Everything around us has mass, but its origin remains one of physics’ biggest unsolved questions. According to modern theory, mass does not simply arise from matter itself. Instead, it is connected to the nature of a vacuum, not an empty space but a dynamic environment with a complex structure. Studying special particle systems can help scientists better understand this hidden framework and how mass is generated.
One promising approach involves mesons, particles consisting of quarks and antiquarks bound to an atomic nucleus. This combination is known as a mesic nucleus. By studying these systems, researchers can study the structure of the vacuum and the mechanisms that give mass to particles. Now, new experimental results have uncovered evidence for an entirely new type of mesic nucleus, bringing scientists closer to that goal.
Evidence of rare and exotic particle states
An international research team has reported the signature of a never-before-seen, theoretically predicted state called an η’mesic nucleus. Their findings, to be published in Physical Review Letters, indicate that this unusual binding system may exist.
Under certain conditions, short-lived particles known as mesons (which exist for less than 1/10 millionth of a second) can become temporarily trapped within an atomic nucleus. When this happens, a rare and exotic condition is formed. Studying these mesic nuclei helps scientists understand how the strong nuclear force behaves and how the vacuum changes in extremely dense environments.
“One particularly interesting particle is the η’ meson,” says senior author Kenta Itabashi. “Because it is unusually heavy compared to related particles, physicists expect its mass to change when it resides in nuclear material. Observation of this phenomenon could provide valuable information about how particle mass is generated in the universe.”
High-precision experiments inside particle accelerators
To search for η’-mesic nuclei, the research team conducted high-precision experiments at the GSI Helmholtzzentrum für Schwerionenforschung in Germany.
The researchers aimed a beam of high-energy protons at a carbon target. This process excited the carbon nucleus and produced η’ mesons, which in some cases became attached to the nucleus. To study these interactions, the researchers measured the excitation energy of carbon nuclei by analyzing deuterons, the simplest atomic nuclei consisting of one proton and one neutron, released during the reaction. These measurements were made using a high-resolution spectrometer called a fragment separator (FRS).
The experiment also relied on a special detector known as WASA, originally developed in Uppsala, Sweden. The device allowed scientists to detect high-energy protons leaving their target and identify a signal indicating that η’ mesons were produced and trapped within the nucleus. These signals, known as decay signatures, were important for identifying exotic conditions.
“Using a new experimental setup that combines FRS and WASA, we are able to identify structures in our data that are consistent with the theoretical features of η’-mesic nuclei,” explains lead author Ryohei Sekiya. “Our analysis suggests that these bound states did indeed form.”
What the results reveal about mass
The experimentally measured excitation spectra of carbon nuclei show a pattern consistent with the formation of η’-mesic nuclei. This result also suggests that if the η’ meson is inside the nuclear material, its mass may decrease. This finding supports theoretical predictions and provides valuable experimental insight into how particle properties change under extreme conditions.
“Our measurements provide important new clues about how mesons behave in nuclear material,” Itabashi says. “This brings us closer to answering deep, fundamental questions about how matter acquires mass and how the vacuum structure inside the nucleus changes.”
what happens next
The research team plans to perform further experiments to improve the accuracy of their measurements and look for additional decay signals that could confirm the presence of η’mesic nuclei. Each new result helps further our understanding of the fundamental laws governing matter and the universe.
The paper “Excitation spectrum of 12C(p,d) reaction near the η’ meson emission threshold measured simultaneously with high-momentum protons” is physical review letter.
