For decades, the rule for creating quark - gluon plasma (QGP) - the ultra-hot soup of unbound quarks and gluons that filled the universe in its first millionths of a second - was simple: bring the biggest nuclei you've got. Lead, specifically, which is more than 200 times heavier than the protons typically used in LHC experiments. The assumption was that only such heavyweights could generate the tremendous pressure and heat required, temperatures exceeding 100,000 times those at the Sun's center, where protons and neutrons stop holding themselves together and dissolve into their constituent quarks and gluons.
It turns out the universe may have been more frugal than that. One year after the first-ever oxygen collisions at the Large Hadron Collider (LHC), all four of its major research collaborations - ALICE, ATLAS, CMS and LHCb - have now reported signs that this primordial form of matter can emerge when oxygen and neon nuclei collide. Yes, oxygen and neon. The lightweight stuff. The participation-trophy nuclei.
The results challenge a long-standing assumption about the enormous forces needed to create such an extreme substance, and offer scientists a new way to investigate the conditions that prevailed during the universe's earliest moments.
One of the clearest ways to identify QGP is to watch what happens when energetic quarks and gluons try to plough through it. As these particles move through the extremely hot and dense material, they lose some of their energy - a process scientists call parton energy loss, and a handy clue that QGP has formed. The ATLAS collaboration clearly detected this effect by studying pairs of particle jets - concentrated streams of particles produced when energetic quarks and gluons emerge from a collision - generated during oxygen - oxygen and neon - neon collisions. Researchers observed differences in energy between the paired jets, indicating that some of the particles had lost energy while traveling through the surrounding material. The effect was especially pronounced during more direct, head-on (central) collisions, which are expected to create a larger volume of plasma, giving particles more material to travel through and causing greater energy loss. Preliminary ATLAS results back this up: by examining charged particles that recoil in the opposite direction from photons, researchers found a similar relationship between energy loss and how centrally the nuclei collided.
ALICE, CMS, and LHCb went looking for the same underlying process with a different approach. Rather than focusing primarily on jet energy, they examined how the plasma appears to reduce the production of certain energetic particles. CMS found that charged-particle production was suppressed in both oxygen - oxygen and neon - neon collisions compared with proton - proton collisions - consistent with particles losing energy inside QGP, and further evidence that the exotic material can form in collisions involving lighter nuclei. LHCb reported another important clue in a separate study, investigating particles containing a charm quark and a lighter quark and comparing how strongly their production was suppressed in oxygen and neon collisions. The suppression was more pronounced in the heavier neon collisions, precisely the behavior scientists would expect if larger collisions produce greater volumes of plasma, causing particles to lose more energy as they pass through it.
Reduced particle production, however, does not necessarily prove that energy loss is responsible; other physical processes could potentially produce similar effects. So ALICE compared the production of neutral pions, another type of subatomic particle, in oxygen - oxygen and proton - oxygen collisions. The comparison provided unambiguous evidence of parton energy loss in oxygen - oxygen collisions, strengthening the case that even relatively small nuclear collisions can generate conditions associated with QGP.
Researchers also looked for another distinctive signature involving particles made from a heavy quark and its corresponding antiquark. These pairs can briefly bind together following high-energy collisions, and some are more tightly bound than others, making them differently susceptible to disruption by the surrounding plasma. By examining which bound states are suppressed and by how much, physicists can gather additional evidence about whether QGP is present. The CMS collaboration found evidence of this varying suppression in particles called upsilon mesons, which consist of a bottom quark paired with its antiquark, by comparing oxygen - oxygen and neon - neon collisions. LHCb also reported preliminary evidence of similar behavior, this time using data from proton - oxygen and oxygen - oxygen collisions.
ALICE has identified yet another possible signature, this time involving the directions in which particles travel after a collision. In preliminary findings, researchers observed that particles containing three quarks (baryons) showed a stronger preference for certain directions than particles composed of two quarks (mesons) during oxygen - oxygen collisions. This behavior is known as anisotropic flow: rather than spreading evenly in every direction, particles emerge in patterns influenced by the conditions inside the material created during the collision. The leading explanation involves QGP formation - when particles with intermediate momenta emerge from this plasma, they can inherit collective motion from the quarks that make them up. Because baryons contain three quarks while mesons contain only two, baryons can inherit more of this flow, producing the difference observed in the experiment.
Earlier this year, the ALICE collaboration, which specializes in studying QGP, identified another indication of QGP formation in proton - proton and proton - lead collisions; the previous year, LHC researchers had also reported the first hints of this plasma emerging from oxygen - oxygen collisions. The picture, in other words, has been assembling itself for a while.
Scientists are continuing to analyze data from collisions involving lighter atomic nuclei, searching for additional evidence of how QGP forms and behaves. The findings are reshaping the understanding of the conditions needed to produce one of the most extreme states of matter known to physics. What once appeared to require collisions between massive nuclei can now be investigated through much smaller systems, giving researchers new opportunities to explore the behavior of matter under extraordinary conditions.
Meanwhile, CERN is upgrading the LHC into the more powerful High-Luminosity LHC, which will allow scientists to collect substantially more collision data and investigate the properties of QGP in greater detail. With each new discovery, researchers are gaining a clearer picture of the remarkable state of matter that dominated the universe during its first fleeting moments after the Big Bang - now apparently reproducible with nuclei that don't even qualify as heavyweight contenders. Materials provided by CERN. Original written by Rory Harris.