In the earliest moments after the universe’s birth, matter looked nothing like the stars, planets and galaxies we observe today. Before protons, neutrons, or even atoms existed, the cosmos was dominated by an incredibly hot and dense state comprised of elementary particles moving at near the speed of light. Understanding how this primordial matter behaved remains one of the biggest challenges in modern physics, offering clues about the conditions that governed the origin of the universe.
A new study leveraging data from the Large Hadron Collider at CERN has allowed researchers to explore this early state of matter by briefly recreating it in a laboratory setting. The research, conducted by the CMS collaboration and published in Physics Letters B, details an analysis of how high-energy particles interact with what’s known as quark-gluon plasma. The findings provide a new way to observe how this extreme medium responds when a particle passes through it. This function represents a significant step forward in our understanding of the universe’s earliest moments.
Recreating the Universe’s Primordial Matter in the Lab
The theory describing the interactions of quarks and gluons, known as quantum chromodynamics, predicts that at extremely high temperatures, these particles cease to be confined within protons and neutrons. Under these conditions, a state of matter called quark-gluon plasma emerges, where the fundamental components of nucleons move freely in an extremely dense and hot medium. This plasma is believed to have dominated the universe during its first few microseconds.
Particle accelerators allow scientists to recreate this environment, albeit momentarily. At the Large Hadron Collider, heavy nuclei—such as those of lead—are accelerated to near the speed of light and collided with each other. The impact releases an enormous amount of energy, generating a tiny droplet of quark-gluon plasma that exists for only a minuscule fraction of a second.
During this brief instant, detectors register thousands of particles produced in the collision. Analyzing their trajectories and energies allows researchers to indirectly reconstruct the properties of the plasma. According to the study, these collisions allow investigation into how high-energy particles interact with the medium created in the experiment, helping to understand the dynamics of the primordial plasma.
A New Way to Observe Plasma Behavior
The work focuses on analyzing events in which a Z boson is produced, an elementary particle belonging to the weak interaction family. Unlike other particles generated in the collisions, the Z boson barely interacts with the quark-gluon plasma. This makes it a kind of experimental reference point for studying what happens in the surrounding environment.
When a Z boson appears in a collision, it is usually generated at the same time as a parton—a high-energy quark or gluon—that moves in the opposite direction. That parton does interact intensely with the plasma and loses energy as it passes through it. Physicists leverage this situation to study how the medium responds to this passage.
The analysis is based on measuring the correlations between the Z boson and the charged particles produced in the collision. By comparing these data with ordinary proton collisions, where no plasma is formed, researchers can identify specific signals of the medium’s effect. As the study explains, the goal is to examine how the quark-gluon plasma medium affects the parton receding in the opposite direction to the Z boson.
The detailed analysis of the data reveals a characteristic pattern in the distribution of the produced particles. Under certain conditions, a decrease in particles appears in the direction of the Z boson and an excess in the opposite region—a behavior that theoretical models associate with the medium’s response.
As the study explains, a minimum in the distribution of particles near the direction of the Z boson indicates the presence of a negative wake of the medium. The article clarifies that “a depression in Δφ near 0 indicates the presence of a negative wake of the medium or ‘holes’ in the medium.”
This phenomenon is interpreted as a sign that the parton drags and redistributes energy within the plasma as it passes through it. The interaction causes the medium to react collectively, generating regions where the particle density decreases and others where it increases. These modifications are recorded in the angular distribution of the detected particles.
The results show that these signals appear more clearly in the most central collisions, where the created plasma is denser. The effects are observed especially in particles with relatively low energies, which is consistent with the predictions of several theoretical models that describe how energy is redistributed in the plasma.
Direct Evidence of the Medium’s Response
The study concludes that the differences observed between heavy-ion collisions and proton collisions provide a clear signal of the interaction between the parton and the plasma. In the study’s own words, “the differences between PbPb and pp data provide the first evidence of medium recoil effects and ‘holes’ in the medium caused by a hard probe”.
This result has an key implication for high-energy physics. It confirms that quark-gluon plasma does not simply behave as a cloud of independent particles, but as a system that responds collectively to disturbances. Understanding this response is essential for determining fundamental properties of the plasma, such as its viscosity or how it transports energy.
Researchers also compared their data with different theoretical models describing particle energy loss in the plasma. Those models that explicitly include the medium’s response best reproduce the experimental observations. This reinforces the idea that collective effects play a central role in the plasma’s dynamics.
While the result constitutes strong evidence, the authors note that measurements with greater statistical precision will be needed to discriminate between the different models. Nevertheless, the work opens a new avenue for studying the properties of quark-gluon plasma, one of the most extreme forms of matter known.
