Multiple experiments at the Large Hadron Collider have independently reported new evidence that collisions involving oxygen and neon nuclei can produce quark–gluon plasma, the ultra-hot state of matter believed to have filled the universe during its first microseconds. The findings suggest that this primordial matter may form in much lighter collision systems than researchers once thought possible, broadening the ways scientists can investigate the early universe.
For decades, physicists believed that recreating quark–gluon plasma (QGP) required smashing together extremely heavy atomic nuclei such as lead. That assumption has steadily weakened in recent years, and fresh results from all four of the Large Hadron Collider’s major experiments now strengthen the case that much lighter atomic nuclei can also generate this extraordinary state of matter.
Just one year after the first oxygen collisions at the LHC, the ALICE, ATLAS, CMS, and LHCb collaborations have each reported new signs pointing toward QGP formation in oxygen–oxygen and neon–neon collisions. Together, the independent observations provide multiple lines of evidence supporting the same conclusion.
QGP forms only under extraordinary conditions, where matter is compressed and heated to temperatures more than 100,000 times hotter than the center of the Sun. Under these extremes, particles that normally remain bound together break apart into their fundamental building blocks—quarks and the gluons that bind them. Scientists believe the entire universe existed in this form during the first microseconds after the Big Bang before cooling into the matter seen today.
Evidence builds from multiple experimental approaches
Rather than relying on a single measurement, the four collaborations searched for several distinct signatures expected if QGP had formed.
One of the clearest indicators is parton energy loss. In this process, fast-moving quarks and gluons lose energy while traveling through the hot, dense plasma. Detecting this energy loss provides an important clue that the medium has formed.
The ATLAS Collaboration observed this effect by studying pairs of particle jets produced during oxygen–oxygen and neon–neon collisions. Researchers found an imbalance between the two jets, indicating that one had lost energy while passing through the dense medium. The imbalance became even stronger in more central, head-on collisions, where scientists expect a larger volume of QGP to develop.
Preliminary ATLAS studies examining charged particles recoiling against photons showed the same dependence on collision centrality, further supporting the interpretation that the particles were losing energy while traversing quark–gluon plasma.
Independent measurements point to the same phenomenon
The ALICE, CMS, and LHCb collaborations investigated the problem from another direction by examining whether energetic particle production becomes suppressed in these lighter-ion collisions.
CMS found that the production of charged particles was suppressed in both oxygen–oxygen and neon–neon collisions when compared with proton–proton collisions. This pattern is consistent with quarks and gluons losing energy inside QGP before emerging from the collision.
LHCb focused on particles made from a charm quark paired with a lighter quark. The collaboration found evidence that suppression became more pronounced in neon–neon collisions than in oxygen–oxygen collisions. Researchers consider this an expected consequence if larger collision systems produce a greater volume of quark–gluon plasma, increasing the amount of energy lost by passing particles.
ALICE designed an analysis intended to separate parton energy loss from other possible effects that might also reduce particle production. Instead of comparing only different collision systems, the collaboration measured the production of neutral pions in oxygen–oxygen and proton–oxygen collisions. According to the researchers, this comparison provided unambiguous evidence that parton energy loss occurs in oxygen–oxygen collisions.
Heavy-quark particles provide another clue
Researchers also searched for another well-established signature of QGP involving short-lived particles composed of a heavy quark and its antiquark.
These particles, known as bound states, can exist with different binding strengths. If quark–gluon plasma forms, it suppresses these states by different amounts. Measuring this pattern allows scientists to infer whether the plasma was present.
CMS found evidence for this varying suppression by studying upsilon mesons, which consist of a bottom quark and its antiquark. Comparing oxygen–oxygen and neon–neon collisions revealed the expected differences in suppression among the various bound states.
LHCb reported preliminary evidence of a similar suppression pattern using data collected from proton–oxygen and oxygen–oxygen collisions, adding another independent indication that QGP may be forming in these lighter systems.
Particle flow offers an additional hint
The ALICE Collaboration also presented preliminary findings involving anisotropic flow, another phenomenon associated with quark–gluon plasma.
In oxygen–oxygen collisions, researchers found that baryons, particles made of three quarks, exhibited a stronger preferred emission direction than mesons, which contain two quarks. The leading explanation attributes this behavior to QGP, whose collective motion is transferred to particles produced during the collision. Because baryons contain one additional quark compared with mesons, they are expected to inherit more of this collective flow.
Although preliminary, these observations add another independent signature consistent with the presence of quark–gluon plasma.
Expanding the search for the universe’s earliest matter
The latest findings continue a shift in researchers’ understanding of where QGP can emerge.
Heavy-ion collisions involving lead were long regarded as the only environment capable of generating the extreme temperatures and densities needed for the plasma to form. More recently, however, that picture has changed. Earlier this year, ALICE reported a new indication of QGP even in proton–proton and proton–lead collisions. Last year’s oxygen collision results provided the first hints from light-ion systems, and the latest analyses deepen that evidence by uncovering multiple, complementary signatures across oxygen–oxygen and neon–neon collisions.
Instead of depending on a single experimental result, the current picture is built from independent measurements that all point toward the same physical process. Energy loss among quarks and gluons, suppression of energetic particles, differing suppression of heavy-quark bound states, and collective particle flow all align with expectations for quark–gluon plasma.
Researchers continue to analyze the growing volume of LHC collision data in search of even stronger evidence. At the same time, the accelerator itself is being upgraded into the High-Luminosity LHC, which is expected to provide more powerful opportunities to probe the properties of quark–gluon plasma and further explore the extreme state of matter that dominated the universe immediately after the Big Bang.
