First measurements of the branching fractions of and
Using data collected by the BESIII detector, this paper reports the first observation of the hadronic decays and , measuring their branching fractions and finding that their ratio is consistent with the 12%-rule while revealing no significant intermediate states or threshold enhancements.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the subatomic world, matter is built from a handful of fundamental particles that interact through forces far more complex than gravity or magnetism. Among these, a family of particles called charmonia acts as a unique laboratory for physicists. These are heavy, short-lived particles made of a charm quark and its antimatter twin, bound together so tightly that they serve as a clean starting point to study how the strong nuclear force—the glue that holds atomic nuclei together—behaves when it shifts from a predictable, mathematical regime to a chaotic, messy one. When these heavy particles decay, they break apart into lighter, more common particles, such as protons, neutrons, and their cousins, the hyperons. By watching exactly how often a specific heavy particle breaks into a specific set of lighter ones, scientists can test a long-standing rule of thumb. This rule suggests that if a heavier version of the particle decays into a certain combination of fragments, it should do so at a rate roughly twelve percent of the rate of its lighter sibling. For decades, this "twelve-percent rule" has held true for many decays, but it has also been famously broken in a few specific cases, leaving physicists puzzled about what hidden forces or new particles might be interfering.
A team of researchers using the BESIII detector at the BEPCII collider in China has now added a new chapter to this story by observing a rare decay that had never been seen before. They examined billions of collisions involving two specific heavy particles, known as J/psi and psi(3686), which are essentially the lighter and heavier siblings in the charmonium family. In their massive dataset, containing over ten billion J/psi events and more than two billion psi(3686) events, they looked for a very specific outcome: the simultaneous creation of a neutral eta meson and a pair of neutral sigma hyperons, one made of matter and the other of antimatter. This is a difficult process to catch because the particles involved are unstable and decay almost instantly into other things, requiring the researchers to piece together the original event from a trail of secondary particles like photons, protons, and pions. By carefully filtering through the noise of background collisions and reconstructing the paths of these fleeting fragments, the team successfully identified the first clear evidence of this specific decay happening in nature.
The researchers found that the lighter J/psi particle produced this specific trio of fragments in about seven and a half out of every hundred thousand decays. The heavier psi(3686) particle did the same, but much more rarely, appearing in about one and three out of every hundred thousand decays. When they compared these two rates, they calculated a ratio of roughly seventeen percent. This result is significant because it aligns closely with the expected twelve-percent rule, differing by only a small margin that falls within the range of normal statistical fluctuation. This finding suggests that, unlike some other rare decays that have defied expectations, this particular process follows the standard patterns predicted by theory, offering a rare moment of consistency in a field often defined by surprises.
Beyond simply counting the events, the team also looked closely at the energy and mass of the particles involved to see if there were any hidden structures lurking in the data. Sometimes, when particles decay, they pass through a brief, intermediate state where they form a temporary, excited version of a known particle before breaking apart completely. The researchers scanned the data for signs of these intermediate states or for sudden spikes in the number of events at specific energy levels, which could indicate the presence of new, undiscovered particles. They found nothing of the sort. The data showed a smooth distribution with no unexpected bumps or peaks, indicating that the decay happens directly without forming any exotic, short-lived intermediates. This absence of hidden structures is itself an important result, as it helps rule out several theoretical models that predicted such features would be present.
The measurement was made possible by the sheer volume of data collected and the precision of the detector, which can track the paths of charged particles and measure the energy of light particles with extreme accuracy. The team had to account for many potential sources of error, from the efficiency of their detectors to the mathematical models used to simulate the collisions, ensuring that their final numbers were robust. They confirmed that the difference between the observed rates and the theoretical prediction is not large enough to claim a violation of the rule, nor is it large enough to suggest a new discovery. Instead, the work provides a solid, measured data point that fits neatly into the existing framework of particle physics. By confirming that this specific decay follows the expected pattern and showing no signs of exotic interference, the study helps refine our understanding of how the strong force operates in the transition between the orderly and the chaotic, adding a reliable piece to the complex puzzle of the subatomic world.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.