An unstable atomic nucleus is like a drop of water on a sizzling skillet: it exists for only a moment, and its shape is constantly changing. Physicists have learned to describe such drops — nuclei that live for less than a billionth of a second and decay almost as soon as they are born.
Standard calculation methods fail for these nuclei because there are too many possible ways the particles inside can move and interact. The authors of the new work have brought order to this chaos. They proposed a new way to sort the possibilities, as if in a huge library, books were arranged not alphabetically but by how often readers request them. This sped up the calculations by tens of times and, for the first time, allowed them to directly compute the ground state of hydrogen-5—a system of one proton and four neutrons that decays in a mere 10⁻²² seconds.
The new approach opens the way to systematically testing theories of nuclear forces on the most exotic nuclei. When theory and experiment converge, we will better understand how matter behaves in neutron stars — where nuclei are compressed to the limit and obey the same quantum laws.
as written by the authors arXiv:2601.16168
\textbf{Background} The reach of \textit{ab initio} theory has greatly increased in recent decades. However, predicting the location of the drip lines remains challenging due to uncertainties in nuclear forces and difficulties in describing nuclei that behave as open quantum systems. \textbf{Purpose} In this work, we extend the \textit{ab initio} Gamow Density Matrix Renormalization Group (G-DMRG) approach to the regime of broad many-body resonances to pave the way for systematic tests of nuclear forces in light exotic nuclei. \textbf{Methods} To stabilize calculations, we introduce a new truncation scheme in the reference space, and propose an orbital ordering based on entanglement considerations. We then show how continuum couplings increase entanglement in the many-body problem, and propose a new truncation scheme to stabilize the renormalization and accelerate calculations in extreme conditions. Finally, we demonstrate that natural orbitals can be used to efficiently describe broad resonances by introducing a new ordering scheme and by redefining the reference space based on occupations. \textbf{Results} Leveraging our findings, we propose a recipe to converge \textit{ab initio} G-DMRG calculations and apply it in low-lying states of \isotope[5,6]{He} and \isotope[4]{H}, demonstrating control of the renormalization and the emergence of convergence patterns. We also obtain the first direct \textit{ab initio} calculation of the $$J^\pi = {1/2}^+$$ ground state of \isotope[5]{H}. \textbf{Conclusions} We demonstrate that entanglement due to continuum couplings can be controlled in extreme conditions and successfully extend the G-DMRG approach in the regime of broad many-body resonances.
The helium-5 nucleus, also calculated in the work, decays into an alpha particle and a neutron in 10⁻²¹ seconds, a billion times faster than a human blink.
In the series 'The Expanse', exotic nuclei are mentioned that can only exist in the depths of neutron stars.