One year after historic first-ever oxygen collisions at the Large Hadron Collider (LHC), all four of the facility’s major research collaborations—ALICE, ATLAS, CMS, and LHCb—have reported compelling signs that the primordial matter of the early universe can emerge when oxygen and neon nuclei collide.
The findings challenge long-standing assumptions about the enormous forces and heavy atomic structures historically thought necessary to create such an extreme substance. By demonstrating that this exotic state of matter can form in smaller nuclear systems, the results offer physicists a novel pathway to investigate the extreme conditions that prevailed during the universe’s earliest moments, reshaping our understanding of high-energy physics.
Recreating the Extreme Matter of the Early Universe
Quark–gluon plasma (QGP) is an unusual state of matter that appears only under conditions of tremendous pressure and heat. At temperatures exceeding 100,000 times those at the center of the Sun, composite particles such as protons and neutrons can no longer maintain their normal internal structures. Instead, they melt or break apart into their fundamental constituents: free-moving quarks and the gluons that ordinarily bind them together in a tight subatomic embrace.
Cosmologists and particle physicists widely believe that this exact state of matter permeated the entire universe during the first millionths of a second following the Big Bang, long before atoms could form. Nearly 14 billion years later, researchers can briefly recreate these extraordinary, primordial conditions by smashing atomic nuclei together at ultra-high velocities and energies inside the massive detectors of the LHC at CERN.
For decades, the prevailing scientific consensus held that generating this dense plasma required collisions involving very heavy atomic nuclei, with lead being the primary candidate. Lead nuclei are more than 200 times heavier than the individual protons typically used in standard LHC proton-proton experiments. This massive scale was thought to be uniquely capable of producing the intense temperatures and pressures necessary for QGP to form and survive long enough to be detected.
However, a series of recent discoveries at CERN has increasingly called that restrictive assumption into question, forcing scientists to rethink the thresholds required for collective subatomic phenomena.
Earlier this year, the ALICE collaboration, which specializes in studying quark–gluon plasma, identified intriguing indicators of QGP formation in proton-proton and proton-lead collisions. The previous year, LHC researchers also reported the first cautious hints of this plasma emerging from oxygen-oxygen collisions.
Now, more detailed and comprehensive investigations have revealed multiple, mutually reinforcing signs that QGP can form reliably in collisions involving both oxygen and neon nuclei. This provides much stronger evidence that the conditions required to create this primordial matter are not strictly limited to ultra-heavy atomic collisions, opening up new avenues for comparative nuclear physics.
Scientists Detect Evidence of Energy Loss in Extreme Collisions
One of the most reliable and direct methods physicists use to identify quark–gluon plasma is to examine what happens when energetic quarks and gluons travel through it immediately after a collision.
As these high-energy particles move through the extremely hot and dense material, they interact with the surrounding medium and lose a portion of their energy. Physicists refer to this phenomenon as parton energy loss, and observing it provides a critical, quantitative clue that QGP has successfully formed and influenced its environment.
The ATLAS collaboration clearly detected this suppression effect by studying pairs of particle jets generated during high-energy oxygen-oxygen and neon-neon collisions. These jets are concentrated, collimated streams of particles produced when energetic quarks and gluons emerge from the primary collision point and hadronize.
By measuring the system, researchers observed distinct asymmetries and energy differences between the paired jets. This indicated that some of the constituent particles had systematically lost energy while plowing through the dense surrounding material before escaping the interaction zone.
Furthermore, this energy-loss effect was especially pronounced during more direct, head-on central collisions. Such high-impact collisions are expected to create a larger, denser volume of plasma, giving outgoing particles a longer path of material to travel through and naturally causing greater energy loss.
Preliminary ATLAS results provide additional robust support for this interpretation. By examining charged particles that recoil in the exact opposite direction from prompt photons, researchers uncovered a similar, consistent relationship between energy loss and the geometric centrality of the nuclear collisions.
Multiple CERN Experiments Confirm the Pattern
To cross-verify these observations, the ALICE, CMS, and LHCb collaborations searched for the same underlying physical processes using entirely different experimental approaches and detection apparatuses. Rather than focusing primarily on the direct energy measurements of particle jets, they examined how the presence of the plasma appears to suppress the production rates of certain energetic particles.
The CMS collaboration found that overall charged-particle production was suppressed in both oxygen-oxygen and neon-neon collisions when compared against baseline proton-proton collisions. This reduction in particle yield is entirely consistent with high-energy particles losing momentum and energy inside a quark–gluon plasma, providing further weight to the argument that the exotic material can form in collisions involving significantly lighter nuclei.
The LHCb collaboration reported another crucial clue in a separate study. The team investigated particles containing a charm quark paired with a lighter quark, comparing how strongly their production was suppressed in oxygen collisions versus neon collisions.
The observed suppression was visibly more pronounced in the heavier neon collisions compared to oxygen. This is precisely the kind of scaling behavior scientists would expect if larger collision volumes produce greater amounts of plasma, causing particles to lose more energy as they traverse the medium.
However, reduced particle production counts alone do not definitively prove that energy loss is the sole culprit, as alternative physical mechanisms could potentially mimic similar suppression patterns.
To investigate and rule out alternative explanations, ALICE compared the production rates of neutral pions—another common type of subatomic particle—in oxygen-oxygen collisions versus proton-oxygen collisions.
This direct comparison yielded unambiguous evidence of parton energy loss specific to oxygen-oxygen collisions, strengthening the scientific case that even relatively small nuclear systems can generate the extreme thermodynamic conditions associated with quark–gluon plasma.
More Clues Emerge From Exotic Subatomic Particles
To build an airtight case, researchers also looked for another distinctive signature of QGP involving exotic particles made from a heavy quark and its corresponding antiquark.
These heavy quark-antiquark pairs can briefly bind together following high-energy collisions. Some of these bound states are more tightly bound than others, making them differently susceptible to thermal melting or color-screening disruption by the surrounding plasma medium.
By examining which bound states are suppressed, and by precisely how much, physicists can gather nuanced, high-resolution evidence about whether a thermalized QGP medium is genuinely present.
The CMS collaboration found clear evidence of this differential suppression in particles known as upsilon mesons, which consist of a bottom quark paired with its antiquark. The researchers identified this pattern by systematically comparing oxygen-oxygen and neon-neon collision datasets.
The LHCb collaboration also reported preliminary evidence of similar behavior, utilizing specialized data compiled from proton-oxygen and oxygen-oxygen collisions.
Together, these independent observations add another vital piece to the growing body of evidence indicating that primordial plasma can emerge in nuclear collisions on scales much smaller than those traditionally utilized by heavy-ion physics.
Unexpected Particle Patterns Offer Another Hint
Expanding their analytical scope, the ALICE collaboration identified yet another potential signature of quark–gluon plasma, this time examining the specific azimuthal angles and directions in which various particles travel after a collision event.
In preliminary findings, researchers observed that particles containing three quarks, known as baryons, showed a significantly stronger preference for specific directions compared to particles composed of just two quarks, known as mesons, during oxygen-oxygen collisions.
This collective behavior is known in nuclear physics as anisotropic flow. Rather than spreading out evenly and randomly in every direction, particles emerge in coordinated geometric patterns heavily influenced by the pressure gradients and hydrodynamic conditions inside the dense material created during the collision.
The leading theoretical explanation for this phenomenon involves the formation of QGP. When particles with intermediate momenta emerge from this flowing plasma, they inherit collective fluid-like motion directly from the underlying quarks that constitute them.
Because baryons contain three valence quarks while mesons contain only two, baryons can inherit a larger share of this collective flow, producing the systematic mass and constituent-dependent differences observed in the experiment.
This distinctive hydrodynamic pattern provides yet another compelling indication that the extreme matter of the early universe is indeed being successfully recreated in oxygen and neon collisions.
A New Window Into the Universe’s First Moments
Physicists and data analysts are continuing to process and analyze vast quantities of collision data involving lighter atomic nuclei, searching for deeper insights into how quark–gluon plasma forms, evolves, and dissipates.
These findings are fundamentally reshaping our understanding of the threshold conditions needed to produce one of the most extreme states of matter known to modern physics. What once appeared to require the brute-force collisions of massive lead nuclei can now be investigated through much smaller, highly versatile systems, giving researchers unprecedented opportunities to explore the behavior of matter under extreme pressures and temperatures.
Meanwhile, CERN is actively upgrading the Large Hadron Collider into the vastly more powerful High-Luminosity LHC. This enhanced accelerator will enable scientists to collect substantially more collision data, improve statistical precision, and investigate the micro-properties of quark–gluon plasma with unprecedented detail.
With each new discovery and technological advancement, researchers are gaining a sharper, more comprehensive picture of the remarkable state of matter that dominated our universe during its first fleeting moments after the Big Bang.
