particle accelerator trace singularity
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Physicists sifting through old particle accelerator data have found evidence of a highly-elusive, never-before-seen process: a so-called triangle singularity.

First envisioned by Russian physicist Lev Landau in the 1950s, a triangle singularity refers to a rare subatomic process where particles exchange identities before flying away from each other. In this scenario, two particles — called kaons — form two corners of the triangle, while the particles they swap form the third point on the triangle.

"The particles involved exchanged quarks and changed their identities in the process," study co-author Bernhard Ketzer, of the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn, said in a statement.

And it's called a singularity because the mathematical methods for describing subatomic particle interactions break down.

If this singularly weird particle identity-swap really happened, it could help physicists understand the strong force, which binds the nucleus together.

Pointing the COMPASS

In 2015, physicists studying particle collisions at CERN in Switzerland thought that they had caught a brief glimpse of a short-lived exotic collection of particles known as a tetraquark. But the new research favors a different interpretation — something even weirder. Instead of forming a new grouping, a pair of particles traded identities before flying off. This identity swap is known as a triangle singularity, and this experiment may have unexpectedly delivered the first evidence of that process.

The COMPASS (Common Muon and Proton Apparatus for Structure and Spectroscopy) experiment at CERN studies the strong force. While the force has a very simple job (keeping protons and neutrons glued together), the force itself is dizzyingly complex, and physicists have had a difficult time completely describing its behavior in all interactions.

So to understand the strong force, the scientists at COMPASS smash particles together at super-high energies inside an accelerator called the Super Proton Synchrotron. Then, they watch to see what happens.

They start with a pion, which is made of two fundamental building blocks, a quark and an antiquark. The strong force keeps the quark and antiquark glued together inside the pion. Unlike the other fundamental forces of nature, which get weaker with distance, the strong force gets stronger the farther apart the quarks get (imagine the quarks in a pion attached by a rubber band — the more you pull them apart, the harder it gets).

Next, the scientists accelerate that pion to nearly the speed of light and slam it into a hydrogen atom. That collision breaks the strong force bond between the quarks, releasing all that pent-up energy. "This is converted into matter, which creates new particles," Ketzer said. "Experiments like these therefore provide us with important information about the strong interaction."

four fundamental forces atoms physics
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There are four fundamental forces of nature, including gravity, the weakest of the bunch (illustrated in upper-left corner); electromagnetism, which works on far smaller scales; the weak nuclear force, which is responsible for nucleons within atoms converting from protons into neutrons and emitting beta radiation in the process; and the strong force, which holds together the nucleons in an atomic nucleus as well as the quarks within nucleons themselves
Four quarks or a triangle?

Back in 2015, the COMPASS analyzed a record 50 million such collisions and found an intriguing signal. In the aftermath of those collisions, less than 1% of the time a new particle appeared. They dubbed the particle "a1(1420)" and initially thought it was a new grouping of four quarks — a tetraquark. That tetraquark was unstable, however, so it then decayed into other things.

Quarks normally come in groups of three (which make up protons and neutrons) or in pairs (such as the pions), so this was a big deal. A group of four quarks was a rare find indeed.

But the new analysis, published in August in the journal Physical Review Letters, offers an even weirder interpretation.

Instead of briefly creating a new tetraquark, all those pion collisions produced something unexpected: the fabled triangle singularity.

Here come the triangles

Here's what the researchers behind the new analysis think is going on. The pion smashes into the hydrogen atom and breaks apart, with all the strong force energy producing a flood of new particles. Some of those particles are kaons, which are yet another kind of quark-antiquark pair. Very rarely, when two kaons are produced, they begin to travel their separate ways. Eventually those kaons will decay into other, more stable particles. But before they do, they exchange one of their quarks with each other, transforming themselves in the process.

It's that brief exchange of quarks between the two kaons that mimics the signal of a tetraquark.

"The particles involved exchanged quarks and changed their identities in the process," said Ketzer, who is also a member of the Transdisciplinary Research Area "Building Blocks of Matter and Fundamental Interactions" (TRA Matter). "The resulting signal then looks exactly like that from a tetraquark."

If you chart the paths of the individual particles after the initial collision, the pair of kaons form two legs, and the exchanged particles make a third between them, making a triangle appear in the diagram, hence the name.

While physicists have predicted triangle singularities for more than half a century, this is the closest any experiment has gotten to actually observing one. It's still not a slam dunk, however. The new model of the process involving triangle singularities has fewer parameters than the tetraquark model, and offers a better fit to the data. But it is not conclusive, since the original tetraquark model could still explain the data.

Still, it's an intriguing idea. If it holds up, it will be a powerful probe of the strong nuclear force, since the appearance of triangle singularities is a prediction of our understanding of that force that has yet to be fully examined.
Paul M.Sutter is a research professor in astrophysics at the Institute for Advanced Computational Science at Stony Brook University and the Flatiron Institute in New York City. He is also the host of several shows, such as How the Universe Works on Science Channel, Space Out on Discovery, and his hit Ask a Spaceman podcast. He is the author of two books, Your Place in the Universe and How to Die in Space, as well as a regular contributor to Space.com, LiveScience, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.