In some types of helium and hydrogen, protons are more than six times as likely to pair up than they are in other atoms – which may mean there is something we don’t understand about the strong nuclear force

Physics 31 August 2022

Artist’s impression of a helium-3 atom


Inside the nucleus of some atoms, protons appear to be doing very unexpected things. They are pairing up far more often than usual when they get extremely close to each other, and physicists don’t fully understand why. Getting to the bottom of this phenomenon could help us better understand the strong nuclear force, which governs interactions on extremely small scales.

John Arrington at Lawrence Berkeley National Laboratory in California and his colleagues directed a beam of very energetic electrons at a target made of a lighter version of helium called helium-3 and tritium, the radioactive version of hydrogen, to get an insight into previously unexplored interactions between protons and neutrons in their nuclei.

When protons and neutrons inside a nucleus get as close to each other as a quadrillionth of a metre, they briefly pair up, then fly away with lots of momentum. Arrington says that by measuring the speed or energy of electrons in the beam ricocheting off the pairs, the researchers could count the number of particle duos that were either proton-proton or proton-neutron pairs.


The ultimate tally was unexpected, says Arrington. Similar experiments in the past that used atoms such as carbon or lead found that only about 3 per cent of pairings in each nucleus were between two protons, but for helium-3 and tritium, the researchers found that number to be closer to 20 per cent.

Arrington says that helium-3 and tritium nuclei are less tightly packed with particles than previously investigated nuclei, which may mean that particles approach each other closely less often, but with more preference for protons to pair up. Such an imbalance could be a property of how nuclear forces work at very small distances, which isn’t yet fully understood, he says.

Lawrence Weinstein at Old Dominion University in Virginia says that the large number of proton pairs may hint at some new wrinkle in the strong nuclear force, but that more refined and detailed theoretical models of the experiment must be developed before the finding is considered definitive.

Mark Strikman at Pennsylvania State University says that if future studies confirm these findings, they may influence how physicists think about neutron stars. Particles are packed so closely together in these stars that they are the densest objects in the universe. How massive a neutron star can be then partly depends on how neutrons and protons interact when they are so close to each other, says Strikman.

Journal reference: Nature, DOI: 10.1038/s41586-022-05007-2

Article amended on 31 August 2022

We corrected the percentage of proton-proton pairings in carbon and lead atoms

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