A Fresh Look at Quark-Gluon Plasma Soup

Theoretical physicists come up with a new way to analyze the firework of signals coming from particle collision experiments.

Yuen Yiu, Staff Writer

(Inside Science) -- Quarks and gluons are the building blocks for larger particles such as protons and neutrons, which in part make up atoms in all the ordinary matter in the universe, from moon rocks to the center of the sun. They are classified as fundamental particles like their more famous cousin, the electron, but remain relatively mysterious. Scientists had to build city-sized particle accelerators to smash protons together and send the quarks and gluons flying apart long enough to study them. A recent paper published in the journal Physical Review Letters describes a new approach that may help the effort to understand the closely tangled quarks and gluons.

Different particles, similar signatures

The most powerful particle accelerator in the world -- the Large Hadron Collider under the France-Switzerland border near Geneva -- can smash protons together with such accuracy and such ferocious speed that they split apart into quarks and gluons. The quarks and gluons then recombine within a fraction of a second and spray out hundreds of other particles by turning the energy from the collision into mass.

The only way for scientists to figure out what happened during the short time when quarks and gluons coexisted is to look at the spray of particles in the aftermath, like police forensic experts trying to figure out what happened at a crime scene by examining ballistic trajectories and scattered debris.

These particle sprays are aptly named "quark jets" and "gluon jets," and since the particles themselves aren't that distinguishable from one another, scientists have struggled with telling the two sprays apart.

"People have been asking the question for more than 30 years, and people have come up with a range of different techniques, but they almost always rely on modeling," said Gavin Salam, a theoretical particle physicist from CERN who was not involved in the paper. "The major advance of this paper is that they've identified a way of using the data itself to figure this out."

Sorting hay straws in a hay stack

Here is a classic math problem you may have encountered in school: A farmer has X number of sheep and Y number of chickens. Among the animals there are a total of 20 heads and 50 legs -- how many sheep and chickens are there?

This is kind of similar to the quark versus gluon jet problem, except the scientists "don’t even know what kind of animals are in the pens, so they'd need to discover how many feet does each animal have from the data itself," said Jesse Thaler, a theoretical particle physicist from MIT and one of the authors of the paper.

In each collision, the quarks and gluons could spew out different numbers and types of particles, each carrying different energies going in different directions. If one can learn about the differences between the jets produced by quarks and gluons, one can then go back and try to figure out what kind of interactions took place when the two were tangled together in a hot soup known as the quark-gluon plasma.

The Big Bang and dark matter

During the first milliseconds after the Big Bang, it is believed that our universe was so hot that ordinary atoms couldn’t exist, and all matter was a quark-gluon plasma similar to the plasmas created in particle accelerators.

"If we want to understand the properties of the plasma in the early universe, one way of doing so is to study these collisions," said Thaler. "By slamming together lead nuclei we can in some sense recreate the conditions of the Big Bang, at least very instantaneously."

The Large Hadron Collider can create what are essentially mini Big Bangs by smashing particles at near light speed, such that the temperature during the collision can rise to almost a million times hotter than the center of the sun. The ability to tell quark and gluon signatures apart can help scientists learn more about how the two fundamental particles interact with each other, which in turn will help them better understand what happened just after the universe was born.

"Another reason you'd want to distinguish quarks from gluons is the searches for dark matter for example, which might interact differently with quarks versus gluons," said Salam. It may also help fundamental physicists search for supersymmetry, an as yet unconfirmed theory that attempts to describe the relationship between the two basic classes of fundamental particles known as bosons and fermions, according to Salam.

Editor’s Note: This article was updated on July 11 to clarify that supersymmetry is an unconfirmed theory.