LHC Collisions of Oxygen and Neon Show Signs of Early-Universe Plasma, Challenging Lead-Only Assumptions

One year after the Large Hadron Collider ran oxygen nuclei through its beams for the first time, all four of its major experiments, ALICE, ATLAS, CMS and LHCb, have reported signs of the same thing: quark-gluon plasma, the state of matter thought to have filled the universe in the first millionths of a second after the Big Bang. The signs appear in collisions of oxygen with oxygen and neon with neon, systems far lighter than the ones physicists long believed were necessary.

Quark-gluon plasma forms at temperatures more than 100,000 times hotter than the center of the Sun. Under that heat and pressure, protons and neutrons lose their structure and break into quarks and the gluons that normally bind them. Nearly 14 billion years on, researchers can briefly recreate the conditions by colliding nuclei at extreme energies. For decades, the working assumption was that this required very heavy nuclei, especially lead, which is more than 200 times heavier than the protons typically used at the LHC.

That assumption has been eroding. Earlier this year, ALICE, the collaboration that specializes in quark-gluon plasma, identified another indication of it in proton-proton and proton-lead collisions. The year before, researchers had reported the first hints from oxygen-oxygen collisions. The new, more detailed analyses point to multiple signs in both oxygen and neon collisions.

One of the clearest signatures is parton energy loss: energetic quarks and gluons shed energy as they plow through the hot, dense medium. ATLAS saw this by studying pairs of particle jets, the concentrated streams of particles that emerge from such quarks and gluons, in oxygen-oxygen and neon-neon collisions. The two jets in a pair differed in energy, indicating that one had lost some on its way out. The effect was strongest in central, head-on collisions, which should create a larger volume of plasma and give particles more material to cross. Preliminary ATLAS results looking at charged particles recoiling opposite photons showed a similar link between energy loss and how centrally the nuclei met.

ALICE, CMS and LHCb came at the same process differently, looking at how the plasma suppresses production of certain energetic particles. CMS found suppressed charged-particle production in both oxygen-oxygen and neon-neon collisions compared with proton-proton collisions. LHCb studied particles containing a charm quark and a lighter quark and found stronger suppression in the heavier neon collisions, the pattern expected if bigger collisions make more plasma.

Suppression alone does not prove energy loss, since other physical processes could mimic it. To address that, ALICE compared neutral pion production in oxygen-oxygen and proton-oxygen collisions and reported unambiguous evidence of parton energy loss in the oxygen-oxygen case.

A further signature involves a heavy quark bound to its antiquark. Some such pairs are more tightly bound than others and so are disrupted differently by plasma. CMS found this varying suppression in upsilon mesons, made of a bottom quark and its antiquark, by comparing oxygen-oxygen and neon-neon collisions. LHCb reported preliminary evidence of similar behavior in proton-oxygen and oxygen-oxygen data.

ALICE also reported, in preliminary findings, a directional pattern called anisotropic flow. In oxygen-oxygen collisions, three-quark baryons showed a stronger directional preference than two-quark mesons. The leading explanation is that particles of intermediate momentum emerging from plasma inherit collective motion from their constituent quarks, and baryons, having three, inherit more.

Analysis of light-nucleus data continues. Meanwhile CERN is upgrading the LHC into the High-Luminosity LHC, which will collect substantially more collision data and allow closer study of the plasma's properties.