First determination of fragmentation functions in an exotic-hadron candidate

For decades, physicists have mapped the building blocks of matter using a simple rulebook: protons and neutrons are made of three quarks, while other particles like pions are made of a quark and an antiquark. This model worked so well that when scientists began finding particles that didn't fit these patterns, they called them "exotic." These strange candidates seemed to be made of four or five quarks, or perhaps molecules of other particles held together loosely. But proving what these exotic particles actually are has been a stubborn puzzle. The usual way to identify a particle—measuring its weight, spin, or how it falls apart—often isn't enough to distinguish between a true four-quark state and a more complex arrangement. To solve this, researchers needed to look at these particles not just as static objects, but as dynamic systems formed at incredibly high speeds, where the fundamental ingredients of quarks and gluons become visible.

A researcher has now taken a fresh look at one of the most famous exotic candidates, a particle called the f0(980). For years, low-energy experiments suggested this particle was likely a tetraquark, a cluster of four quarks, or a molecule made of two other particles bound together. However, a new analysis of high-energy collision data tells a different story. By examining how the f0(980) is created when electrons and positrons smash together, the scientist determined the probability of this particle forming from different types of quarks. Their findings suggest that at high energies, the f0(980) behaves not like a four-quark exotic, but rather like a simple pair of strange quarks. This discovery does not necessarily mean the old low-energy theories were wrong; instead, it points to a fascinating possibility that the internal structure of a particle can change depending on the energy at which it is observed.

The key to this discovery lies in a concept called fragmentation functions. When high-energy particles collide, they do not simply bounce off each other; they shatter into a spray of new particles. The fragmentation function is essentially a map that tells us how likely it is for a specific type of quark or gluon to turn into a specific final particle. Think of it like a recipe that lists the probability of a chef turning a specific ingredient into a finished dish. For common particles like protons or pions, scientists have already mapped these probabilities with great precision. But for exotic candidates like the f0(980), the map was blank because there simply wasn't enough data. Until recently, the measurements were too scattered and imprecise to draw a clear picture.

The situation changed dramatically with new, highly accurate measurements from the Belle collaboration in 2025. Using a powerful particle accelerator, they recorded thousands of events where electrons and positrons collided to produce the f0(980). The researcher combined this new data with older measurements from other laboratories to perform a global analysis. They tested four different theoretical models for what the f0(980) could be: a simple pair of up and down quarks, a pair of strange quarks, a four-quark cluster, or a glueball made entirely of gluons. By fitting the experimental data to these models, they calculated the "second moment" of the fragmentation functions, a statistical value that reveals how much of the particle's energy comes from each type of quark.

The results were striking. The analysis showed that the f0(980) is overwhelmingly likely to be formed from strange quarks, while the probability of it forming from up or down quarks is significantly smaller. Furthermore, the data indicated that the strange quarks carry a significantly larger share of the particle's energy compared to the other components. If the f0(980) were truly a four-quark state or a molecule as low-energy studies had suggested, the data would have shown a much more balanced mix of quark types. Instead, the pattern matched perfectly with a simple pair of strange quarks. The researcher also noted that the strange quark contribution was concentrated in a specific region of energy distribution, a signature that strongly supports the idea of a direct, favored formation process rather than a complex, multi-step assembly.

This finding creates an apparent paradox. Low-energy experiments, which look at the particle when it is relatively calm, strongly suggest it is a tetraquark or a molecular state. Yet, this high-energy analysis suggests it is a simple quark-antiquark pair. The author proposes that these two pictures are not contradictory but are two sides of the same coin. Just as a material might look like a solid block from a distance but reveal a complex lattice structure under a microscope, the f0(980) might appear as a complex four-quark state at low energies but resolve into a simple quark pair when probed at high energies. This idea is supported by similar observations in other particles, such as the Lambda(1405), which also appears to shift its internal configuration depending on the energy scale.

The study does not claim to have solved the mystery of exotic hadrons once and for all, but it provides the first clear evidence that the internal structure of these particles can transition as energy increases. It suggests that the "exotic" nature of these particles might be a low-energy phenomenon, while at the high energies where the fundamental rules of quantum chromodynamics dominate, they may revert to the standard configurations of ordinary matter. This opens a new direction for physics, suggesting that the distinction between ordinary and exotic hadrons might not be a fixed property, but a fluid one that depends on how we look at them. The work relies on the precision of the 2025 Belle data, which allowed the researcher to determine these fragmentation functions with an accuracy that was impossible just a few years ago, turning a vague hypothesis into a concrete, data-driven picture of the particle's true nature.