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Lifetime and fragment correlations for the two-neutron decay of $^{26}$O ground state

The structure and decay of $^{26}$O are investigated in a three-body $^{24}$O+$n+n$ model suitable for studies of the long-lived (including radioactivity timescale) states. We have found extremely strong effect of the subbarrier configuration mixing on the decay width of true $2n$ emitters due to core recoil and neutron-neutron final state interaction. This effect is far exceeding analogous effect in the true $2p$ emitters. Our calculations provide reasonably narrow boundaries for the lifetime vs.\ decay energy dependence for the true $2n$ emission. An upper limit of $\sim 1$ keV for the decay energy of the unbound $^{26}$O is inferred based on the recent experimental lifetime value.