Heavy collisions at the Large Hadron Collider (LHC) have given scientists a peek into the early universe. Researchers using the Compact Muon Solenoid (CMS) collaboration found clear evidence of a “wake” left by a quark moving through a type of matter called quark-gluon plasma. This plasma is thought to have existed right after the Big Bang, just moments after the universe began.
Published on December 25, 2025, in Physics Letters B, this study emphasizes how the universe’s first moments might have been more complex than we imagined.
When heavy atomic nuclei crash into each other at nearly the speed of light in the LHC, they briefly transform into this exotic state known as quark-gluon plasma. In this extreme environment, regular atomic structures break down. Yi Chen, a physicist at Vanderbilt University and a member of the CMS team, explains that “all the nuclei overlap, forming a state where quarks and gluons can move freely, almost like a liquid.”
This plasma droplet, incredibly tiny—about 10,000 times smaller than an atom—disappears almost instantly. Inside it, quarks and gluons interact in a way that mirrors the behavior of liquid rather than gas. Scientists are keen to understand how high-energy particles behave within this unique medium.
Chen notes, “We want to observe how these particles interact with the quark-gluon plasma.” As a quark travels through the plasma, it should theoretically leave a detectable wake similar to a boat moving in water. The challenge lies in differentiating the signals from the quark and the plasma due to the extreme densities involved.
To study the wake, researchers used Z bosons, which are carriers of the weak nuclear force. Chen highlights that Z bosons hardly interact with the plasma, allowing them to serve as clean indicators of the quark’s direction and energy. This unique setup enables scientists to analyze how many particles, or hadrons, appear in the “backward” direction of the quark’s movement.
The findings indicate a subtle but significant change in plasma density—less than 1%. While this may seem minor, it aligns with expectations about how a quark transfers energy and momentum to the plasma. This discovery marks the first clear detection of such a dip in Z-tagged events.
Understanding the shape and depth of this dip can reveal details about the plasma’s properties. If it behaves like water, the dip fills quickly; if it acts more like honey, it remains longer. Analyzing these characteristics can give researchers deeper insights into how the quark-gluon plasma functions.
The implications of these findings extend beyond the lab. The early universe, shortly after the Big Bang, is believed to have been primarily quark-gluon plasma before cooling down to form protons, neutrons, and eventually atoms. “We can’t see that era with telescopes, as the universe was opaque,” Chen says, calling heavy-ion collisions a unique way to glimpse how the universe operated during those formative moments.
This discovery is just the beginning. Chen believes that further data will enhance understanding of the properties of quark-gluon plasma, providing new avenues for exploration in physics. As the team continues its research, the insights gained might help paint a clearer picture of the universe’s earliest days.
For a glimpse into the complex behaviors of quark-gluon plasma, check out additional resources from CERN and Live Science.
