In Brief
Physicists working with the Large Hadron Collider have found an entire set of particles with four heavy quarks, further confirming the existence of tetraquarks. The four structures do not follow the characteristics of particles dictated by pre-existing laws of physics.

“It’s the First Time We’ve Seen This”

While technically protons have tons of quarks (and anti-quarks), three of those quarks, known as valence quarks, make up the positive charge of a proton. Hence, the three-quark label.

Throughout the history of physics, we have been familiar with two and three-quark particles. This made the recent discovery of four-quark particles called tetraquarks, and five-quark particles or pentaquarks, a slow uphill battle as it is met with severe skepticism. In 2003, the Belle experiment in Japan first observed particles in a four-quark state but lacked sufficient evidence to definitively prove it. Belle, Fermilab, and other research facilities since have announced similar observations, but none have been able to provide irrefutable proof of their existence.

Tetraquark in comparison with ordinary matter. Credit: Nature
Tetraquark in comparison with ordinary matter. Credit: Nature

In 2014, the Large Hadron Collider finally confirmed tetraquarks, and now has identified four more of these particles—a discovery that stands as solid evidence that would permanently cement their existence. “It was a long road to get here,” says University of Iowa physicist Kai Yi of the Collider Detector at Fermilab (CDF) and Columbia-MIT-Fermilab (CMF) experiments.

The exotic particles are named based on their respective masses in mega-electronvolts: X(4140), X(4274), X(4500) and X(4700). They are each composed entirely of heavy quarks: two charm quarks and two strange quarks arranged in a unique way, each with a different internal structure by mass and quantum numbers. “The quarks inside these particles behave like electrons inside atoms,” says Syracuse University physics professor Tomasz Skwarnicki says. “They can be ‘excited’ and jump into higher energy orbitals. The energy configuration of the quarks gives each particle its unique mass and identity.”

“What we have discovered is a unique system,” Skwarnicki continues. “We have four exotic particles of the same type; it’s the first time we have seen this and this discovery is already helping us distinguish between the theoretical models.”

Are They Even Particles?

Our current laws of physics cannot explain this groundbreaking discovery. “We looked at every known particle and process to make sure these four structures couldn’t be explained by any pre-existing physics. It was like baking a six-dimensional cake with 98 ingredients and no recipe—just a picture of a cake,” Syracuse University researcher Thomas Britton says.

The researchers are now working on models that would help make sense of these new particles, which may not even be particles, as they do not behave in accordance with our standard models of particles. “The molecular explanation does not fit with the data,” Skwarnicki adds. “But I personally would not conclude that these are definitely tightly bound states of four quarks. It could be possible that these are not even particles. The result could show the complex interplays of known particle pairs flippantly changing their identities.”

The bizarre particles (or whatever else they may eventually turn out to be) are possibly heralding a new era of expansion for quantum physics, thanks to the Large Hadron Collider. “The huge amount of data generated by the LHC is enabling a resurgence in searches for exotic particles and rare physical phenomena,” Britton says. “There’s so many possible things for us to find and I’m happy to be a part of it.”