Ripples in the Quark Soup: Discoveries Reinforce Liquid Nature of Primordial Matter

Scientists at CERN have detected signs that quarks create waves as they pass through quark-gluon plasma, akin to ripples on water, affirming the liquid-like properties of this extreme form of matter. Physicists are recreating quark-gluon plasma (QGP)-the state of matter that existed in the first microseconds after the Big Bang-at the Large Hadron Collider (LHC) at CERN by colliding heavy ions at nearly the speed of light. QGP existed for only a few millionths of a second and comprised a “soup” of quarks and gluons at trillions of degrees Celsius. By analyzing the results of these collisions, scientists aim to understand the properties of this “primordial matter.”

A group of physicists from MIT, working at CERN, developed a new data analysis method that allowed them to see for the first time how individual quarks interact with QGP. They observed signs of wave effects occurring as the quark moves through the plasma, providing direct evidence of its liquid behavior. It was previously believed that QGP should respond to passing particles as a unified liquid, rather than as a random set of individual particles. This new study confirms that hypothesis.

“There have long been debates about whether the plasma should respond to a quark. Now we see that the plasma is incredibly dense, capable of slowing down quarks and producing splashes and swirls like a liquid. So the quark-gluon plasma truly is a primordial soup,” says Professor Yen-Ji Lee from MIT.

For these effects to be detected, the authors of the study analyzed data obtained using the Compact Muon Solenoid (CMS) detector at LHC. CMS is one of two major general-purpose detectors designed to study a wide range of physical phenomena arising from particle collisions. Physicists searched for events in which a quark and a Z boson are simultaneously produced. The Z boson is a neutral elementary particle that interacts minimally with its environment and is easily detected. In lead ion collisions, scientists selected events where a Z boson and one quark were born, flying in opposite directions. Unlike the quark, the Z boson does not interact with the plasma, allowing the “trail” left by the quark to be isolated.

By analyzing approximately 2,000 such events out of 13 billion collisions, scientists discovered characteristic wave patterns corresponding to the predictions of a theoretical model. The study results open new opportunities for studying the properties of QGP. By measuring the characteristics of wave effects, scientists will gain a more accurate understanding of how this exotic form of matter behaved in the first moments of the universe’s existence.

Recent advancements in theoretical research have also posed challenges to previously accepted views on quark-gluon interactions. New models suggest intricate behaviors that could redefine our understanding of QGP’s viscosity, including potential variations in how scatterings occur at different energy levels within the plasma.

Furthermore, as CERN plans to enhance the sensitivity and resolution of its detectors, particularly the CMS, future experiments might unveil even more intriguing findings about the early universe, potentially capturing phenomena beyond current models’ predictions. The global physics community continues to eagerly anticipate these developments, contributing fresh insights and methodologies to collectively elevate our comprehension of the cosmos’ foundational ingredients.