Physics Breakthrough in 50-Year Search for 'Holy Grail' Particle Tetraquark

An abstract artist's illustration shows a high-energy hadron collision. LiveScience/Giroscience/Shutterstock

It's been 100 years since scientists first split open the atom and realized it was not the smallest thing to exist. It's been more than 50 years since they started to suspect that the atom's components— protons, neutrons, and electrons—weren't either. They have components of their own, called quarks, which are particles so tiny they're almost impossible to study.

A combination of three quarks creates a hadron, which, when stable, we know as a proton or neutron. But a combination of four quarks is something scientists have never been able to prove exists. That would be the tetraquark, which scientists have been chasing without success since the 1960s.

Now, researchers are about to publish what they say is proof that the tetraquark exists, that it's stable, and that they can generate it in the Large Hadron Collider at the CERN particle physics lab in Switzerland.

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Quarks themselves come in six varieties: up, down, charm, top, strange, and bottom. While the origins of those names are delightful, the more relevant information you need to know about them is that 'up' and 'down' are lighter than the other four, which means they're not as good at bonding into hadrons. The heavier ones can better form into three-quark groups in various combinations, but according to laws of physics they shouldn't be stable enough to do groups of four.

But because quarks are complex and strange, they're subject an extra charge besides the regular positive/negative that holds atoms together. This is called the strong force, which has charges of its own that come in red, green, and blue—all of which sounds vaguely like Star Wars fan fiction but is not. To form a four-quark hadron, the colors would need to be compatible in addition to everything else, and even then it could still be unstable.

"The suspicion had been for many years that [the tetraquark] is impossible," physicist Marek Karliner of Tel Aviv University told LiveScience.

Karliner, along with Jonathan Rosner of the University of Chicago, calculated the outcome of different quark combinations and found one that theoretically could remain stable: doubly charmed baryon and double-bottom baryon. They then took their research to CERN, and over the summer found the first doubly charmed baryons, making the first real road map to creating a stable tetraquark. A paper to be published in a forthcoming issue of the journal Physical Review Letters.

"The upshot of all this is that we now have a robust prediction for the mass of this object which had been the holy grail of this branch of theoretical physics," Karliner told LiveScience.

From here, the research moves into the Large Hadron Collider particle smasher. Karliner predicts two-bottom-quark baryon will follow within the next couple of years. After that, all the pieces will be in place. The next big discovery should be the tetraquark itself.