MIT physicists leading a team at CERN have observed the first unambiguous signs that speeding quarks create wakes in the trillion-degree quark-gluon plasma, confirming this primordial state of matter flows and reacts as a near-perfect liquid. The discovery, using a novel technique with Z boson tagging, provides a direct snapshot of how the infant universe behaved in its first microseconds.
For decades, physicists have theorized about the state of the universe moments after the Big Bang: a seething, trillion-degree-hot soup of fundamental particles called a quark-gluon plasma (QGP). Recreating this primordial goo is a primary goal of facilities like CERN’s Large Hadron Collider (LHC). But a fundamental question remained: did this exotic plasma behave like a collection of individual particles or a unified, flowing liquid? Now, a team led by MIT researchers has the answer, and it’s decidedly soupy.
“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.” The team’s findings, published in the journal Physics Letters B, offer the first clean evidence of these liquid-like wake effects.
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The challenge in seeing a quark’s wake was akin to trying to spot the ripples from one duck while another duck is splashing right beside it. Previous searches looked for pairs of quarks and antiquarks moving in opposite directions, but their overlapping signals muddied the view. The MIT-led team, working within the global CMS Collaboration, devised an ingenious workaround. They scoured data from 13 billion heavy-ion collisions to find rare events where a single high-energy quark was produced back-to-back with a Z boson—a neutral particle that zips through the plasma without interacting.
“We have figured out a new technique that allows us to see the effects of a single quark in the QGP,” explained Lee, as reported by MIT News. The Z boson acted as a perfect reference point. By mapping the energy distribution in the plasma opposite each detected Z boson, the researchers consistently found a fluid-like pattern of splashes and swirls—the unmistakable wake of a solitary quark.
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This experimental observation brilliantly confirms theoretical predictions, notably the hybrid model developed by MIT’s Professor Krishna Rajagopal. The model posited that the QGP should flow like a frictionless fluid in response to passing particles. “What Yen-Jie and CMS have done is to devise and execute a measurement that has brought them and us the first clean, clear, unambiguous, evidence for this foundational phenomenon,” said collaborator Daniel Pablos of Oviedo University.
The implications are profound. By measuring the size and dissipation of these quark wakes, scientists can now precisely quantify the properties of this exotic fluid—its density, viscosity, and how it transported energy in the early universe. “Studying how quark wakes bounce back and forth will give us new insights on the quark-gluon plasma’s properties,” Lee stated. “With this experiment, we are taking a snapshot of this primordial quark soup.”
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This breakthrough not only settles a longstanding debate but also opens a new window into the universe’s first moments. It confirms that the hottest liquid ever known behaved with a seamless, liquid unity, painting a vivid picture of the cosmic infancy that shaped everything to come.
