Odd gluon compounds may be lurking in the protons

February 9, 2018 by Iva Raynova, CERN
The TOTEM experiment studies protons that stay intact after collisions in the LHC. Credit: Maximilien Brice/CERN

Protons are known to contain quarks and gluons. But are gluons behaving as expected?

Scientists from the TOTEM (Total, elastic and diffractive cross-section measurement) collaboration may have found indirect evidence of a subatomic -compound in proton-proton collisions. First theorised in the 1970s, such a state, then dubbed "Odderon," consists of an odd number of gluons.

Usually, the protons that collide in the LHC shatter and create new particles. Sometimes though, in about 25 percent of the time, they survive the encounter intact. Instead of breaking in pieces, they only change their direction and emerge from the detector at very small angles to the beampipe – their deviation at a 200-metre distance is in the order of one millimetre. This kind of interaction is called "elastic scattering" and it is the specialty of TOTEM, CERN's longest experiment. To be able to detect the survived protons, its detectors are spread across almost half a kilometre around the CMS interaction point.

The quarks in the proton are bound by gluons, the carriers of the strong force. Physicists have successfully explained elastic scattering at low-momentum transfer and high energies with the exchange of a "Pomeron," which in modern language is a state of two teamed-up gluons.

TOTEM precisely measured the elastic-scattering process at 13 TeV to extract the total probability for as well as the so-called rho parameter that helps to explain the difference in proton-proton and antiproton-proton scattering.

Combining these two measurements, TOTEM finds better agreement with theoretical models that indicate the exchange of three aggregated gluons. Although this exchange has been predicted by the Quantum Chromodynamics (QCD) theory back in the 1980s, no experimental evidence had been presented to date.

The measurements also hint towards a slow-down of the total probability of scattering with . While somewhat expected at the very highest energy, there has been no indication of such an effect in previous data.

"These measurements explore for the first time the behaviour of protons in elastic interactions at the highest energy of 13 TeV. These results obtained with a record precision were made possible by the excellent performance of the TOTEM detectors and the exceptional capabilities of the Large Hadron Collider," observed Simone Giani, the TOTEM spokesperson.

If three gluons really were to form a compound, it should appear in other scattering experiments. Physicists are hence looking forward to dedicated experiments to establish whether such a compound is actually being formed. In order to further explore and confirm the theoretical interpretations, a special LHC run at an energy of 900 GeV is planned to take place in 2018 to collect more data and it will involve also other LHC experiments.

Explore further: Great expectations from fewer collisions

Related Stories

LHC achieves record luminosity

November 14, 2017

It's the end of the road for the protons this year after a magnificent performance from the Large Hadron Collider (LHC). On Friday, the final beams of the 2017 proton run circulated in the LHC. The run ended, as it does every ...

Recommended for you

Pond dwellers called Euglena swim in polygons to avoid light

September 25, 2018

In any seemingly quiet pond the still waters actually teem with tiny pond dwellers called Euglena gracilis. Unseen to the naked eye, the single-celled organism spirals through the water, pulled along a relatively straight ...

A new way to count qubits

September 24, 2018

Researchers at Syracuse University, working with collaborators at the University of Wisconsin (UW)-Madison, have developed a new technique for measuring the state of quantum bits, or qubits, in a quantum computer.

Explainer: The US push to boost 'quantum computing'

September 24, 2018

A race by U.S. tech companies to build a new generation of powerful "quantum computers" could get a $1.3 billion boost from Congress, fueled in part by lawmakers' fear of growing competition from China.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.