Results from all four major Large Hadron Collider collaborations—ALICE, ATLAS, CMS and LHCb—indicate that quark-gluon plasma, a primordial state of matter, may form when oxygen and neon nuclei collide. The findings build on the first oxygen collisions at the Large Hadron Collider (LHC) and challenge the long-held view that creating this extreme material requires collisions between very heavy atomic nuclei.
Recreating matter from the early universe
Quark-gluon plasma (QGP) is an unusual state of matter produced under enormous pressure and heat. At temperatures exceeding 100,000 times the temperature at the center of the Sun, protons and neutrons can no longer maintain their normal structure. Instead, they break down into quarks and the gluons that ordinarily bind them together.
Physicists believe the universe existed in this state during the first millionth of a second after the Big Bang. Almost 14 billion years later, researchers can briefly recreate similar conditions by colliding atomic nuclei at extremely high energies in the LHC.
For decades, scientists believed that producing QGP required collisions involving very heavy atomic nuclei, particularly lead. Lead nuclei are more than 200 times heavier than the protons typically used in LHC experiments, and were thought to provide the powerful conditions necessary to create the plasma.
Recent discoveries have increasingly called that assumption into question. Earlier this year, the ALICE collaboration, which specializes in studying QGP, reported another sign of QGP formation in proton-proton and proton-lead collisions. LHC researchers also reported the first hint of QGP arising from oxygen-oxygen collisions last year.
More detailed investigations now provide multiple indications that QGP can form in collisions involving oxygen and neon nuclei. The results suggest that the conditions needed to create this primordial material are not limited to collisions between heavy nuclei.
Evidence of energy loss in extreme collisions
One of the clearest ways to identify QGP is to observe what happens when high-energy quarks and gluons pass through it. These particles lose some of their energy as they move through extremely hot and dense matter. Physicists refer to this process as parton energy loss, and it is an important clue that QGP has formed.
The ATLAS collaboration clearly detected this effect by studying pairs of particle jets produced in oxygen-oxygen and neon-neon collisions. Jets are concentrated streams of particles created when high-energy quarks and gluons emerge from a collision.
The researchers observed energy differences between paired jets, indicating that some particles lost energy while passing through the surrounding material. The effect was especially pronounced in more direct, head-on collisions. Such collisions are expected to produce a larger amount of plasma, giving the particles more material to traverse and increasing their energy loss.
Preliminary ATLAS results provide additional support for this interpretation. By examining charged particles recoiling in the direction opposite to photons, researchers found a similar relationship between energy loss and the centrality of the nuclear collision.
Evidence across multiple CERN experiments
ALICE, CMS and LHCb used different methods to investigate the same underlying processes. Rather than focusing mainly on the energy of particle jets, they examined whether QGP reduces the production of certain high-energy particles.
CMS found that, compared with proton-proton collisions, both oxygen-oxygen and neon-neon collisions suppressed the production of charged particles. This reduction is consistent with particles losing energy in QGP and provides further evidence that the exotic state of matter can form in collisions involving lighter nuclei.
In a separate study, LHCb examined particles containing charm and light quarks and compared how strongly their production was suppressed in oxygen and neon collisions.
The suppression was more pronounced in more intense neon collisions. This is the behavior scientists would expect if a larger collision produced more plasma and caused particles to lose more energy as they passed through it.
However, a reduction in particle production does not by itself prove that energy loss is responsible. Other physical processes can produce similar effects.
To investigate this possibility, ALICE compared the production of neutral pions, another type of subatomic particle, in oxygen-oxygen and proton-oxygen collisions. The comparison provided evidence of parton energy loss in oxygen-oxygen collisions and strengthened the case that relatively small nuclear collisions can generate conditions associated with QGP.
Clues from exotic subatomic particles
The researchers also looked for another distinctive signature of QGP involving particles made from heavy quarks and their antiquarks. These pairs can briefly form bound states after high-energy collisions. Some bound states are more strongly connected than others and are therefore differently susceptible to destruction by the surrounding plasma.
By examining which bound states are suppressed, and by how much, physicists can gather additional evidence about whether QGP is present.
CMS found evidence of these different suppression patterns in upsilon mesons, which consist of a bottom quark paired with an antiquark. The researchers identified the effect by comparing oxygen-oxygen and neon-neon collisions.
LHCb also reported preliminary evidence of similar behavior using data from proton-oxygen and oxygen-oxygen collisions.
Together, these observations add to a growing body of evidence that primordial plasma can be produced in much smaller nuclear collisions than those traditionally used to study it.
Particle patterns offer another possible signature
ALICE has identified another possible signature of QGP based on the directions in which particles move after a collision. In its preliminary findings, the collaboration observed that, during oxygen-oxygen collisions, particles containing three quarks, known as baryons, showed a stronger preference for certain directions than particles containing two quarks, known as mesons.
This behavior is called anisotropic flow. The particles do not spread uniformly in every direction; instead, their distribution reflects the internal conditions of the matter created in the collision.
One explanation involves the formation of QGP. When particles with intermediate momentum emerge from the plasma, they can inherit collective motion from their constituent quarks.
Because baryons contain three quarks while mesons contain two, baryons may inherit more of this collective flow, producing the difference observed in the experiments. The pattern is another potential indication that matter resembling the extreme state of the early universe is being recreated in oxygen collisions.
A new window into the universe’s first moments
Scientists are continuing to analyze data from collisions involving lighter atomic nuclei in search of further evidence about how QGP forms and behaves.
The findings could reshape understanding of the conditions required to produce one of the most extreme states of matter known to physics. A state once thought to require collisions between giant atomic nuclei can now be investigated in much smaller systems, giving researchers new opportunities to study matter under unusual conditions.
CERN is also upgrading the LHC to the more powerful, high-luminosity LHC. The enhanced accelerator will allow scientists to collect substantially more collision data and investigate the properties of QGP in greater detail.
Each new result offers a clearer view of the state of matter that dominated the universe during the first moments after the Big Bang.
Source: www.sciencedaily.com