Physics researchers have discovered evidence of an elusive and undiscovered process in old data from particle accelerators. It is called the triangle singularity.
A triangle singularity was first proposed by Lev Landau, a Russian physicist in the 1950s. It is a rare subatomic phenomenon where particles swap identities before moving away from one another. This scenario sees two particles called Kaons, which form two corners of a triangle. The particles they swap form the third corner.
Bernhard Ketzer (study co-author at the Helmholtz Institute for Radiation and Nuclear Physics, University of Bonn) stated in a statement that "The particles involved exchanged quiarks and changed the identities in the process."
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It's also called a singularity, because mathematical methods used to describe subatomic particle interactions fail.
This singularly bizarre particle identity-swap could be used to help physicists better understand the strong force that binds the nucleus together.
Pointing to the COMPASS
In 2015, CERN scientists in Switzerland believed they had seen a glimpse of an exotic collection of particles called a Tetraquark. The new research suggests a completely different interpretation. Instead of creating a new grouping of particles, a pair exchanged identities before they flew off. This is called a triangle singularity and the experiment may have been the first to show it.
CERN's COMPASS experiment (Common Muon and Protein Apparatus to Structure and Spectroscopy), studies the strong force. Although the force does a simple job, keeping protons and neutrons together, the force is complex enough that physicists have struggled to describe its behavior in all interactions.
To understand the force of the strong force, scientists from COMPASS smash particles together using super-high energies in an accelerator called Super Proton Synchrotron. They then watch and wait to see what happens.
They begin with a pion. It is composed of two basic building blocks: a quark or an antiquark. The strong force holds the quarks and antiquark together within the pion. Contrary to other fundamental forces of nature, which decrease in strength with distance, this strong force is stronger as the quarks are further apart (imagine quarks attached to a pion by a rubber band, and the harder they pull apart).
The scientists then accelerate the pion to almost the speed of light, and smash it into a hydrogenatom. The collision breaks the strong force bonds between quarks and releases all the stored energy. Ketzer stated that this energy is transformed into matter and creates new particles. These experiments provide important information about strong interaction.
There are four fundamental forces in nature: gravity, which is the weakest, and electromagnetism which operates on smaller scales. The weak nuclear force, responsible for the conversion of protons into neutrons in atoms, and emitting beta radiation during the process, and the weak force which holds together nucleons in atomic nuclei as well as quarks within them. Image credit: MARK Garlick/SCIENCEPHOTO LIBRARY via Getty Images
Are you looking for four quarks, or a triangular?
In 2015, COMPASS analyzed a record number of collisions with 50 million particles and discovered an interesting signal. A new particle was discovered in less than 1% percent of the collisions. The particle was initially called "a1(1420")" by the scientists. They thought it was a new grouping four quarks, a tetraquark. However, the tetraquark became unstable and eventually decayed into other particles.
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Quarks are usually found in groups of three (which make protons and neutrons), or in pairs (such the pions). This was quite significant. It was rare to find a group of four quarks.
The new analysis, published in August by Physical Review Letters, suggests a more bizarre interpretation.
All those pion collisions created something surprising instead of creating a new Tetraquark for a short time: the fabled triangular singularity.
These are the triangles
Here are the findings of the new analysis. The pion is smashed into the hydrogen atom, and then it breaks apart. All the strong force energy produced a flood of particles. Some of these particles are called kaons. They are another type of quark-antiquark couple. Rarely, two kaons will travel their own ways. These kaons eventually will become other, more stable particles. They exchange one of their quarks and transform themselves before that happens.
This is the brief exchange of quarks among the two kaons, which mimics the signal from a tetraquark.
Ketzer is also a member of Transdisciplinary Research Area "Building Blocks of Matter and Fundamental Interactions". The resulting signal looks very much like a tetraquark.
Charting the paths of individual particles following the collision will show that the pair of kaons forms two legs and the exchanged particles create a third. This makes the diagram look like a triangle, which is why the name.
Although physicists have been predicting triangle singularities for over 50 years, this experiment is the closest to actually seeing one. However, it's not an easy task. The new model for the process involving triangular singularities has fewer parameters that the tetraquark models and is better suited to the data. However, it is not conclusive as the original tetraquark could still explain the data.
It's still an interesting idea. It will provide a powerful probe into the strong nuclear force if it is successful. The appearance of triangle singularities is a prediction about our understanding of this force.
Original publication on Live Science
