Violent collisions at the Large Hadron Collider (LHC) have revealed faint traces of wakes left by quarks tearing through trillions of degrees of nuclear material. This suggests that the primordial soup of the universe may have been more literally soupy than we think.
New discoveries from the LHC’s Compact Muon Solenoid (CMS) collaboration provide the first clear evidence of a subtle “dip” in particle production behind high-energy quarks as they traverse the quark-gluon plasma, a droplet of primordial matter thought to have filled the universe microseconds after the Big Bang.
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Reproducing the early universe conditions in the laboratory
When heavy atomic nuclei collide at near the speed of light in the LHC, they briefly melt into an exotic state known as quark-gluon plasma.
In this extreme environment, “densities and temperatures are so high that regular atomic structures can no longer be maintained,” Yi Chen, an assistant professor of physics at Vanderbilt University and a member of the CMS team, told Live Science in an email. Instead, “all the atomic nuclei overlap to form what is called a quark-gluon plasma, and quarks and gluons can move beyond the confines of the nucleus. They behave more like a liquid.”
These plasma droplets are very small, about 10 to 14 meters in diameter, or 10,000 times smaller than an atom. And it disappears almost instantly. But within that fleeting droplet, quarks and gluons (the fundamental carriers of the powerful nuclear force that binds atomic nuclei) flow together in a form that resembles an ultra-hot liquid more than just a gas of particles.
Physicists want to understand how high-energy particles interact with this strange medium. “In our research, we want to study how different objects interact with small droplets of liquid produced in collisions,” Chen said. “For example, how do high-energy quarks pass through this hot liquid?”
Theory predicts that quarks leave detectable wakes in the plasma behind them, similar to how a boat cuts through water. “Water will be pushed in the same direction as the boat, but we also expect the water level behind the boat to drop a little because the water is being pushed away,” Chen said.
But in reality, disentangling “boat” and “water” is by no means easy. Because plasma droplets are very small, the resolution of the experiment is limited. At the front of the quark path, the quarks and plasma interact so intensely that it becomes difficult to distinguish which signal is coming from which. But if a wake exists behind the quark, it must be a property of the plasma itself.
“So we want to find this little depression on the back side,” Chen said.
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Clean probe containing Z boson
To isolate that wake, the team focused on a special partner particle. It is a Z particle that is one of the carriers of the weak nuclear force, one of the four fundamental interactions, along with the electromagnetic force, the strong force, and gravity, that are involved in the decay process of certain atoms and subatomic atoms. In certain collisions, Z bosons and high-energy quarks are produced together and recoil in opposite directions.
This is where the Z boson becomes important. “The Z particles are responsible for the weak force, and as far as the plasma is concerned, the Z particles just escape and disappear from the screen,” Chen said. Unlike quarks and gluons, Z bosons have little interaction with the plasma. They leave the collision zone intact and provide a clear indication of the original direction and energy of the quarks.
This setup allows physicists to focus on quarks traveling through the plasma without worrying that their counterpart particles are being distorted by the medium. Essentially, the Z boson acts as a calibrated marker, facilitating the search for subtle changes in particle production behind the quarks.
The CMS team measured the correlation between the Z boson and hadrons, composite particles made of quarks. By analyzing how many hadrons appear in the “opposite direction” to the quark’s motion, it will be possible to search for the predicted wake.
A small but important signal
The results are mixed. “On average, we see less than a 1% change in plasma volume in the reverse direction,” Chen said. “It’s a very small effect (and also one of the reasons it took so long for people to demonstrate it experimentally).”
Still, that less than 1% suppression is exactly the kind of sign you would expect from quarks transferring energy and momentum to the plasma, leaving a depletion region in their wake. The researchers report that this is the first time such a dip has been clearly detected in Z-tagged events.
The shape and depth of the depression encode information about the properties of the plasma. Chen went back to the analogy, saying that when water flows easily, the depression behind the boat quickly fills up. If it behaves like honey, the depression will last longer. “So studying what this dip looks like gives us information about the plasma itself without the complexity of the boat,” she said.
Looking back at the early days of the universe
This discovery also has cosmological implications. The early universe, shortly after the Big Bang, is thought to have been filled with quark-gluon plasma before cooling into protons, neutrons, and eventually atoms.
“At this time, we can’t directly observe it with a telescope,” Chen said. “The universe was opaque back then,” she added, and heavy ion collisions “give us a little glimpse into how the universe behaved at this time.”
For now, Chen concluded that the observed decline is “just the beginning.” “An interesting implication of this study is that it opens a new field to gain more insight into the properties of plasmas. As more data accumulates, we will be able to study this effect more precisely and learn more about plasmas in the near future.”
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