Results from all four major Large Hadron Collider (LHC) collaborations—ALICE, ATLAS, CMS and LHCb—provide multiple indications that quark-gluon plasma, a primordial state of matter created under extreme temperatures and pressures, can form in collisions involving relatively light nuclei such as oxygen and neon. The findings challenge the long-standing assumption that this material requires collisions between very heavy nuclei, while offering new ways to study conditions that existed during the universe’s earliest moments.
Recreating matter from the early universe
Quark-gluon plasma (QGP) is an unusual state of matter that appears under immense heat and pressure. 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 their constituent quarks and the gluons that normally bind them together.
Physicists believe that QGP filled the universe 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 between very heavy atomic nuclei, particularly lead. Lead nuclei are more than 200 times heavier than the protons typically used in LHC experiments, and their collisions were thought to generate the powerful conditions needed to form the plasma.
Recent discoveries have increasingly challenged that assumption. Earlier this year, the ALICE collaboration, which specializes in studying QGP, reported another sign of its formation in proton-proton and proton-lead collisions. Last year, LHC researchers also reported the first hint of QGP arising from oxygen-oxygen collisions.
The LHC has since produced its first oxygen collisions. More detailed analyses now reveal several indications that QGP can form in collisions involving oxygen and neon nuclei, suggesting that the conditions needed to create this primordial material are not limited to collisions between heavy atoms.
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 travel 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.
In an ATLAS collaboration analysis, researchers studied pairs of particle jets produced during 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 differences in the energies of paired jets, indicating that some of the particles had lost energy while traveling through the surrounding material. The effect was particularly noticeable in more direct, head-on collisions involving the centers of the nuclei.
Such central collisions are expected to produce a larger volume of plasma. Particles would then pass through more of the material, leading to greater 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 collisions.
Similar patterns across CERN experiments
ALICE, CMS and LHCb used different methods to investigate the same underlying processes. Rather than focusing primarily on the energy of particle jets, these collaborations 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. The reduction is consistent with particles losing energy in QGP and provides further evidence that this exotic state of matter can form in collisions involving lighter nuclei.
LHCb reported another clue in a separate study. The collaboration examined particles containing charm and light quarks, comparing 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 a greater amount of plasma, causing 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 examine this possibility, ALICE compared the production of neutral pions—another type of subatomic particle—in oxygen-oxygen and proton-oxygen collisions. The comparison provided clear evidence of parton energy loss in oxygen-oxygen collisions and strengthened the case that relatively small nuclear collisions can produce conditions associated with QGP.
Further clues from exotic subatomic particles
Researchers also looked for another distinctive QGP signature involving particles made from heavy quarks and their antiquarks. These pairs can briefly form bound states after high-energy collisions. Some states are more tightly bound than others, making them differently susceptible to destruction by the surrounding plasma.
By measuring which bound states are suppressed, and by how much, physicists can gather additional evidence about whether QGP is present.
CMS found evidence of different levels of suppression in a particle called an upsilon meson, which consists of a bottom quark paired with a bottom 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.
These observations add to a growing body of evidence that primordial plasma can be generated in much smaller nuclear collisions than those traditionally used to study it.
Unexpected particle patterns offer another signal
ALICE has identified another possible signature of QGP, this time based on the directions in which particles move after a collision.
In its preliminary findings, the collaboration reported that, during oxygen-oxygen collisions, particles containing three quarks—known as baryons—show a stronger preference for certain directions than particles containing two quarks, known as mesons.
This behavior is called anisotropic flow. Instead of spreading uniformly in every direction, the particles follow a pattern influenced by 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.
This distinctive pattern is another potential indication that the extreme matter present in the early universe can be 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, searching for further evidence of how QGP forms and behaves.
The findings reshape the understanding of the conditions needed to produce one of the most extreme states of matter known to physics. What once appeared 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 upgraded accelerator will allow scientists to collect substantially more collision data and examine the properties of QGP in greater detail.
With each new result, researchers are building a clearer picture of the remarkable state of matter that dominated the universe during the first moments after the Big Bang.
Source: www.sciencedaily.com
