One year after historic breakthroughs involving oxygen collisions at the Large Hadron Collider (LHC), all four of the collider’s major research collaborations—ALICE, ATLAS, CMS, and LHCb—have reported robust new signs that the primordial form of matter known as quark-gluon plasma can emerge when lighter oxygen and neon nuclei collide.
The findings fundamentally challenge a long-standing assumption in modern physics regarding the enormous forces required to create such an extreme substance. By demonstrating that this primordial soup can emerge from smaller atomic nuclei, the discoveries offer scientists a novel, highly versatile way to investigate the extreme conditions that prevailed during the universe’s earliest, formative moments.
Recreating the Extreme Matter of the Early Universe
Quark-gluon plasma (QGP) represents an unusual and highly exotic state of matter that appears only under conditions of tremendous pressure and heat. At temperatures exceeding 100,000 times those measured at the center of the Sun, composite subatomic particles such as protons and neutrons are fundamentally unable to maintain their normal, bound structure. Instead, under these extreme regimes, they melt apart into a liberated soup of free quarks and the gluons that ordinarily bind them together in everyday matter.
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. Nearly 14 billion years later, researchers are able to briefly recreate these extraordinary, primeval conditions by smashing atomic nuclei together at ultra-high energies inside the multi-kilometer ring of the LHC at CERN.
For decades, the prevailing scientific consensus held that generating this transient 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 utilized in standard LHC physics runs. Because of this massive mass disparity, they were long considered uniquely capable of producing the intense energy densities and spatial volumes thought necessary for QGP to stably form and evolve.
However, a wave of recent discoveries at CERN has increasingly called that restrictive assumption into question. Earlier this year, the ALICE collaboration, which specializes extensively in the study of quark-gluon plasma, identified intriguing new indications of QGP formation in proton-proton and proton-lead collisions. That followed milestones from the previous year, when LHC researchers reported the first tantalizing hints of plasma emerging from oxygen-oxygen collisions.
Now, comprehensive and detailed investigations conducted across multiple detector systems have revealed multiple, mutually reinforcing signs that QGP can form reliably in collisions involving both oxygen and neon nuclei. This provides much stronger empirical evidence that the physical conditions required to forge primordial matter are by no means restricted to heavy-ion collisions alone.
Scientists Detect Evidence of Energy Loss in Extreme Collisions
One of the most definitive and reliable experimental methods for identifying the presence of quark-gluon plasma is to examine the behavior of energetic quarks and gluons as they travel through the newly created medium.
As these high-momentum particles move through the extremely hot, dense material, they interact with the surrounding color-charged medium and lose a fraction of their initial energy. Particle physicists refer to this vital diagnostic process as parton energy loss, and its observation serves as a cornerstone clue that a deconfined QGP medium has successfully formed.
The ATLAS collaboration clearly detected this suppression effect by studying pairs of energetic particle jets generated during oxygen-oxygen and neon-neon collisions. These jets manifest as highly collimated, concentrated streams of particles produced when energetic quarks and gluons fragment upon emerging from a high-energy collision.
By analyzing the data, researchers observed distinct asymmetries and energy discrepancies between the paired jets. This imbalance indicated that some of the constituent particles had systematically lost energy while plowing through the surrounding nuclear matter.
Significantly, this energy-loss effect was found to be especially pronounced during more direct, head-on central collisions. Such high-impact collisions are expected to create a larger, denser volume of plasma, forcing particles to traverse a longer path through the medium and thereby causing greater overall energy loss.
Preliminary ATLAS results provide additional, independent support for this interpretation. By examining charged particles that recoil in the exact opposite direction from prompt photons, researchers uncovered a remarkably similar, quantifiable relationship between energy loss and the geometric centrality of the nuclear impact.
Multiple CERN Experiments Confirm the Pattern
To cross-verify these phenomena, the ALICE, CMS, and LHCb collaborations investigated the same underlying physical processes through alternative experimental lenses. Rather than focusing primarily on the calorimeter energies of high-momentum particle jets, they studied how the dense plasma appears to systematically suppress the production of certain identifiable energetic particles.
The CMS collaboration discovered that overall charged-particle production was notably suppressed in both oxygen-oxygen and neon-neon collisions when compared against baseline proton-proton collisions. This systematic reduction is entirely consistent with particles losing momentum and energy inside a nascent quark-gluon plasma, offering another layer of confirmation that the exotic material can materialize in lighter nuclear systems.
Meanwhile, the LHCb collaboration reported another crucial clue in a separate study. The collaboration investigated composite particles containing a heavy charm quark alongside a lighter quark, comparing the degree to which their production was suppressed in oxygen versus neon collisions.
The suppression proved to be markedly more pronounced in the heavier neon collisions compared to oxygen. This behavior matches precisely what theoretical models predict if larger nuclear overlap volumes produce greater quantities of plasma, causing traversing particles to forfeit more energy along their trajectory.
Because reduced particle production can occasionally arise from alternative, non-QGP physical mechanisms, researchers took pains to rule out confounding variables. To investigate this alternative possibility, ALICE compared the production of neutral pions—another fundamental subatomic particle—in oxygen-oxygen collisions against reference proton-oxygen data.
The resulting comparison yielded unambiguous evidence of parton energy loss exclusively in the oxygen-oxygen collisions, strongly reinforcing the argument that even relatively small nuclear systems can generate the extreme conditions historically associated only with heavy-ion physics.
More Clues Emerge From Exotic Subatomic Particles
Seeking further verification, researchers also hunted for another distinctive, multi-faceted signature of QGP involving bound states made from a heavy quark and its corresponding antiquark.
These heavy quark-antiquark pairs can briefly bind together following high-energy collisions to form distinct mesonic states. Some of these bound states are held together more loosely than others, rendering them differentially susceptible to being melted or disrupted by the surrounding thermal plasma.
By systematically examining which of these bound states are suppressed, and quantifying the exact degree of that suppression, physicists can gather sophisticated diagnostic evidence regarding the temperature and density of the intervening QGP medium.
The CMS collaboration found clear evidence of this differential suppression pattern when analyzing particles known as upsilon mesons, which consist of a bottom quark paired with its antiquark. Researchers identified the effect by carefully contrasting oxygen-oxygen collisions with neon-neon datasets.
Complementing these findings, LHCb reported preliminary evidence of analogous behavior utilizing data drawn from proton-oxygen and oxygen-oxygen runs. Together, these independent observations add vital pieces to the growing puzzle, cementing the conclusion that primordial plasma can emerge in nuclear collisions scaled vastly smaller than those traditionally relied upon.
Unexpected Particle Patterns Offer Another Hint
Beyond jet quenching and particle suppression, the ALICE collaboration has identified yet another potential signature of quark-gluon plasma, this time looking closely at the azimuth angles and directions in which particles travel subsequent to a collision.
In preliminary findings, researchers observed that composite particles containing three quarks, known as baryons, exhibited a significantly stronger preference for specific trajectories than particles composed of two quarks, known as mesons, during oxygen-oxygen collisions.
This complex behavior is referred to by physicists as anisotropic flow. Rather than dispersing evenly in every direction, particles emerge in structured patterns strongly influenced by the hydrodynamic pressure and collective gradients inside the matter created during the collision.
The leading theoretical explanation for this phenomenon points directly to the formation of a localized QGP fluid. When particles with intermediate momenta emerge from this flowing plasma, they inherit collective fluid motion from the liberated quarks that coalesce to form them.
Because baryons contain three valence quarks while mesons contain only two, baryons inherit a proportionally larger share of this collective flow, resulting in the distinct momentum-dependent patterns observed in the detector data. This behavioral pattern supplies yet another compelling indication that the extreme matter of the early universe is indeed being successfully recreated in these modest oxygen collisions.
A New Window Into the Universe’s First Moments
Buoyed by these successes, scientists are continuing to comb through vast sets of collision data involving lighter atomic nuclei, actively searching for further insights into how quark-gluon plasma forms, evolves, and dissolves.
These findings are actively reshaping contemporary theoretical understandings of the energy thresholds and geometrical constraints required to produce one of the most extreme states of matter known to physics. Phenomena that once appeared exclusively accessible via massive lead-lead collisions can now be probed with precision using much smaller, highly tunable systems, granting researchers unprecedented new opportunities to map out the phase diagram of nuclear matter.
Meanwhile, CERN is advancing preparations to upgrade the Large Hadron Collider into the significantly more powerful High-Luminosity LHC. This technological enhancement will empower scientists to collect substantially larger volumes of collision data, allowing them to probe the transient properties of quark-gluon plasma with unprecedented statistical fidelity.
With each new experimental discovery, researchers are piecing together a sharper, more comprehensive picture of the remarkable state of matter that dominated our universe during its first fleeting, energetic moments after the Big Bang.
