After years of failed searches, CERN has finally caught a needle in a subatomic haystack: a heavy relative of the proton called Ξcc⁺. Ξ (or Xi) is a Greek letter pronounced like the word “Zye” (rhymes with “eye” or “pie”).
This discovery lands at the crossroads of an old mystery and a new machine. Scientists have been trying to detect it for two decades, but the particle stubbornly evaded detection. Its appearance now proves that CERN’s upgraded detectors are working perfectly. More importantly, it gives us a new lens through which to view the “strong force”—the cosmic glue that builds every atom in existence.
A Rare Cousin of the Proton
The Ξcc⁺ belongs to the same family as the proton and neutron, the particles found in the nucleus of atoms.
A proton may seem simple, but it is built from three even smaller particles called quarks (two up quarks and one down quark). Quarks themselves come in six types (up, down, charm, strange, top, and bottom). Ordinary matter, everything we see around us, is built from the lightest ones (up and down). The heavier types of quarks are rarer and far less stable.
Ξcc⁺ is unusual because while it has one down quark (like the proton), instead of the proton’s two up quarks, it has two charm quarks. This makes it a much heavier and extremely rare cousin of the proton.
Researchers study these particles because they allow them to test ideas about how matter holds together. But these particles are extremely difficult to find.
The Long Hunt
Physicists don’t just go randomly looking for particles. They had good reasons to think the Ξcc⁺ should exist.
A closely related particle had already turned up at LHCb in 2017: the Ξcc⁺⁺. It contains the same pair of heavy charm quarks as the newly discovered Ξcc⁺, but pairs them with an up quark instead of a down quark. Because up and down quarks are very similar, physicists expected the two particles to have nearly the same mass. That made the absence of the Ξcc⁺ increasingly hard to explain.
The particle should be there, and yet, it refused to appear.
For more than 20 years, it sat in an awkward category: expected by theory, hinted at in the past, but never cleanly confirmed. When that kind of gap emerges, scientists start to wonder whether the theory is wrong or whether there’s a problem with the experiment. As it turns out, we needed was a better detector.
Now the upgraded LHCb detector has produced a signal strong enough for the collaboration to report the particle’s observation.
“This is just the first of many expected insights that can be gained with the new LHCb detector,” said Prof Tim Gershon at the University of Warwick, according to The Guardian. “The improved detection capability allowed us to find the particle after only one year, while we could not see it in a decade of data collected with the original LHCb.”
Why anyone beyond particle physics should care
The very fact that Ξcc⁺ exists is important.
There are four fundamental forces in nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. These forces govern everything that happens in the universe, from the subatomic scale to the intergalactic events. The strong nuclear force, as the name implies, is the strongest one, but it only acts over extremely short distances. It’s what holds protons and neutrons together in the atomic nucleus and acts only over extremely short distances.
Because the scale is so small, it’s difficult to study this force in detail. Rare heavy particles such as the Ξcc⁺ give scientists a new way to test their ideas.
“The result will help theorists test models of quantum chromodynamics, the theory of the strong force that binds quarks into not only conventional baryons and mesons but also more exotic hadrons such as tetraquarks and pentaquarks,” said LHCb spokesperson Vincenzo Vagnoni.
In other words, the more kinds of particles scientists can confirm and compare, the better they can check whether their picture of matter is complete. That is why this discovery has real appeal. It closes a long-running search, gives CERN’s upgraded detector an early success, and opens another route into one of physics’ deepest questions: how the basic pieces of matter hold together at all.