactions
• Few-body systems
• Nuclear reactions
• Resonance reactions
• Unstable nuclei induced nuclear reactions