Physicists analyzing old data from the particle accelerator have found evidence of a highly elusive and never-before-seen process: the so-called triangular singularity.
First imagined by Russian physicist Lev Landau in the 1950s, a triangular singularity refers to a rare subatomic process where particles exchange identities before flying off each other.
In this scenario, two particles (called caons) form two corners of the triangle, while the interchanging particles form the third point of 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.
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It is called singularity because mathematical methods for describing subatomic particle interactions are broken down.
If this exchange of identity of foreign particles really took place, physicists could help understand the strong force that binds the nucleus together.
Pointing to the compass
In 2015, physicists studying particle collisions at CERN, Switzerland, thought they had seen a brief glimpse of an exotic collection of short-lived particles known as tetraquarks. But the new research favors a different interpretation, which is even stranger.
Instead of forming a new grouping, a pair of particles traded with identities before flying. This exchange of identity is known as triangular singularity and it is possible that this experiment has unexpectedly provided the first evidence of this process.
The COMPASS (Common Muon and Proton Apparatus for Structure and Spectroscopy) experiment at CERN studies strong force. Although force has a very simple job (keeping protons and neutrons stuck together), force itself is dizzyingly complex and physicists have had difficulty fully describing its behavior in all interactions.
Thus, to understand the strong force, COMPASS scientists break particles together with super-high energies inside an accelerator called Super Proton Synchrotron. Then they look to see what happens.
They start with a pawn, which consists of two fundamental blocks, a quark and an antiquark. The strong force keeps the quark and antiquark stuck inside the pawn.
Unlike the other fundamental forces of nature, which become weaker with distance, the stronger the force the farther the quarks are distinguished (imagine the quarks in a pawn joined by a rubber; the more you separate them, the harder it becomes).
Scientists then accelerate this pion to almost the speed of light and place it in a hydrogen atom. This collision breaks the strong bond of force between the quarks and releases all this accumulated energy.
“This becomes matter, which creates new particles,” Ketzer said. “Therefore, experiments like these provide us with important information about strong interaction.”
Four quarks or a triangle?
In 2015, COMPASS analyzed a record 50 million such collisions and found an intriguing signal. After these collisions, a new particle appeared less than 1 percent of the time.
They named the particle “a1 (1420)” and initially thought it was a new grouping of four quarks: a tetraquark. However, this tetraquark was unstable, so it decayed into other things.
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Quarks usually occur in groups of three (forming protons and neutrons) or in pairs (such as pions), so this posed a big problem. A group of four quarks was a rare find.
But the new analysis, published in August in the journal Physical review letters, offers an even stranger interpretation.
Instead of briefly creating a new tetraquark, all those pawn collisions produced something unexpected: the uniqueness of the fabled triangle.
Here come the triangles
This is what the researchers think behind the new analysis.
The pion breaks to the hydrogen atom and breaks, with all the strong force energy producing a flood of new particles. Some of these particles are kaons, which are another type of quark-antiquark pair.
Very rarely, when two canyons occur, do they begin to travel their separate paths. Eventually, these canons will decay into other more stable particles. But before they do, they exchange one of their quarks with each other, transforming into the process.
It is 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 “Basic Components of Matter and Fundamental Interactions” (TRA matter).
“The resulting signal looks exactly like that of a tetraquark.”
If you trace the paths of the individual particles after the initial collision, the pair of beads forms two legs and the exchanged particles form a third, causing a triangle to appear in the diagram, hence the name.
Although physicists have predicted triangular singularities for more than half a century, this is the closest any real observation has experienced.
However, it’s not a slam dunk yet. The new process model involving triangle singularities has fewer parameters than the tetraquark model and offers a better fit to the data. But it is not conclusive, as the original tetraquark model could still explain the data.
Still, it’s an intriguing idea. If maintained, it will be a powerful probe of the strong nuclear force, as the appearance of triangular singularities is a prediction of our understanding of this force that has not yet been fully examined.
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