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Unfolding of exotic near-threshold structure and decay dynamics in $^{17}$B

Neutron-rich boron isotopes provide a valuable testing ground for threshold-driven structure and reaction phenomena, including halo formation and exotic decay modes. In particular, the structure of $^{17}\mathrm{B}$ and its relation to unbound $^{16}\mathrm{B}$ are of special interest. The $^{16}\mathrm{B}$ nucleus is slightly unbound by approximately $50~\mathrm{keV}$, while $^{17}\mathrm{B}$ is bound with a neutron separation energy of about $1.4-1.6~\mathrm{MeV}$. The observation of a $1640~\mathrm{keV}$ $γ$ ray in $^{17}\mathrm{B}$, which we argue originates from a $1/2^-$ excited state, points to a remarkable situation in which $γ$ decay and two-neutron decay can compete. We analyze and identify the main reasons for this competition: $L=2$ emission of the neutron pair, and structural realignment driven by the proximity of the one-body threshold, in particular the nearby $s$-wave neutron decay channel. The decay is a unique near-threshold $L=2$ process in which multiple structural components contribute, each with coexisting direct and virtual sequential amplitudes whose interference governs the observables. Because threshold dynamics, continuum coupling, and interference of multiple quantum pathways are universal, closely related scenarios arise in ultracold atoms near Feshbach resonances, few-body atomic and molecular breakups, mesoscopic and photonic open systems, and other areas where open-quantum-system effects impact observables. We employ advanced theoretical models to study this first-of-its-kind case and provide a coherent theoretical perspective based on configuration interaction and complex-energy formalisms that incorporate both reaction continuum and structural effects near threshold.