Physicists find signs of four-neutron nucleus

Potential existence of ‘tetraneutron’ defies theoretical expectations


NO PROTONS ALLOWED  The collision of two varieties of helium nuclei at a Japanese lab produced beryllium nuclei and tetraneutrons, nuclei with four neutrons but no protons. This illustration depicts the number of protons and neutrons but not their actual positions within each nucleus.

Source: APS; Credit: J. Hirshfeld

The suspected discovery of an atomic nucleus with four neutrons but no protons has physicists scratching their heads. If confirmed by further experiments, this “tetraneutron” would be the first example of an uncharged nucleus, something that many theorists say should not exist. “It would be something of a sensation,” says Peter Schuck, a nuclear theorist at the National Center for Scientific Research in France who was not involved in the work. Details on the tetraneutron appear in the Feb. 5 Physical Review Letters.

Researchers spotted the signature of tetraneutrons at RIKEN in Wako, Japan, after firing a beam of neutron-rich helium nuclei (two protons, six neutrons) at a liquid composed of the most common form of helium (two protons, two neutrons). Occasionally, the reaction produced beryllium nuclei with four protons and four neutrons, leaving four neutrons missing in action. Although the scientists could not see the other product directly, its properties fit the description of a clumped neutron quartet. The four-neutron nuclei lasted about a billionth of a trillionth of a second before decaying into other particles.

Physicists will need to see more detections before agreeing that tetraneutrons exist, though Schuck says this study offers better evidence than several past claims. Tetraneutrons are puzzling because neutrons should not cluster unless there are protons too. Theorists would probably have to propose some kind of interneutron force to explain the exotic nuclei, Schuck says.

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