One year on from the first-ever collisions of oxygen at the Large Hadron Collider (LHC), the main LHC Collaborations - ALICE, ATLAS, CMS and LHCb - have each reported signs of the state of matter known as quark-gluon plasma (QGP) originating from these collisions.
QGP is a state of matter that forms under intense pressure and at temperatures over 100 000 times hotter than the centre of the Sun. Under these extreme conditions, composite particles break down into quarks and the gluons that ordinarily hold them together. Scientists believe that this was the state of the Universe in the first millionths of a second after the Big Bang. And in the present-day Universe, nearly 14 billion years later, they can recreate and study QGP with high-energy nuclear collisions at the LHC.
It was previously thought that colliding heavy ions such as lead - which is over 200 times heavier than the protons typically collided at the LHC - was the only way to create the conditions necessary to form QGP. But recently this premise has been thoroughly challenged, including earlier this year when the ALICE Collaboration, which specialises in the study of this extreme state of matter, reported a new sign of QGP from proton-proton and proton-lead collisions. And last year, the LHC Collaborations opened up a new probe of QGP when they found the first hints of QGP from oxygen-oxygen collisions. Now, having searched even more deeply, the LHC experiments have seen multiple signs of QGP formation in oxygen-oxygen and neon-neon collisions.
One of the clues to QGP formation is that fast-moving quarks and gluons lose energy as they pass through this hot, dense medium in a phenomenon known as parton energy loss. The ATLAS Collaboration unambiguously observed the effect of parton energy loss through an imbalance between pairs of particle jets produced in oxygen-oxygen and neon-neon collisions. This effect became more pronounced in more head-on (central) collisions, where a larger volume of QGP leads to greater energy loss. Preliminary ATLAS studies of charged particles recoiling against photons also show the same dependence on collision centrality, consistent with the energy loss of the recoiling particles as they traverse the QGP.
The ALICE, CMS and LHCb Collaborations took a different approach to finding signs of parton energy loss by studying how it suppresses the production of different types of energetic particles.
CMS observed a suppression of charged-particle production in oxygen-oxygen and neon-neon collisions when compared to proton-proton collisions, suggesting parton energy loss and the presence of QGP in the light-ion collisions. In another study, LHCb compared how particles consisting of a charm quark and a light quark were suppressed in oxygen-oxygen and neon-neon collisions, finding evidence that the suppression becomes more prominent in the heavier neon collisions. This is an expected feature of parton energy loss and is consistent with the formation of a larger-volume QGP as collision size increases.
To account for the possibility that mechanisms other than parton energy loss may contribute to the suppression of particle production, ALICE compared the production of another type of particle - neutral pions - in oxygen-oxygen and proton-oxygen collisions. This comparison provided unambiguous evidence for parton energy loss in oxygen-oxygen collisions.
Another clue to QGP formation is the suppression of briefly bound states, consisting of a heavy quark and its antiquark, which are often produced as a result of high-energy collisions. The quarks in these pairs can be bound to each other to varying degrees and researchers can deduce the presence of QGP by measuring how it suppresses each of the differently bound states. CMS found evidence of this varying suppression for upsilon mesons - bound states of a bottom quark and its antiquark - by comparing oxygen-oxygen and neon-neon collisions. LHCb found preliminary evidence of the same suppression but with data from proton-oxygen and oxygen-oxygen collisions.
Finally, the ALICE Collaboration has released preliminary results suggesting another hint of QGP. The Collaboration found that particles made of three quarks (baryons) produced in oxygen-oxygen collisions were emitted with a preferred direction - a phenomenon known as anisotropic flow - more strongly than particles composed of two quarks (mesons). The leading explanation for this is the presence of QGP, which causes particles produced in a collision, moving at intermediate momenta, to exhibit anisotropic flow. The fact that baryons contain one more quark than mesons means that they inherit more of this flow.
Studies of possible QGP formation in light-ion collisions continue as researchers comb through the LHC data. In the meantime, the LHC is being transformed into the even more powerful High-Luminosity LHC, which will undoubtedly probe even deeper into the nature of QGP.
