The universe was created 13.8 billions years ago without stars, planets or even atoms. It was instead in a dense, hot state made up of quarks and other tiny building blocks, called quark-gluon Plasma (QGP).
Quarks and neutrons were free to move in the first few moments following the Big Bang. They seemed like swimming freely through a cosmic ocean. They merged together as the universe cooled to create protons and neutrons.
CERN scientists and MIT physicists have discovered strong evidence that “primordial liquid” acted as a fluid. When quarks speed through the plasma they produce ripples and spirals. The plasma is shown to flow smoothly and densely as fluid, rather than breaking up into particles.
It has been debated for a very long time in our field whether plasma would respond to quarks. Yen Jie Lee is a professor of physics and mathematics at MIT. We can see that the plasma has an incredible density, allowing it to produce splashes, swirls, and even slow down quarks. Quark-gluon Plasma is really primordial soup.
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This study answers a question that has been a source of debate for a very long time: does QGP act more like a liquid or a gas when it is disturbed?
Quark-gluon Plasma (QGP), the first liquid in the universe, was also one of its most extreme. The liquid reached temperatures in the trillions and flowed nearly perfectly.
It was difficult to detect these wakes. Scientists studied pairs of antiquarks and quarks that were produced by high-energy collisions to try and spot these wakes. Due to their overlap, it is difficult to determine the effects of one quark.
Lee’s group took a completely different approach. They did not chase quark-antiquark pair collisions, but instead looked for those that produce a Z boson along with a quark. It is the ideal “silent companion” because it’s a neutral, non-interacting particle.
In this soup of plasma quarks, many quarks and other gluons pass by each other and are colliding. Lee explains. Sometimes, if we’re lucky, one collision creates both a Z boson, and a high-energy quark.
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Both particles fly off in opposing directions. The Z boson does not leave a trace. However, the quark drags the plasma along, leaving behind a trail. The Z boson can be tracked to reveal the location of the quark. This makes it possible to track the Z boson and measure its ripples.
They sifted data collected by the CMS experiment, which smashes lead-like heavy ions at speeds close to light. They found that out of 13 billion collisions they had about 2,000 instances where a Z-boson was produced.
They mapped out the patterns of energy within each droplet. Each one lasted less than a quadrillionth second. They saw the same fluid splashes, which were unmistakable signals of quark wakes, on both sides of the Z-boson.
According to Rajagopal’s hybrid model which has been around for a while, quarks are supposed to stir up the plasma in a similar way that boats cut through water.
Many of us had argued for many years that this must exist, and many experiments were conducted to find it. Krishna Rajagopal is the William A. M. Burden professor of Physics at MIT. He was not involved directly in the research.
Daniel Pablos is a professor of Physics at Oviedo university in Spain. He called the results “What Yen Jie and CMS did was devise and implement a measure that brought us and them the first clear, clean and unambiguous proof for this fundamental phenomenon.”
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Lee added We’ve gained direct proof that quarks do indeed carry more plasma along with them as they travel. It will allow us to examine the behavior and properties of this fluid at a level never before seen.
The quark-gluon Plasma is much more than an interesting curiosity. This plasma is an excellent window to the forces of nature that form matter. Scientists hope that by studying the interaction of quarks with the fluid they can uncover the process whereby the universe went from chaos to order and the reason why it exists today.
This discovery also shows the importance of creative experiment design. By using Z bosons to “tag” single quarks’ elusive trails, scientists were able to isolate them. This breakthrough could lead to new ways of understanding plasma properties.
Models and independent experiments have suggested for a long time that QGP is the first and possibly most perfect liquid in the universe. New evidence presents a very clear picture. The evidence shows an ocean of quarks, gluons and other particles flowing together in a smooth manner. When disturbed, they spewed and swirled.
Journal Reference
- The Cms collaboration. Correlations between Z bosons and hadrons during heavy ion collision provide evidence of medium response. Physics Letters B. DOI: 10.1016/j.physletb.2025.140120
