High-energy collisions of oxygen nuclei in the LHC accelerator at CERN produce the plasma of quarks and gluons that also filled the early universe shortly after the Big Bang. Previously, this was only seen in lead collisions.
This is evident from analyses of all four major LHC detectors, based on the first oxygen-oxygen collisions in the accelerator, which took place a year ago. Physicists from Nikhef are closely involved in these detector experiments, with the exception of CMS.
QGP is a state of matter that forms under intense pressure and at temperatures more than 100,000 times higher than those at the center of the Sun. Under these extreme conditions, composite particles break down into quarks and the gluons that normally hold them together.
Scientists believe 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 simulate and study QGP using high-energy nuclear collisions in the LHC.
It was previously assumed that colliding heavy ions, such as lead—which is more than 200 times heavier than the protons typically collided in the LHC—was the only way to create the conditions necessary for the formation of QGP.
But recently, this assumption has been thoroughly called into question, including earlier this year when the ALICE collaboration—which specializes in the study of this extreme state of matter—reported a new sign of QGP from proton-proton and proton-lead collisions.
Last year, the LHC collaborations opened a new avenue for studying QGP when they found the first evidence of QGP from oxygen-oxygen collisions. Now, after even more thorough investigation, the LHC experiments have observed multiple signs of QGP formation in oxygen-oxygen and neon-neon collisions.
One of the signs of QGP formation is that fast-moving quarks and gluons lose energy as they pass through this hot, dense medium, a phenomenon known as parton energy loss. The ATLAS collaboration has unequivocally observed that parton energy loss occurs due to 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 scattering off photons also show the same dependence on collision centrality, which corresponds to the energy loss of the scattered particles as they traverse the QGP.
The ALICE, CMS, and LHCb collaborations took a different approach to finding evidence of parton energy loss by studying how it suppresses the production of various types of energetic particles.
CMS observed a suppression of charged-particle production inoxygen-oxygen andneon-neon collisions compared to proton-proton collisions, indicating parton energy loss and the presence of QGP in light-ion collisions. Nikhef is not part of the CMS collaboration.
In another study, LHCb compared the suppression of particles consisting of a charm quark and a light quark in oxygen-oxygen and neon-neon collisions and found evidence that the suppression becomes more pronounced in the heavier neon collisions. This is an expected feature of parton energy loss and is consistent with the formation of a QGP with a larger volume as the collision scale increases.
To account for the possibility that mechanisms other than parton energy loss might contribute to the suppression of particle production, ALICE performed a comparison between the production of another type of particle—neutral pions—in oxygen–oxygen and proton–oxygen collisions. This comparison provided unequivocal evidence of parton energy loss in oxygen–oxygen collisions.
Another indication of the formation of a QGP is the suppression of short-lived 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 infer the presence of a QGP by measuring how it suppresses each of the differently bound states.
CMS found evidence of this varying suppression in 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 using data from proton–oxygen and oxygen–oxygen collisions.
Finally, the ALICE collaboration has published preliminary results suggesting a new indication of QGP. The collaboration discovered that particles consisting of three quarks (baryons), produced in oxygen–oxygen collisions, were emitted in a specific direction—a phenomenon known as anisotropic flow—and to a greater extent than particles consisting of two quarks (mesons).
The most likely explanation for this is the presence of QGP, which causes particles produced in a collision and moving with an average momentum to exhibit anisotropic flow. The fact that baryons contain one more quark than mesons means that they contribute more to this flow.
Source: CERN
