This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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. 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°.
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