Physicists Discover the Elusive Odderon, First Predicted 50 Years Ago

Physicists Discover the Elusive Odderon, First Predicted 50 Years Ago
Observations that contributed to the discovery of the odderon were done at Fermi National Accelerator Laboratory before its Tevatron particle accelerator shut down in 2011. (Photo: Fermilab, Reidar Hahn)

Scientists are celebrating the long-sought discovery of the odderon, a strange phenomenon that appears only rarely when protons collide at high energies, such as inside particle accelerators. Though the odderon was first predicted to exist in the early 1970s, it wasn’t until recently that physicists finally gathered the data they needed at CERN’s Large Hadron Collider to confirm a true discovery.

The discovery contributes to physicists’ understanding of how all the matter in the universe interacts at the smallest levels. Unlike the famous Higgs boson, which was officially discovered in 2012, the odderon isn’t a particle exactly. Instead, it’s the name for a compound of three gluons that gets exchanged between protons (or a proton and its antimatter twin, the antiproton) when they collide violently but aren’t destroyed. Gluons are subatomic particles so named because they “glue” together other particles called quarks; quarks are the tiny things that make up the bigger particles like protons and neutrons that form the atoms we all know and love.

Gluons are funny in that they don’t like to be alone; they’re almost always found together. When it’s an even-numbered group of gluons (two, four, etc.), we call it a pomeron. When the number of gluons in the group is odd (three, five, etc.), well, you guessed it: That’s an odderon. The odderon, for mysterious reasons, is very rarely produced, and though hints of it have popped up over the decades, the evidence was never quite strong enough to say it existed for sure. But the generally accepted theory of quantum physics says odderons should exist, so scientists have continued to hunt for them.

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An international team of physicists announced earlier this month that their data reached a level of statistical significance known as “five sigma,” a threshold that most scientists agree means you can be 99.999+% sure that you really made a discovery. After all, it’s not like physicists can just look inside their particle collider and see an odderon smiling back at them. Instead, they’ve got to go through staggering amounts of data recorded when protons and antiprotons bounce into the walls of their detectors.

If you put all that data onto CDs and stacked them on top of each other, “you will cover more than the distance between the Earth and the Moon,” said Christophe Royon, a professor of physics and astronomy at the University of Kansas who is part of the team behind the new research. “You are collecting a huge number of data. And then you have to do some analysis to identify, among all this data, what is interesting for you.”

Where the protons and antiprotons strike the detector after smashing into each other tells the researchers how the particles interacted when they collided. Physicists sort through records of millions and millions of these collisions, looking for enough of the right data points to be able to say confidently that what they’re seeing could only be explained if the odderon exists. If they kept up this research for years and never found evidence of the odderon, they’d have to go back to the drawing board and come up with a new theory of how the universe works.

Fortunately, the researchers were able to gather their results from the particle colliders before the covid-19 pandemic halted in-person work, and then the data analysis could be done remotely. But they haven’t been able to celebrate together yet.

“With the covid situation, it’s a bit tricky — everybody is working from home and so on,” Royon said in a video call. “But when we are back to normal, I think we deserve a party.”

The research involved a careful comparison of data sets: One created a decade ago at the now-closed DØ experiment at Fermilab in Illinois and others taken in 2015, 2019, and 2020 (before pandemic lockdowns) at the Large Hadron Collider’s TOTEM experiment. The Fermilab experiment collided protons and antiprotons, while the LHC work looked at protons hitting protons. It was by comparing the data from these two different colliders that they were able to be so certain about the existence of the odderon.

Though the team, which involved researchers in countries around the world, suspected they had something big last year, they didn’t want to rush an announcement. They asked independent researchers in the field to check their work for potential biases or problems before making their paper public. The article is now published as a preprint by CERN and Fermilab and has been submitted to the journal Physical Review Letters.

“The odderon is a solid prediction of the theory of strong interactions, made almost half a century ago,” said physicist Yuri Kovchegov from The Ohio State University, who was not involved in the new work. “At the same time, it has been avoiding experimental detection for decades. The new DØ and TOTEM result, if it holds, is likely to indicate that the odderon has at last been found.”

Kovchegov said in an email that the paper “appears to be the first solid experimental evidence for the existence of the odderon,” though he would still like to see more experiments confirm the finding. He said the upcoming Electron-Ion Collider, a major new experiment to be built in New York and set to open in the early 2030s, might be able to answer ongoing questions about the odderon.

Royon agrees that the work of studying the odderon is far from over. “It’s not something which we close and say we are happy, finished, done,” he said. “In physics, when you find something new, usually it’s a door which opens completely new domains.”