Quark-Gluon Plasma: First Evidence of Quark Wakes Confirms ‘Primordial Soup’

For decades, physicists have theorized about the state of matter that existed in the universe just moments after the Big Bang – a searingly hot, dense mix of fundamental particles known as quark-gluon plasma (QGP). Now, researchers at CERN’s Large Hadron Collider (LHC) are reporting the clearest evidence yet that this primordial “soup” didn’t just exist as a chaotic collection of particles, but actually flowed like a liquid, exhibiting ripples and swirls as quarks moved through it. The findings, published in Physics Letters B, offer a crucial glimpse into the conditions of the early universe and validate long-held theoretical predictions about the behavior of QGP.

Recreating the Universe’s First Moments

The universe, in its earliest fractions of a second, wasn’t composed of the atoms, stars, and galaxies we see today. Instead, it was an incredibly hot and dense environment where quarks and gluons – the fundamental building blocks of protons and neutrons – existed in a free state, unbound by the strong force that normally confines them. This state, the quark-gluon plasma, lasted for only a few millionths of a second before cooling sufficiently for quarks and gluons to combine and form the matter we know. CERN physicists are attempting to recreate these conditions by colliding heavy ions, such as lead nuclei, at near-light speeds.

These collisions generate temperatures exceeding trillions of degrees Celsius, briefly liberating quarks and gluons from ordinary matter and creating minuscule droplets of QGP. Studying the debris from these collisions – particles like pions, kaons, protons, and antiprotons – allows scientists to infer the properties of the plasma itself. The challenge lies in observing the fleeting interactions within this incredibly short-lived state.

A Wake in the Primordial Soup

The team, led by MIT physicists, developed a novel technique to observe the effects of individual quarks moving through the QGP. Previous attempts to detect “wakes” – disturbances created by a quark’s passage – were hampered by the presence of paired quarks and antiquarks, where one particle’s wake overshadowed the other. To overcome this, the researchers focused on events involving a single quark produced in conjunction with a Z boson, a neutral particle that doesn’t interact with the plasma.

“We have figured out a new technique that allows us to see the effects of a single quark in the QGP, through a different pair of particles,” explained Yen-Jie Lee, professor of physics at MIT. By using the Z boson as a “tag” to identify the quark’s trajectory, they were able to map the energy distribution within the QGP and consistently observe fluid-like patterns of splashes and swirls – a clear indication of a wake effect. From analyzing 13 billion collisions, the team identified approximately 2,000 events exhibiting this behavior.

Confirming a “Perfect” Fluid

The observed wake effects align with predictions from the “hybrid model” developed by Krishna Rajagopal, a professor of physics at MIT. This model posits that QGP should respond to particles moving through it as a dense fluid, creating ripples and splashes. As CERN notes, the QGP is thought to be a near-“perfect” liquid, meaning that the quarks and gluons flow together smoothly with minimal friction. This new evidence provides strong support for this idea.

“Here’s something that many of us have argued must be there for a great many years, and that many experiments have looked for,” said Rajagopal, who was not directly involved in the new study. The findings demonstrate that the plasma isn’t simply a loose collection of particles, but a cohesive medium capable of influencing the motion of quarks passing through it.

Implications for Understanding the Strong Force

The study has significant implications for our understanding of the strong force, one of the four fundamental forces of nature. As CERN explains, the strong force is responsible for binding quarks and gluons together within hadrons, such as protons and neutrons, and ultimately holds atomic nuclei together. By studying the QGP, physicists can gain insights into the behavior of the strong force under extreme conditions, shedding light on the fundamental laws governing the universe.

The ability to observe and characterize the wake effects of quarks within the QGP opens up new avenues for research. Measuring the size, speed, and dissipation rate of these wakes can provide valuable information about the plasma’s properties, such as its viscosity and density. This, in turn, can help refine theoretical models and deepen our understanding of the early universe.

What Comes Next: Refining the Snapshot

The researchers plan to apply their new technique to analyze more data from the LHC, aiming to pinpoint other quark wakes and further refine their understanding of the QGP. Future studies will focus on characterizing the detailed properties of these wakes, including their shape, size, and how they evolve over time. The CMS Collaboration, the international team of physicists responsible for the experiment, will continue to collect and analyze data from the LHC, seeking to unravel the mysteries of the quark-gluon plasma and the early universe. The open-access study appears in the journal Physics Letters B.