Four major research collaborations at the Large Hadron Collider (LHC) have reported evidence that quark gluon plasma can form in surprisingly small nuclear collisions. The findings from the ALICE, ATLAS, CMS, and LHCb experiments indicate that this primordial state of matter, which existed fractions of a second after the Big Bang, emerges when oxygen and neon nuclei collide.
These results challenge a long-standing assumption in high-energy physics that creating such extreme conditions requires collisions involving very heavy atomic nuclei, such as lead. Lead nuclei are more than 200 times heavier than the protons typically used in LHC experiments, and scientists previously believed their massive size was necessary to generate the intense pressure and heat required for quark gluon plasma to form.
Challenging Assumptions About Collision Mass
Quark gluon plasma is a state of matter that appears under tremendous heat and pressure. At temperatures exceeding 100,000 times those at the center of the Sun, protons and neutrons break apart into their constituent quarks and gluons. Physicists believe this was the state of the universe during its first millionths of a second.
For decades, researchers relied on heavy ion collisions to recreate these conditions. However, recent discoveries have increasingly questioned this requirement. Earlier this year, the ALICE collaboration identified signs of quark gluon plasma in proton-proton and proton-lead collisions. The previous year, LHC researchers reported the first hints of this plasma emerging from oxygen-oxygen collisions.
The new, more detailed investigations provide stronger evidence that the conditions for creating this primordial matter are not limited to heavy atomic collisions. The findings come one year after the first-ever oxygen collisions at the Large Hadron Collider, marking a significant expansion in how scientists can study the early universe.
Multiple Signatures Detected Across Collaborations
The four collaborations used different methods to identify the plasma, all pointing to the same conclusion. The ATLAS collaboration detected parton energy loss, a process where energetic quarks and gluons lose energy as they travel through the hot, dense material. ATLAS researchers studied pairs of particle jets generated during oxygen-oxygen and neon-neon collisions and observed differences in energy between the paired jets. This effect was especially pronounced in direct, head-on collisions, which create a larger volume of plasma.
CMS and LHCb examined the suppression 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. This reduction is consistent with particles losing energy inside the plasma.
LHCb reported that the suppression was more pronounced in the heavier neon collisions. The collaboration also investigated particles containing a charm quark, finding behavior consistent with larger collisions producing greater volumes of plasma. To rule out other physical processes, ALICE compared the production of neutral pions in oxygen-oxygen and proton-oxygen collisions, providing unambiguous evidence of parton energy loss in oxygen-oxygen collisions.
Additional signatures further support the presence of quark gluon plasma. CMS found evidence of varying suppression in upsilon mesons, which consist of a bottom quark paired with its antiquark. LHCb reported preliminary evidence of similar behavior using proton-oxygen and oxygen-oxygen collisions. ALICE also observed anisotropic flow, where particles containing three quarks showed a stronger directional preference than those composed of two quarks during oxygen-oxygen collisions.
These observations reshape 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.
Source: ScienceDaily
