Large Hadron Collider detects elusive ‘odderon’ quasiparticle 50 YEARS after it was first predicted

Concluding a hunt that began almost 50 years ago, particle physicists from CERN and the US have detected an elusive and weird phenomenon called the ‘odderon’.

It was formed by firing protons at each other at high energies in the Large Hadron Collider, the 17-mile-long particle accelerator circling under the French-Swiss border.

This effort built on work undertaken ten years ago at the now-inactive Tevatron accelerator at Fermilab, in Illinois, which instead collided protons and anti-protons.

The odderon’s existence was first predicted back in 1973 by the Polish and Romanian particle physicists Leszek Łukaszuk and Basarab Nicolescu. 

The discovery not only validates the duo’s hypothesis, but will also help scientists to better understand how matter interacts at the smallest, subatomic scales. 

Unlike the famous Higgs Boson discovered by CERN in 2012, the odderon is a ‘quasiparticle’ — not a real particle, but a phenomena that behaves rather like one. 

Specifically, the recently-spotted odderon is a gang of three gluons, the subatomic particles that bind quarks together to form the protons and neutrons that make up all atomic nuclei.

When protons collide at high energies, they don’t always break apart, but instead, around a quarter of the time, glance off each other, shifting course.

During these ‘elastic’ collisions, they also exchange gluons. Until now, such exchanges had only been seen with an even number of gluons.

(In fact, one might say that gluons are one of the universe’s more clingy phenomena, as they do not like to be alone and appear in multiples dubbed ‘glueballs’.)

The odderon takes its name from the fact that it appears as an odd-number of gluons moving together, an outcome which would seem to be considerably rarer. 

Meanwhile, the more commonly seen quasiparticle made up of an even number of gluons is called a pomeron, after the Soviet theoretical physicist Isaak Pomeranchuk.

To finally reveal the odderon, physicists studied data both from the so-called TOTEM and DØ scientific collaborations at the LHC and the Tevatron, which are respectively the most and second-highest energy particle colliders ever built.

The two TOTEM detectors sit 722 feet (220 m) either side of ‘CMS’ — one of the LHC’s four main experiments — which occupies one of the points along the LHC beamline where beams of high-energy protons, guided by magnets, can be made to collide.

After the collisions, TOTEM measures the resulting, tiny deflections in the paths of the protons. These typically measure only around 0.04 inches (1 mm), but allow experts to extrapolate details about the nature of the proton–proton interactions.

The DØ experiment used a similar set-up on the Tevatron (so named because it accelerated particle up to energies of 1 TeV, or tera election volts), albeit colliding beam of protons with their antimatter counterparts, which have an opposite charge.

Analysis of data from millions of collisions in both experiments determined that the probability that an odderon was indeed produced during particle collisions exceeded the so-called ‘5-sigma’ value used as threshold for statistical significance.

‘This means that if the odderon did not exist, the probability that we observe an effect like this in the data by chance would be 1-in-3.5 million,’ said paper author and high-energy nuclear physicist Cristian Baldenegro of the University of Kansas.

Previously, in 2018, TOTEM had reported measurements that hinted at an odderon detection, but these were not enough to confirm a definite observation.

In the latest study, the researchers compared LHC proton–proton collision data — recorded at energies of 2.76, 7, 8 and 13 TeV — with proton–antiproton data from the Tevatron at collision energies of 1.96 TeV to reveal further evidence for the odderon.

What finally clinched the result and achieved a discovery level statistical significance was the further combination of these results from measurements by TOTEM at 13 TeV but with much smaller proton beam scattering angles.

‘When combined with the measurements at 13 TeV, the significance of the result is in the range of 5.2–5.7 standard deviations and thus constitutes the first experimental observation of the odderon,” said paper author Christophe Royon, also of Kansas

‘This is a major discovery by CERN and Fermilab,’ he concluded. 

For physicists, though, this achievement is just the beginning of plumbing the mysteries of the odderon. This will likely be assisted by the Electron-Ion Collider, an experimental machine to be built in New York that will begin operation in the 2030s. 

A pre-print of the researcher’s article, which has been submitted for publication in the journal Physical Review Letters, can be read on the CERN document server.