Physicists working at CERN’s Large Hadron Collider have obtained the first direct evidence that quark-gluon plasma truly behaves like a liquid. As it passes through this superheated matter, a single quark leaves behind a distinctive trail — waves, splashes, and vortices, much like a boat or a duck moving through water.

Quark-gluon plasma existed in the first moments after the Big Bang, when the temperature of the Universe reached several trillion degrees. It lasted for only a few millionths of a second, after which it rapidly cooled, and quarks and gluons combined into protons, neutrons, and other particles.

Now scientists have effectively managed to see how this primordial “substance” responded to particles passing through it, Sciencedaily reports.

The Hottest Liquid in the Universe

Quark-gluon plasma forms at extreme temperatures and densities, when quarks and gluons are no longer bound inside protons and neutrons. It is believed that this was the state of matter that filled the Universe during the first microseconds of its existence.

It can be recreated in particle accelerators. At CERN, heavy ions such as lead nuclei are accelerated to nearly the speed of light and collided with one another. For an infinitesimally short moment, the collision produces a tiny droplet of quark-gluon plasma.

These droplets exist for less than a quadrillionth of a second. That is why the plasma cannot be observed directly: scientists have to reconstruct what happened from the particles that appear after it decays.

Numerous experiments have already shown that quark-gluon plasma behaves like an extraordinarily “perfect” liquid — its particles move in a coordinated way, and resistance to flow is extremely low. But one important question remained: can this medium respond to a single fast-moving quark as a unified whole?

A Quark Leaves a “Trail” Behind It

Researchers led by MIT physicist Yen-Jie Lee developed a new way to find such a trail.

When a fast quark passes through the plasma, it interacts with the surrounding matter and is expected to create a wave behind it — a so-called wake, or trail. Theoretical models predicted that the plasma should not simply scatter the quark, but collectively respond to its motion by creating waves and vortices.

The problem was that seeing this effect is extraordinarily difficult. In previous experiments, scientists often looked for quark-antiquark pairs. But the two objects move in opposite directions, and each creates its own trail. As a result, one of them can overlap the other’s signal.

The new method makes it possible to study a situation in which effectively only one quark of interest to scientists passes through the plasma.

The Z Boson Became a Reference Point

To do this, physicists used an unusual “beacon” — the Z boson, a neutral elementary particle associated with the weak interaction.

In some heavy-ion collisions, a quark and a Z boson are produced simultaneously. They fly apart in roughly opposite directions. At the same time, the quark interacts strongly with the quark-gluon plasma, while the Z boson interacts with it hardly at all.

This gives scientists a convenient point of reference. If the Z boson indicates one direction, the trail left by the quark can be sought in the opposite one.

“In this soupy medium, many quarks and gluons fly past one another and collide,” Lee explains. Sometimes one of these interactions creates a Z boson and a quark with high momentum.

Since the Z boson has virtually no effect on the plasma, changes in the energy distribution detected nearby on the opposite side can be attributed specifically to the quark’s motion.

The Trail Was Found Among 13 Billion Collisions

Researchers analyzed data from about 13 billion heavy-ion collisions. Among them, they found about 2,000 events in which a Z boson was produced.

For each such event, the scientists reconstructed the energy distribution in the short-lived quark-gluon plasma. In the direction opposite the Z boson, characteristic structures appeared again and again — energy surges and vortices matching the predicted trail of a fast-moving quark.

The resulting picture agrees with a theoretical model developed by MIT physicist Krishna Rajagopal and his colleagues. It predicts that a quark passing through the plasma should drag along some of the surrounding matter and create a liquid-like wave.

Thus, the experiment has for the first time provided direct evidence that quark-gluon plasma really does respond to a single fast particle as a collective liquid.

A Window Into the First Microseconds of the Universe

The new method is important not only because physicists have finally seen the predicted effect. The size, propagation speed, and lifetime of the quark trail can reveal the properties of the plasma itself.

By studying how far such waves spread, how quickly they fade, and how they interact with the surrounding matter, scientists will be able to determine the characteristics of quark-gluon plasma more precisely.

And since this was exactly the state of matter that existed in the first microseconds after the Big Bang, experiments at the LHC provide an opportunity to indirectly investigate the physics of that period.

Researchers now plan to apply the new method to additional collision data and find more such events. The more trails they can gather, the more precisely they will be able to reconstruct the properties of the most unusual liquid that has ever existed in the Universe.