Measurements of the branching fractions of \boldmath $χ_{cJ}\toΣ^0\barΣ^0η$

Deep within the fabric of the universe, matter is held together by a force so powerful it binds the very cores of atoms. This force, known as the strong interaction, is governed by a set of rules called quantum chromodynamics. While scientists have mastered the mathematics of this force at high energies, the low-energy realm remains a foggy landscape of mysteries. Here, particles do not behave like simple billiard balls; they swirl and interact in complex ways that are difficult to predict. To map this hidden territory, physicists often use special, short-lived particles called charmonium states. These are like tiny, heavy laboratories created in particle accelerators, where a charm quark and its antimatter partner, the anti-charm quark, orbit each other. When these pairs decay, they break apart into other particles, offering a glimpse into the rules that govern their disintegration. By studying exactly how these heavy pairs fall apart, researchers can test the limits of their current understanding and look for cracks in the theory that might point to new physics.

A researcher using the BESIII detector at the BEPCII collider in Beijing has recently illuminated a previously dark corner of this landscape. They analyzed a massive collection of data containing over 2.7 billion collisions of electrons and positrons. From this vast dataset, they focused on a specific type of heavy particle known as the chi-c, which comes in three slightly different varieties distinguished by their spin. The researcher was looking for a specific, rare event: the moment when one of these chi-c particles decays into a trio of other particles—a neutral sigma baryon, its antimatter twin, and a neutral eta meson. Until now, this specific decay had never been seen, either in experiments or in theoretical calculations. The researcher reconstructed these events by tracking the debris left behind, specifically looking for a signature pattern of protons, antiprotons, pions, and five photons that would confirm the presence of the desired particles.

After sifting through the billions of collisions, the researcher reported the first observations of this decay for all three varieties of the chi-c particle, with statistical significances of 7.2σ, 5.7σ, and 6.7σ respectively. While these results are strong enough to claim the first observations, the researcher noted that the evidence for one channel (5.7σ) is slightly below the typical 6σ threshold often used for definitive observations in some contexts. They measured how often this decay occurs relative to the total number of chi-c particles produced, determining the probability, or branching fraction, for each case. For the first variety, the decay happens roughly 9.7 times out of every 100,000 chi-c particles. For the second variety, it occurs about 4.9 times out of 100,000, and for the third, about 6.1 times out of 100,000. These numbers provide the first concrete data points for this specific interaction, allowing physicists to compare how the different spin states of the parent particle influence the outcome.

The researcher also examined the arrangement of the particles as they flew apart to see if they were forming any temporary, intermediate structures, such as a new type of excited particle. They looked closely at the mass combinations of the particles in the final group, searching for any unexpected clumps or resonances that would signal a new state of matter. The data showed no significant structures in the mass distributions of the particle pairs under the current statistics; the particles appeared to emerge directly from the decay without forming any intermediate steps. This absence of new structures is just as important as the discovery of the decay itself, as it rules out certain theoretical models that predicted such intermediate states would exist. Furthermore, by comparing these results with previous measurements of similar decays involving charged particles, the researcher found that the rates matched expectations based on the symmetry of nature known as isospin. This suggests that the strong force treats these different particle combinations with a consistent hand, reinforcing the standard model of particle physics.

The findings also offer a subtle clue about the internal mechanics of these decays. When the researcher compared the strength of the decay for the different spin varieties, they noticed that the first variety behaved differently than the other two. This difference in behavior hints that the first variety might be mixing with another, more exotic type of particle predicted by theory, or that it decays through a unique mechanism. While the data does not confirm exactly what is happening, it provides a crucial new piece of the puzzle. The discovery of these decays, along with the precise measurements of their rates, gives theorists a new benchmark to test their models against. It confirms that the strong force operates in a way that allows these specific combinations to form, while simultaneously narrowing the search for the deeper reasons behind the differences in how these particles break apart. As the field moves forward, these measurements will serve as a foundation for future experiments aiming to unravel the remaining mysteries of the strong interaction.