Here’s what you’ll learn when you read this story:

• Ordinary atoms need both the strong force and the electromagnetic force to hold together, but physicists have long wondered whether a nuclear system could be bound by the strong force alone.
• Because the eta prime meson carries no electric charge, it cannot be bound electromagnetically—only via the strong interaction—making it the ideal candidate for such a system.
• By firing a proton beam at carbon-12, researchers created an eta prime meson that briefly bound to a carbon-11 nucleus, forming the first observed system held together by the strong force alone—and offering clues about where the mass of matter comes from.

For decades, every known atomic and nuclear system has relied on at least two fundamental forces working in concert: the strong force binds protons and neutrons inside the nucleus, while electromagnetism holds electrons in orbit around it. Now, an international team of physicists has found the first experimental evidence of a nuclear system bound exclusively by the strong force—confirming a theoretical prediction made twenty years ago and opening a new window onto how matter acquires mass.

Creating a system held together by only one force required a particle with a special property: no electric charge. Ordinary atoms can’t do the job because their components—protons and electrons—are electrically charged, so electromagnetism is always in play. The Standard Model of particle physics, which describes three of the four fundamental forces (the strong force, the weak force, and electromagnetism—gravity isn’t included), predicts that electrically neutral mesons should be able to bind to a nucleus through the strong interaction alone. The eta prime meson (η′) is the ideal test case: it carries no electric charge, so it can’t be bound electromagnetically, and its unusually large mass makes it a uniquely sensitive probe of the strong force’s inner workings.

That large mass is itself a mystery. Known as the U(1) problem since physicist Steven Weinberg raised it in the 1970s, the puzzle is that simple quark models can’t account for how heavy the eta prime is. Modern quantum chromodynamics (QCD)—the theory of the strong force—attributes the extra mass to a phenomenon called chiral symmetry breaking, combined with the quantum dynamics of gluons, the particles that carry the strong force between quarks. In simple terms, “chirality” refers to the handedness of particles: just as your right hand looks like a left hand in a mirror and cannot be superimposed onto it, certain particles come in right-handed and left-handed versions. When the symmetry between those versions breaks down inside nuclear matter, it generates much of the eta prime's mass.

The same theories that predict how massive eta prime mesons are also suggest their mass can be reduced when embedded inside a nucleus. Measuring that mass reduction would be powerful evidence that chiral symmetry breaking really is responsible for the particle's heft—and, by extension, for much of the mass of all hadrons (the protons and neutrons that make up visible matter). Now, an international team co-led by physicists Ryohei Sekiya, Kenta Itahashi, and Yoshiki Tanaka of RIKEN in Saitama, Japan, has actually done just that. They fired a proton beam at the nucleus of an atom of carbon isotope C-12 at velocities a fraction of the speed of light, and this onslaught of protons knocked out a neutron. When that neutron merged with a proton, it formed a stable deuteron that zoomed away. It also left a ridiculous amount of energy in the nucleus of what had just turned into a C-11 atom.

That excess energy can give rise to an eta prime meson which, in rare cases, binds to the carbon-11 nucleus, forming a short-lived quantum state held together by the strong force alone. The eta prime meson only exists for about a thousandth of a trillionth of a trillionth of a second before it decays. Catching such fleeting events required a specially designed detector called WASA, which allowed the team to identify the high-energy protons produced when the eta prime is absorbed by the nucleus. Even so, background events outnumbered signals by a factor of 100 to 1,000, making the measurement extraordinarily challenging.

Despite those odds, the team found peak structures in their data that match theoretical predictions for eta prime–mesic nuclei. Their results, published in Physical Review Letters, indicate that the eta prime meson’s mass drops by about 60 MeV inside nuclear matter—qualitative support for the idea that chiral symmetry breaking and gluon dynamics are responsible for its mass.

“Investigations of the mass distribution in a chiral-symmetry-restored environment provide information on the mass generation mechanisms and the non-trivial structure of the vacuum in the evolution of the universe,” the researchers wrote.

“The results indicate the first direct detection of [eta prime]-mesic nuclei, which provide information on the meson properties in a high-density nuclear medium,” the researchers said, adding that they “aim at measurement of the elementary [eta prime] production cross section” in the future.