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For the first time, researchers directly observed a helium-7 nucleus emitting three neutrons at once.
A high-speed collision knocked a neutron out of a helium-8 nucleus, and for an instant a helium-7 nucleus appeared, like an overloaded shelf shedding three books at once. This fleeting 'sound' of decay was captured for the first time: three neutrons flew apart, leaving behind an ordinary helium-4 nucleus.
A helium-7 nucleus lives for less than a billionth of a second, but such moments hold the secrets of nuclear forces.
It turned out that the neutrons don't leave all at once but one by one: first, two form an excited helium-6 nucleus, and only then does the third fly out. The experiment found no special 'triple embrace' of neutrons, including the hypothetical trineutron — everything is explained by known pairwise interactions.
This work is like a precise recording of three drops falling from a leaf: physicists can now test their models of nuclear forces against reality at the very edge of stability.
