Collaborators at the Large Hadron Collider (LHC) have all reported signs of quark-gluon plasma, an extreme state of matter, in oxygen collisions experiments.
A year after the first collisions of oxygen at the 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).
This is a state of matter that is believed to have existed in the first millionths of seconds after the Big Bang, meaning this research could lead into valuable findings about the earliest moments of our universe’s existence.
It was previously thought QGP could only be observed using heavier ions
Under intense pressure and temperatures 100,000 times hotter than the centre of the Sun, composite particles break down into quarks and the gluons that hold them together.
Lead ions, which are over 200 times heavier than the protons typically collided at the LHC, was thought to be the only way to recreate the conditions needed to form QGP. But this theory was challenged by the ALICE Collaboration, who reported a new sign of QGP from proton-proton and proton-lead collisions. Last year, the LHC Collaborations found signs of QGP in oxygen-oxygen collisions, which has been further supported by the latest round of experiments- as well as finding signs of QGP in oxygen-neon collisions.
Parton energy loss as evidence of QGP
The speed of quarks and gluons means that they lose energy as they pass through the hot, dense conditions of the experiments, which is called parton energy loss. The ATLAS Collaboration observed the effects of parton energy loss through an imbalance between pairs of particle jets produced in oxygen–oxygen and neon–neon collisions- an effect that was more noticeable in more head-on (central) collisions, where a larger volume of QGP leads to greater energy loss.
Other ATLAS studies of charged particles recoiling against photons shows a similar dependence on how central the collision is, matching the energy loss of recoiling particles as they pass the QGP.
Other collaborators examined how parton energy loss impacted different 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.
LHCb found evidence that suppression becomes more prominent in the heavier neon collisions involving particles made of a charm quark and a light quark. This is an expected feature of parton energy loss and is consistent with the formation of a larger-volume QGP as collision size increases.
ALICE compared the production of another type of particle – neutral pions – in oxygen–oxygen and proton–oxygen collision to rule out the possibility that a force other than parton energy loss was contributing to the suppression of particle production. This study found clear 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.
Further signs of QGP in anisotropic flow conditions
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 phenomena known as anisotropic flow – more strongly than particles composed of two quarks (mesons).
The predominant theory for this is the presence of QGP, which causes particles produced in a collision, moving at intermediate momenta, to exhibit anisotropic flow. That baryons contain one more quark than mesons suggests that they inherit more of this flow.
