After 20 years, jet diffusion wakes detected in quark-gluon plasmas

Boats passing over smooth water form a pair of diffusion wakes behind them veering off at a certain angle. Turbulence occurs along the line directly behind the boat, but the two diffusion waves are at a theoretical angle of 19.5° from the same line, for deep, ideally smooth water.

Now physicists have for the first time measured diffusion wakes of jets passing through quark-gluon plasmas (QGP), a phenomenon first predicted 20 years ago after it was realized that QGPs are liquid not a plasma (that is, like a gas). In fact, they are the most perfect liquid in the universe. The result, obtained by the CMS Collaboration, is published in Physical Review Letters.

QGPs were the state of the universe for its first few microseconds, from about a trillionth of a second after the Big Bang to a few microseconds after. It is 200,000 times hotter than the center of the sun, and its viscosity was first inferred in 2005 in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York.

Their shear-viscosity-to-entropy-density ratio is 0.08 to 0.20 Kelvin-seconds, less than one-tenth that of water at room temperature and pressure and about a millionth that of dry air. (Note this is not viscosity, but in a ratio with, essentially, entropy.

These qualities mean the QGP is the “hottest, densest and most perfect fluid in the universe,” says Olga Evdokimov of the University of Illinois, Chicago and a collaborator at the CMS Experiment on the Large Hadron Collider at CERN.

Jets in the primordial fluid

Unlike water or air, a new aspect of QGP creations was jets traveling through it. Jets were first seen as a stream of strong force particles—quarks and gluons—traveling away from a high energy collision of protons and protons (or other “hadrons“).

Because a quark or gluon cannot exist on their own—the strong force grows stronger with distance, not weaker, a property called asymptotic freedom—a quark exiting a hadron-hadron collision inevitably creates other particles around them to ultimately create hadrons.

This stream of conical particles is called a “jet,” and their observations have been vital to understanding quantum chromodynamics (QCD), the quantum field theory that describes the strong force governing the interactions between quarks and gluons.

Collisions create a crowded soup

To look for the diffusion wakes that were expected in QGPs, the CMS collaboration at CERN in Europe smashed lead nuclei into lead nuclei traveling in opposite directions, with a total energy of 5 gigaelectron-volts (GeV), about 40 percent of CERN’s highest proton-proton collisions. These produced hot and dense QGPs lasting about 10^-22 seconds.

During that time interval a quark or gluon occasionally has enough energy to shoot off in one direction, with an equal amount of momentum streaming in the opposite direction, back to back to conserve momentum. Interactions in the QGP then usually put them at angles other than 180°. These become jets.

These jets are fighting through, to put it technically, a big mess. Quarks and gluons and other energy are whizzing around through this “soup,” everything interacting with everything else due to the complicated nature of QCD.

“The QGP droplet explodes and expands in a complicated way with a typical expansion velocity of order half the speed of light,” Evdokimov told Phys.org. The QGP is so hot that particles “boil” out of the vacuum, too.

The result is thousands of particles in a region typically a few to ten times the diameter of a proton. (The nuclei before the collision are large, typically three to four times the diameter of a proton; the proton’s radius, while not sharp, is approximately 8 x 10^-16 m.)

Separating wakes from larger waves

As the jets travel away from the collision point, amidst the expanding, riotous QGP, large-scale changes begin to appear in the distribution of particles. “Ridges, bumps and valleys that are much bigger than the tiny dip we expect from a single wake” occur, says Evdokimov.

These are akin to ocean waves and must be subtracted out from the data if the smaller wakes are to be seen, something CMS and other experimental groups have learned to do over the last two decades.

Even then, the wakes are so thin they cannot usually be seen in a single nuclei-nuclei collision. So, the experiment looks at millions of collisions, finds those with a transverse (to the collision direction) momentum above a certain cutoff, and analyzes the resulting mass of data to statistically find patterns that identify the wakes. Unlike a boat on smooth water, these wakes are in a much more complex environment due to the enormous number of interactions taking place in the QGP.

A long-predicted signal emerges

“The biggest surprise, or rather, excitement for us was being able to finally observe the phenomenon,” says Evdokimov. “Many previous searches, including some conducted by my group,” did not observe the wake signals or yield conclusive results.”

But, she explains, the theoretical predictions showed for over two decades: the evolving QGP should have a diffusion wake behind fast-moving quarks and gluons. “We were thrilled to confirm this experimentally.”

Who’s behind this story?

David Appell

David Appell is an Oregon-based freelance science writer whose work has appeared in Scientific American, New Scientist, Physics World, and The Washington Post. He holds a Ph.D. in physics from Stony Brook University.

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Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

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