In the universe's earliest moments, temperatures reached trillions of degrees, creating an intensely hot mixture of quarks and gluons. These elementary particles raced around at nearly the speed of light in a state of matter known as quark-gluon plasma (QGP). This primordial material existed for only a few millionths of a second before cooling rapidly, allowing quarks and gluons to combine into protons, neutrons, and other particles found throughout the universe today.
At CERN's Large Hadron Collider in Switzerland, physicists are recreating quark-gluon plasma to investigate the ingredients that filled the young universe. By colliding heavy ions at nearly the speed of light, researchers can briefly separate quarks and gluons and produce tiny amounts of the same kind of matter that existed during the universe's first microseconds. A CERN team led by MIT physicists has now found clear evidence that quarks generate wakes as they travel through this plasma, much like a duck creating ripples as it moves across water.
The observations provide the first direct evidence that quark-gluon plasma responds to fast-moving particles as a unified fluid, producing waves, splashes, and swirling motion instead of simply behaving as a collection of independently scattering particles. "It has been a long debate in our field, on whether the plasma should respond to a quark," says Yen-Jie Lee, professor of physics at MIT. "Now we see the plasma is incredibly dense, such that it is able to slow down a quark, and produces splashes and swirls like a liquid.
So quark-gluon plasma really is a primordial soup." Lee and his colleagues developed a new method for detecting these quark wakes. They plan to use the technique on additional particle collision data to search for more examples and study them in greater detail. By measuring how large the wakes become, how quickly they travel, how far they extend, and how long they take to fade, scientists may be able to determine important properties of quark-gluon plasma.
Those measurements could also offer clues about how the plasma behaved during the first microseconds after the universe began. "Studying how quark wakes bounce back and forth will give us new insights on the quark-gluon plasma's properties," Lee says. "With this experiment, we are taking a snapshot of this primordial quark soup." The study's co-authors are members of the CMS Collaboration, a worldwide group of particle physicists who conduct and analyze experiments using the Compact Muon Solenoid (CMS), one of the general-purpose particle detectors at CERN's Large Hadron Collider.
Researchers used the CMS experiment to identify signs of quark wakes in this study. The open-access findings appear in Physics Letters B. Quark-gluon plasma is thought to have been the first liquid in the universe.
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