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Researchers recently discovered evidence of a rare quirk of matter buried in data almost a decade old—the πK atom. Observations of these manufactured atoms could help scientists understand how some particles lose their electric charge.

Physicists have long suspected that pion (π) and kaon (K) particles could stick together to form πK atoms, but no one had ever seen such an exotic particle pairing. It’s rare enough to spot pions and kaons on their own—scientists have only ever witnessed these exotic particles in high-energy radiation inside atom smashers or from outer space.

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“You can’t buy a bottle of pions or kaons in any store,” said Gerald Miller, a physicist at the University of Washington who was not involved in the new study.

Physicists first thought they’d glimpsed πK atoms splintering out of particle collisions in 2007. The researchers were working on the Dimeson Relativistic Atom Complex (DIRAC) experiment at CERN, shooting protons into metal foil at nearly the speed of light.

The DIRAC experiment is fed by CERN's Proton Synchrotron.

But the scientists analyzed the data “quickly by using only part of the detectors,” said Juerg Schacher, a physicist at the University of Bern and co-author on the new study, so they weren’t certain of their discovery.

Now, CERN researchers have re-examined the data from 2007 as well as data collected in similar experiments between 2008 and 2010. This time around, using observations from all of DIRAC’s sub-detectors and a more sophisticated method of data analysis, the scientists saw clear evidence of hundreds of πK atoms. Their work is described in a paper recently published in Physical Review Letters .

“Of course, we were excited to be able to demonstrate in a sophisticated experiment the existence of such [an] exotic atom,” Schacher said.

Now that they have found πK atoms lurking in near-decade-old data, physicists want to see what these particle duos reveal about a fundamental theory of nature called quantum chromodynamics (QCD).

QCD is a mathematical theory that describes how quarks—the building blocks that construct pions and kaons, as well as familiar particles like protons and neutrons—stick together to form matter in the universe. That is, if the complete picture of how quarks behave in nature is a puzzle, then QCD is the box cover that tells physicists what the finished puzzle should look like. Unfortunately, the math of QCD is so gnarly that physicists have trouble using it to make testable predictions about the natural world. In other words, the QCD box cover is pretty much unreadable.

Still, each observation of a new quark composite gives experimental physicists another puzzle piece to work with. And now that they have the πK piece in hand, the next step is to study the πK’s characteristics to see what it contributes to the full picture of quark interaction.

One key characteristic they hope to measure is the πK atom’s lifetime. When a πK atom is born in a particle collision, electromagnetic force binds the pion and the kaon together because one is positively charged and one is negatively charged. But because pions and kaons are made of quarks, they have the curious ability to shed their electric charge and go neutral, said Eric Swanson, a physicist at the University of Pittsburgh who was not involved in the new study. When that happens, the electromagnetic force no longer holds the pion and kaon together, and the atom falls apart.

Theories that explain how pions and kaons lose their charge are based on QCD, Swanson said. Measuring how quickly the πK atom falls apart provides physicists with new insight into how pions and kaons—and thereby the quarks that compose them—behave.

“By collecting observations like this, we can learn something about how QCD manifests itself in nature,” Swanson said.