The helium-5 nucleus is like an overtightened string: touch it, and it falls apart into helium-4 and a neutron. Catching such a decay and measuring its rate is no easy task. Standard calculation methods stumble over mathematical noise.
The researchers took a reliable approach to describing nuclear forces and added a clever trick — analytic continuation. It's like guessing how long a swing will keep swinging from its behavior in the first few seconds. To keep the calculation on track, they used a special mathematical filter that cuts off errors.
In the end, for helium-5 they obtained a decay energy of 0.80 MeV and a width of 1.05 MeV. The width is a measure of how quickly the nucleus falls apart: the wider, the shorter the life. Experiments give 0.798 MeV and 0.648 MeV — very close.
Now this method can be applied to other invisible nuclei that exist only for an instant on the edge of stability. It will help us understand how matter is structured in the most extreme corners of the universe.
The helium-5 nucleus is so unstable that it cannot be found in nature — only created in an accelerator for a fraction of a second.
In Poul Anderson's novel Tau Zero, a spaceship accelerates so fast that matter around it becomes unstable — the physics of exotic nuclei there is almost like the real thing.