Resonant and Non-Resonant Three-Body Decay of Borromean ¹¹Li: A Monte Carlo Comparison

Borromean nuclei — bound three-body systems in which no two-body subsystem is itself bound — occupy a distinct position at the edge of nuclear stability; for example, ⁶He = α + n + n and ¹¹Li = ⁹Li + n + n owe their existence entirely to three-body correlations rather than pairwise binding. They serve as natural laboratories for the study of few-body quantum dynamics, continuum coupling, and universal physics. This article reviews the conceptual foundations of Borromean and halo nuclei, the variational conditions required for three-body binding, and the context provided by Efimov universality, before presenting the central result: a Monte Carlo comparison of resonant and non-resonant decay pathways in the breakup ¹¹Li → ⁹Li + n + n. Non-resonant, direct three-body phase-space decay is simulated at the experimental two-neutron separation energy S₂ₙ = 369 keV, while resonant sequential decay is modeled through the p₁/₂ ¹⁰Li resonance (E_r = 0.62 ± 0.04 MeV) identified in a recent one-neutron transfer measurement. The two mechanisms produce cleanly separated core-neutron summed-energy distributions — confined below the 369 keV kinematic limit for direct decay versus centered near 600–650 keV for resonant sequential decay — demonstrating that this observable is a sensitive, quantitative discriminator between resonant and non-resonant three-body decay mechanisms in Borromean nuclei.

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