Study narrows the search for particles that hold secrets of matter
A University of Melbourne researcher has placed the strongest constraints yet on certain rare decays of subatomic particles, narrowing the window for where new "hidden" particles could be lurking.
In research published in Physical Review Letters on 12 June, 2026, Dr Daniel Marcantonio analysed data from the Belle experiment to search for ‘feebly interacting particles’ (FIPs) – a broad class of hypothetical particles that interact extremely rarely with ordinary matter.
FIPs are predicted by many theories that extend our current understanding of particle physics, and some could serve as candidates for dark matter or as messengers between ordinary matter and a hypothetical "dark sector".
The analysis used data from the Belle particle physics experiment at the KEK laboratory in Japan, which collided electrons with their antimatter counterparts, positrons. The collision energy was tuned to produce large numbers of B mesons – heavy, unstable particles containing a bottom quark. B mesons are of particular interest because their decays can be sensitive to the effects of new, yet-undiscovered particles.
Daniel's analysis searched for an invisible new particle produced in B meson decays, accompanied by a known particle such as a pion, kaon, proton, or heavier meson. Five different decay channels were studied, three of which had never been searched for before.
Daniel said no evidence for new particles was found in any of the five decay channels – but in particle physics, not finding something can be just as valuable as finding it.
The logic works by exclusion: if a new particle existed and interacted strongly enough with ordinary matter, it would have shown up as a tell-tale spike in the data. Since no such spike was observed, the analysis can place upper limits on how often these decays can occur. The stronger the interaction, the more decays would be expected, so the absence of a signal rules out interactions above a certain strength. These are the tightest such constraints to date for all five channels.
These limits on decay rates can be translated into constraints on the interaction strength between known particles and various proposed new particles, including axion-like particles and dark scalars. This does not rule out these particles entirely (they could still exist with weaker interactions) but it narrows the range of possibilities that future experiments need to explore.
The search was made possible by Belle's enormous dataset of 711 inverse femtobarns of electron-positron collision data, containing over 770 million pairs of B mesons. The analysis used a technique called "B-tagging", in which one B meson in each event is fully reconstructed, allowing the properties of the other B meson's decay to be tightly constrained.
"Because this is the first search for several of these decay channels, I hope it will motivate further exploration of even more variations of these decays, both at Belle II and at other experiments."
The results also have implications for understanding why the universe contains far more matter than antimatter. One of the decay channels studied, involving a proton in the final state, can constrain a theoretical mechanism called "B-mesogenesis", in which B meson decays in the early universe could have funnelled antimatter into a dark sector, helping to explain the matter-dominated universe we observe today. The analysis rules out this mechanism for a range of masses of the hypothetical dark-sector particle involved.
Daniel believes the findings will help focus future searches for physics beyond the Standard Model.
"These results are applicable to more theoretical models than we were able to cover in the paper, and I hope the broader community will use them to constrain even more scenarios than we have considered."