Quark Shadows

Quark–gluon plasma (QGP) is believed to be the very first liquid that existed in the universe. It is also the hottest liquid ever known, with scientists estimating that during its brief lifetime it reached temperatures of several trillion degrees Celsius. This extremely hot mixture behaved almost like a “perfect” liquid, where quarks and gluons moved together smoothly with almost no friction.

Figure 1. Big Bang’s Primordial Matter Found to Flow Like Liquid.

This understanding of QGP comes from many experiments and theoretical studies. One important theory, developed by Krishna Rajagopal, the William A. M. Burden Professor of Physics at Massachusetts Institute of Technology, along with collaborators, suggests that the plasma should react like a fluid when particles move through it. Their idea, called the hybrid model, predicts that a fast-moving jet of quarks traveling through the QGP would leave a wake behind it, causing the plasma to ripple and splash much like water disturbed by a speeding boat. Figure 1 shows Big Bang’s Primordial Matter Found to Flow Like Liquid.

Scientists have searched for these wake patterns in experiments at the CERN’s Large Hadron Collider and other powerful particle accelerators. In these experiments, heavy ions such as lead are accelerated to nearly the speed of light and then smashed together. The collision briefly creates a tiny droplet of primordial plasma that lasts for less than a quadrillionth of a second. Researchers capture snapshots of these moments to study the properties of QGP.

To detect quark wakes, physicists usually examine pairs of quarks and antiquarks—particles that are nearly identical to quarks but have certain properties with opposite values. When a quark moves through the plasma, an antiquark often travels at the same speed in the opposite direction.

Because of this, researchers have focused on quark–antiquark pairs produced in heavy-ion collisions, assuming that both particles would generate similar detectable wakes in the plasma.

However, this method creates a complication. As Lee explains, when two quarks move in opposite directions, the wake produced by one can overshadow the wake from the other, making it difficult to observe clearly.

Realizing this challenge, Lee and his team proposed a new approach: instead of studying two opposing quarks, they developed a technique that allows scientists to detect the wake created by a single quark using a different pair of particles. This method makes it easier to observe how individual quarks interact with the quark–gluon plasma.

A Wake Tag

Instead of searching for quark–antiquark pairs after lead-ion collisions, Lee’s team focused on events where a single quark travels through the plasma in the opposite direction of a Z boson. A Z boson is a neutral particle associated with the weak force and interacts very little with its surroundings. Because it appears at a well-defined energy, it is relatively easy for researchers to detect.

“In this soup of quark–gluon plasma, countless quarks and gluons are constantly passing by and colliding with one another,” Lee explains. “Occasionally, one of these collisions produces both a Z boson and a quark with very high momentum.”

When this happens, the two particles move away from each other in exactly opposite directions. The quark can disturb the surrounding plasma and create a wake, while the Z boson passes through without affecting it. Any ripples detected in the plasma therefore must come from the single quark moving through it.

Working with the group of Yi Chen at Vanderbilt University, the researchers realized they could use Z bosons as a “tag” to trace the wake produced by individual quarks. For the study, the team analyzed data from heavy-ion collision experiments at the Large Hadron Collider at CERN. Out of roughly 13 billion collisions, they identified about 2,000 events that produced a Z boson.

For each event, they mapped how energy spread through the short-lived quark–gluon plasma. The results consistently revealed swirling, splash-like patterns forming in the direction opposite the Z boson—clear evidence of a wake produced by a single quark moving through the plasma.

The researchers also found that these wake patterns matched predictions from the hybrid model developed by Krishna Rajagopal of Massachusetts Institute of Technology. The agreement suggests that quark–gluon plasma truly behaves like a flowing fluid when fast-moving particles pass through it.

Source: SciTECHDaily

Cite this article:

Priyadharshini S (2026), Physicists Discover Big Bang’s Primordial Matter Behaved Like a Liquid, AnaTechMaz, pp. 454