This exotic particle could finally explain why matter has mass

One promising approach involves mesons, which are particles made of a quark and an anti-quark, bound together with an atomic nucleus. This combination is known as a mesic nucleus. By examining these systems, researchers can probe the vacuum structure and the mechanisms that give particles their mass. Now, new experimental results have brought scientists closer to that goal by revealing evidence for a completely new type of mesic nucleus.

Evidence for a Rare and Exotic Particle State

An international team of researchers has reported signs of a previously unseen but theoretically predicted state called an η′-mesic nucleus. Their findings, which will appear in Physical Review Letters, point to the possible existence of this unusual bound system.

Under certain conditions, short-lived particles known as mesons, which exist for less than ten-millionth of a second, can become temporarily trapped inside an atomic nucleus. When this happens, they form a rare and exotic state. Studying these mesic nuclei can help scientists understand how the strong nuclear force behaves and how the vacuum changes in extremely dense environments.

"One particle of particular interest is the η′ meson," says senior author Kenta Itahashi. "It is unusually heavy compared with related particles, and physicists expect that its mass changes when it exists inside nuclear matter. Observing this phenomenon would provide valuable information about how particle masses are generated in the universe."

High Precision Experiment Inside a Particle Accelerator

To search for η′-mesic nuclei, the team carried out a high-precision experiment at the GSI Helmholtzzentrum für Schwerionenforschung in Germany.

The researchers directed a beam of high-energy protons onto a carbon target. This process excited the carbon nuclei and produced η′ mesons, which in some cases became bound to the nucleus. To study these interactions, the team measured the excitation energy of the carbon nuclei by analyzing deuterons -the simplest atomic nucleus made of one proton and one neutron- that were emitted during the reaction. These measurements were made using a high-resolution spectrometer called the Fragment Separator (FRS).

The experiment also relied on a specialized detector known as WASA, originally developed in Uppsala, Sweden. This device allowed scientists to detect high-energy protons leaving the target and identify signals that indicate an η′ meson had been created and captured within the nucleus. These signals, known as decay signatures, were critical for identifying the exotic state.

"With our new experimental setup combining the FRS and the WASA, we can identify structures in the data that match theoretical signatures of η′-mesic nuclei," explains lead author Ryohei Sekiya. "Our analysis suggests that these bound states were indeed formed."

What the Results Reveal About Mass

The excitation spectrum of the carbon nucleus measured in the experiment shows patterns consistent with the formation of η′-mesic nuclei. The results also suggest that the mass of the η′ meson may decrease when it is inside nuclear matter. This finding supports theoretical predictions and offers rare experimental insight into how particle properties can change under extreme conditions.

"Our measurements provide important new clues about how mesons behave in nuclear matter," says Itahashi. "This brings us closer to answering deep, fundamental questions about how matter acquires mass, as well as how the vacuum structure changes inside atomic nuclei."

What Comes Next

The team plans to carry out further experiments to improve measurement accuracy and look for additional decay signals that could confirm the existence of η′-mesic nuclei. Each new result will help refine our understanding of the fundamental laws that govern matter and the universe.

The article, "Excitation Spectra of the 12C(p,d) Reaction near the η'-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons," was published in Physical Review Letters.