Hints of a Nucleus Irked by a Meson Houseguest

Michael Schirber

Synopsis

Hints of a Nucleus Irked by a Meson Houseguest

April 7, 2026• Physics 19, s27

Experiments with a proton beam striking a carbon target have uncovered events that may be due to a short-lived meson residing within a nucleus.

Researchers from two large international collaborations have found the first experimental indication of a nucleus temporarily hosting a heavy two-quark particle called the $𝜂^{'}$ meson [1]. The results remain preliminary, but if confirmed, this exotic nucleus could give new insights into quantum chromodynamics (QCD), the theory of the strong force that binds quarks together.

Among mesons, the $𝜂^{'}$ meson is an oddball: Its mass of 958 MeV/c² is roughly 7 times greater than that of its close cousin, the pion. Explaining the large mass has challenged theorists since the 1970s [2]. The current understanding is that the $𝜂^{'}$ meson gains some extra mass from the vacuum. Specifically, certain QCD symmetries are broken in the vacuum, leading to a Higgs-like mechanism that imparts mass to the $𝜂^{'}$ meson. But models disagree over the mass gained.

One way to test these models is to search for a nucleus with a bound $𝜂^{'}$ meson. “One can imagine that the $𝜂^{'}$ meson is swimming in the core of the nucleus,” says Kenta Itahashi of the $𝜂$ -PRiME Collaboration and the Super-FRS Experiment Collaboration. In that “nonvacuum” environment, the QCD symmetries are partially restored, and the $𝜂^{'}$ meson would presumably have less mass. Previous searches that bombarded carbon-12 nuclei with protons were confounded by high background rates.

Now Itahashi and colleagues have reperformed the proton–carbon experiment at the GSI Helmholtz Center for Heavy Ion Research in Germany. Using a new background-suppressing detector, they recorded events in which a proton removes a neutron from a carbon-12 nucleus and produces an outgoing deuteron (deuterium nucleus). When the researchers measured the energies of these deuterons, they found spectral features that hinted at the fleeting formation of $𝜂^{'}$ -hosting nuclei in the carbon target. The results suggest an $𝜂^{'}$ -mass-reduction of 60 MeV/c², which could bolster certain QCD models.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.

References

R. Sekiya et al. (η-PRiME Collaboration and Super-FRS Experiment Collaboration), “Excitation spectra of the ¹²C(p,d) reaction near the $𝜂^{'}$ -meson emission threshold measured in coincidence with high-momentum protons,” Phys. Rev. Lett. 136, 142501 (2026).
S. Weinberg, “The U(1) problem,” Phys. Rev. D 11, 3583 (1975).