Researchers at the Large Hadron Collider (LHC) have reported new indications that oxygen and neon collisions can create quark–gluon plasma (QGP), an extreme state of matter believed to have existed during the first microseconds after the Big Bang. According to PHYS, the four main LHC experiments—ALICE, ATLAS, CMS, and LHCb—have each identified signs of this state of matter in light-ion collisions, one year after the first oxygen collisions occurred at the facility.
Challenging the Heavy-Ion Premise
Historically, scientists believed that colliding heavy ions, such as lead—which is more than 200 times heavier than protons—was the only method to generate the conditions necessary for QGP formation. This state of matter exists under intense pressure and at temperatures exceeding 100,000 times those found at the center of the sun. Under these conditions, composite particles decompose into quarks and gluons.
The long-standing premise regarding heavy-ion exclusivity has been challenged by recent findings. Earlier this year, the ALICE Collaboration reported signs of QGP in proton–proton and proton–lead collisions. The latest findings in oxygen and neon collisions provide further evidence that QGP can be generated in smaller collision systems.
Evidence Through Parton Energy Loss
The ATLAS Collaboration observed evidence of parton energy loss, indicated by an imbalance between pairs of particle jets in oxygen–oxygen and neon–neon collisions. This effect is more pronounced in central, or head-on, collisions, where a larger volume of QGP results in greater energy loss. Preliminary ATLAS studies involving charged particles recoiling against photons have shown a consistent dependence on collision centrality.
The ALICE Collaboration also provided evidence for parton energy loss by comparing the production of neutral pions in oxygen–oxygen and proton–oxygen collisions. This comparison offered unambiguous evidence of the phenomenon, which is consistent with the formation of a larger-volume QGP as the collision size increases.
Suppression of Particle Production
Other collaborations utilized different methods to identify signs of QGP by studying the suppression of energetic particles. The CMS Collaboration observed a suppression of charged-particle production in oxygen–oxygen and neon–neon collisions compared to proton–proton collisions, which researchers interpret as a sign of parton energy loss and QGP presence.
Furthermore, researchers can deduce the presence of QGP by measuring how it suppresses differently bound states of heavy quarks and their antiquarks. CMS found evidence of this varying suppression for upsilon mesons, which are bound states of a bottom quark and its antiquark, by comparing oxygen–oxygen and neon–neon collision data. The LHCb Collaboration has reported preliminary evidence of similar suppression using proton–oxygen and oxygen–oxygen collision data. Additionally, the ALICE Collaboration has released preliminary results indicating another potential sign of QGP.
Future Research and Experimental Scope
As researchers continue to analyze data from light-ion collisions, the LHC is undergoing a transition into the High-Luminosity LHC. This upgrade is expected to allow scientists to probe the nature of QGP with greater precision. While studies of potential QGP formation continue, the current findings from the four main collaborations suggest that the conditions for this primordial state of matter are reachable through a broader variety of collision types than previously assumed.
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