Four research collaborations at CERN’s Large Hadron Collider have reported evidence that quark-gluon plasma can form when relatively light atomic nuclei collide. The findings, from ALICE, ATLAS, CMS and LHCb, include results from oxygen-oxygen and neon-neon collisions, as well as comparisons involving protons and oxygen nuclei.
Quark-gluon plasma is an extremely hot, dense state of matter in which protons and neutrons break down into quarks and the gluons that bind them. Physicists believe it filled the universe during the first millionths of a second after the Big Bang. Until recently, researchers generally expected that creating it required collisions between very heavy nuclei, such as lead.
The new results build on earlier indications of plasma-like behavior in proton collisions and the first oxygen collisions at the LHC. Across the latest studies, scientists looked for several effects that can occur when energetic particles travel through the dense material and lose energy.
ATLAS detected differences in the energy of paired particle jets produced in oxygen and neon collisions. The effect was stronger in more central, head-on collisions, which are expected to create a larger volume of plasma. Preliminary ATLAS results using particles recoiling opposite photons showed a similar relationship.
CMS found that charged-particle production was reduced in oxygen-oxygen and neon-neon collisions compared with proton-proton collisions. LHCb observed stronger suppression of particles containing charm quarks in neon collisions than in oxygen collisions, consistent with greater energy loss in a larger collision system. ALICE’s comparison of neutral pions in oxygen-oxygen and proton-oxygen collisions provided evidence of parton energy loss in oxygen-oxygen collisions.
The collaborations also reported other possible signatures. CMS found different levels of suppression among upsilon mesons—particles made of a bottom quark and its antiquark—in oxygen and neon collisions. LHCb reported preliminary evidence of similar behavior in proton-oxygen and oxygen-oxygen data.
In preliminary oxygen-collision results, ALICE also observed that baryons, which contain three quarks, showed a stronger directional preference than mesons, which contain two. This pattern, known as anisotropic flow, may reflect collective motion inherited from quarks in the plasma.
Together, the observations strengthen the case that conditions associated with quark-gluon plasma can arise in collisions involving lighter nuclei, though the reported signatures do not all establish the plasma on their own. CERN is upgrading the LHC to the High-Luminosity LHC, which is expected to provide substantially more collision data for further studies.
Sources: ScienceDaily.