Scientists see single building blocks of matter creating ripples in the universe’s earliest liquid for the first time

For less than a quadrillionth of a second, lead ions colliding at nearly the speed of light inside CERN’s Large Hadron Collider recreate a droplet of matter hotter than anything in today’s universe — the same primordial soup that filled the cosmos in its first microseconds.

For decades, physicists have debated what that soup actually is. Now, a team led by MIT researchers may have found an answer — hidden among 13 billion collisions, in a subtle ripple so faint it had never been directly detected before.

A liquid born in the first microseconds of time

Quark-gluon plasma is unlike anything that exists in today’s universe. In the moments after the Big Bang, temperatures soared to several trillion degrees Celsius — hot enough to stop quarks and gluons from binding together. Instead, they flowed freely as a single unified fluid: the first liquid ever to exist, and the hottest one ever known.

What makes QGP especially unusual is how smoothly it flows. Scientists describe it as a near-perfect liquid — individual quarks and gluons move together with almost no friction, a remarkable property for something built from the universe’s most fundamental building blocks.

This primordial state didn’t last long. Within a few millionths of a second, the plasma cooled, and quarks and gluons combined into protons and neutrons — the ordinary matter that makes up everything we see today. To study what came before that transition, physicists at CERN accelerate heavy lead ions to nearly the speed of light and smash them together inside the Large Hadron Collider, briefly recreating tiny droplets of that ancient plasma.

The long search for quark wakes

The idea that QGP behaves like a true liquid carries a testable prediction. One influential framework — the hybrid model, developed by MIT physicist Krishna Rajagopal and collaborators — suggests that a fast-moving quark should disturb the surrounding plasma, leaving a detectable wake behind it. Think of a duck gliding across a pond, trailing ripples in its path.

Physicists searched for those wakes across multiple high-energy experiments over many years. Most searches focused on quark-antiquark pairs: when a quark moves through the plasma in one direction, an antiquark travels the other way. Both particles should produce wakes — but in practice, one wake drowned out the other.

“When you have two quarks produced, the problem is that, when the two quarks go in opposite directions, the one quark overshadows the wake of the second quark,” explains Yen-Jie Lee, professor of physics at MIT. Finding a single quark’s wake meant isolating one lone particle moving through the plasma, without interference from a counterpart.

Using a Z boson as a wake detector

The breakthrough came from switching the reference particle entirely. Rather than searching for quark-antiquark pairs, Lee’s team looked for a rarer collision type: one producing a single quark paired with a Z boson traveling in the opposite direction.

Z bosons are neutral elementary particles associated with the weak force. Crucially, they interact almost not at all with the surrounding plasma — making them ideal reference points that pass through QGP without leaving any disturbance. They also appear at a distinctive energy that makes them relatively straightforward to identify. Working with Professor Yi Chen’s group at Vanderbilt University, the team turned this Z boson “tagging” technique into a practical detection method.

Ripples found among 13 billion collisions

The team examined 13 billion heavy-ion collisions recorded by the CMS detector at CERN. Among those, roughly 2,000 events contained a detectable Z boson — each one flagging a collision where a lone quark had traveled through the plasma.

For each event, researchers mapped how energy was distributed across the short-lived plasma droplet. On the quark’s side, they repeatedly found fluid-like patterns of splashes and swirling motion that matched the predictions of Rajagopal’s hybrid model, confirming that QGP responds collectively as a liquid when energetic particles pass through it.

“We’ve gained the first direct evidence that the quark indeed drags more plasma with it as it travels,” Lee says. The plasma is so dense, he notes, that it can actually slow a speeding quark — and respond to it the way any liquid responds to an object moving through it.

What these wakes can tell us about the early universe

Detecting a wake is just the beginning. By measuring how large the wakes grow, how fast they travel, and how long they take to fade, scientists can extract concrete physical properties of QGP — measurements that could reveal how the primordial plasma behaved during the universe’s first microseconds.

Lee’s team plans to apply this technique to additional collision datasets. Each new dataset could sharpen the picture of how QGP flows, dissipates energy, and responds to disturbance. Studying how quark wakes bounce back and forth will yield new insights into QGP’s properties — offering a clearer snapshot of the universe’s very first liquid.

More information is available here: V. Chekhovsky, A. Hayrapetyan, V. Makarenko, et al. Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions. Physics Letters B, 2026; 874: 140120 DOI: 10.1016/j.physletb.2025.140120