Physicists recreated the first millisecond after the Big Bang — and found it was surprisingly soupy
Violent collisions Large Hadron Collider (LHC) have revealed the faintest trace of a trail left by a quark passing through nuclear matter billions of degrees away – suggesting that the primordial soup of the universe may have literally been more soup-like than we thought.
New findings from the LHC’s Compact Muon Solenoid (CMS) collaboration show the first clear evidence of a subtle “dip” in particle production behind a high-energy quark as it passes through the quark-gluon plasma – a droplet of primordial matter that would have filled the universe within microseconds of the explosion. Big Bang.
Recreating the conditions of the early universe in the laboratory
When heavy atomic nuclei collide at near the speed of light inside the LHC, they briefly melt into an exotic state called quark-gluon. plasma.
In this extreme environment, “the density and temperature are so high that the regular atomic structure is no longer maintained.” Yi Chenassistant professor of physics at Vanderbilt University and member of the CMS team, told Live Science via email. Instead, “all the nuclei overlap and form what is called quark-gluon plasma, where quarks and gluons can move beyond the boundaries of the nuclei. They behave more like a liquid.”
This plasma droplet is extraordinarily small: about 10-14 meters in diameter, or 10,000 times smaller than an atom – and disappears almost instantly. However, within this ephemeral droplet, there are quarks and gluons, the fundamental carriers of strong nuclear force that holds atomic nuclei together – flow collectively in a way that is more like an ultrahot liquid than a simple gas of particles.
Physicists want to understand how energetic particles interact with this strange environment. “In our studies, we want to investigate how different things interact with the small liquid droplet created during collisions,” Chen said. “For example, how would a high-energy quark pass through this hot liquid?”
The theory predicts that the quark would leave behind a detectable wake in the plasma, much like a boat cutting through the water. “We will have water pushed forward with the boat in the same direction, but we also expect a small drop in the water level behind the boat as the water is pushed back,” Chen said.
In practice, however, disentangling the “boat” from the “water” is far from simple. The plasma droplet is tiny and the experimental resolution is limited. At the front of the quark path, the quark and plasma interact intensely, making it difficult to distinguish which signals come from which signals. But behind the quark, the wake – if present – must be a property of the plasma itself.
“So we want to find that little hollow in the back,” Chen said.
A clean probe with Z bosons
To isolate this wake, the team turned to a special partner particle: the Z boson, one of the carriers of the weak nuclear force – one of four fundamental interactions, along with electromagnetic, strong and gravitational forces – responsible for some atomic and subatomic decay processes. In some collisions, a Z boson and a high-energy quark are produced together, recoiling in opposite directions.
This is where the Z boson becomes crucial. “Z bosons are responsible for the weak force, and when it comes to plasma, Z escapes and disappears from the picture,” Chen said. Unlike quarks and gluons, Z bosons interact little with plasma. They leave the collision zone unscathed, providing a clear indicator of the quark’s original direction and energy.
This setup allows physicists to focus on the quark as it passes through the plasma, without worrying that its partner particle has been distorted by the medium. Essentially, the Z boson serves as a calibrated marker, making it easier to look for subtle changes in particle production behind the quark.
The CMS team measured the correlations between the Z bosons and the hadrons – composite particles made of quarks – emerging from the collision. By analyzing the number of hadrons appearing in the “backwards” direction relative to the quark’s motion, they were able to look for the predicted wake.
A tiny but important signal
The result is subtle. “On average, in the opposite direction, we see less than a 1% change in the amount of plasma,” Chen said. “It’s a very small effect (and that’s part of why it took people so long to demonstrate it experimentally).”
Yet this less than 1% suppression is precisely the type of signature expected of a quark transferring energy and momentum to the plasma, leaving a depleted region in its wake. The team reports that this is the first time such a drop has been clearly detected in Z-marked events.
The shape and depth of the trough encode information about the properties of the plasma. Returning to his analogy, Chen noted that while water flows easily, the trough behind a boat fills quickly. If it behaves more like honey, the depression persists. “So studying what this trough looks like… gives us information about the plasma itself, without the complication of the boat,” she said.
Back to the beginning of the universe
The findings also have cosmological implications. The early universe, shortly after the Big Bang, is thought to have been filled with quark and gluon plasma before cooling into protons, neutrons and, ultimately, atoms.
“This epoch is not directly observable through telescopes,” explains Chen. “The universe was opaque back then.” Heavy ion collisions provide “a little insight into how the universe behaved at that time,” she added.
For now, the decline observed is “just the beginning,” Chen concluded. “The exciting implication of this work is that it opens a new avenue for better understanding the properties of plasma. With more data accumulated, we will be able to study this effect more precisely and learn more about plasma in the near future.”
