Quark-Gluon Plasma: A Window into the Birth of the Universe

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Quark-Gluon Plasma

These particles that carry force are what hold baryonic matter together. Gluons are appropriately named because they act as the “glue” that binds quarks to form protons and neutrons.

They are responsible for the strong force, one of the four fundamental forces in nature. Gluons, along with photons for the electromagnetic force, and W and Z bosons for the weak force, are all massless particles with a quantum spin of 1, and are collectively referred to as “gauge bosons.”

Quarks and gluons work together to form hadrons, such as protons and neutrons that make up atomic nuclei. Quarks have a quantum spin of 1/2 and a small mass, while gluons are massless. Neither quarks nor gluons can exist independently of each other.


Although scientists cannot directly observe gluons, there is indirect evidence of their existence, which can only be explained by their presence.

Gluons were first identified in 1979 through an experiment at the Positron Electron Tandem Ring Accelerator (PETRA) in Germany. The experiment involved smashing electrons and positrons together, which resulted in the release of a quark and an antiquark. The presence of gluons was confirmed by observing that the quark and antiquark created a third “jet” of particles, off to one side of the initial collision point.

In the experiment, when matter and antimatter came together, they annihilated each other and released a quark and an antiquark. The two quarks were unable to escape each other and the strong force between them became stronger the farther apart they moved. This stored energy allowed the quark and antiquark pair to decay into hadron particles that formed in a conic region along the directions of travel of the original quark and antiquark.


“Although physicists can’t see individual gluons, we know they exist because of indirect evidence that can only be explained by the presence of gluons.”

“Gluons were first detected in 1979, in an experiment at the Positron Electron Tandem Ring Accelerator (PETRA) at the Deutsches Elektronen-Synchrotron (DESY) Laboratory in Germany. This experiment confirmed the existence of gluons by observing the formation of a third jet in electron-positron annihilations.”

“We asked Markus Diehl, an expert in quantum chromodynamics at the DESY Theory Group a few frequently asked questions about gluons.”

“Markus Diehl is an expert in quantum chromodynamics (QCD), the theory that covers the interactions of quarks and gluons (the strong force).”


The existence of gluons is inferred from indirect evidence, such as the production of three distinct sprays of particles in electron-positron collisions, which was first observed at DESY’s PETRA collider in 1979.

Gluons are important because they are responsible for binding quarks together and thus for the formation and properties of protons and neutrons, the building blocks of atomic nuclei.

Quarks and gluons cannot be observed as free particles, but they give rise to hadronic jets. By looking closely at the distributions of particles in a jet, it is possible to determine whether it originated from a gluon or a quark.


The theory that explains the physics behind the strong force, which is carried by gluons to bind quarks together, is called quantum chromodynamics (QCD). Developed by Nobel-prize-winning physicist Murray Gell-Mann, QCD focuses on the concept of “color charge” – a property of quarks and gluons that is similar to electric charge in the sense that it is the source of strong force interactions between quarks and gluons, as described by physicists at Georgia State University.

Quarks can have a color charge referred to as red, green or blue, and there are positive and negative versions of each. The quarks can change color in their interactions and the gluons conserve the color charge. For example, if a green quark changes to a blue quark, the gluon must be able to carry a color charge of green-blue. This means that there must be 8 different gluons in total, as described by John Baez, to account for all the different color and anti-color combinations.

It is worth noting that the electromagnetic force operates under the theory of quantum electrodynamics (QED) which only has two possible charges, positive or negative. This makes QCD far more complex than QED.


While it is technically possible to separate quarks and gluons, it would require extremely high-energy conditions that have not occurred naturally since the early moments after the Big Bang.

In the initial moments after the Big Bang, the universe’s temperature was incredibly high at a thousand trillion degrees. During this time, the universe was filled with free quarks and gluons known as the quark-gluon plasma. The high temperature of the universe caused these particles to move at light speed and interact with too much energy for the strong force to bind them together.

As the universe expanded and cooled, the temperature dropped to 2 trillion degrees, which allowed the strong force to bind quarks and gluons together to form the first hadrons.

Scientists can recreate the quark-gluon plasma in laboratories using particle accelerator experiments, such as those at CERN or the Relativistic Heavy Ion Collider at Brookhaven National Laboratory.

By smashing the atomic nuclei of heavy elements such as gold or lead together at high speeds, scientists are able to create a mini fireball that briefly dissolves hadrons into a quark-gluon plasma.

As the fireball cools, the quarks and gluons recombine to form jets of hadrons, including mesons and baryons. The quark-gluon plasma is extremely dense, so the hadron jets often lose energy as they struggle to pass through, a process known as ‘quenching’.

By studying the amount of quenching and the distribution and energy of the hadron jets, scientists can gain insight into the properties of the quark-gluon plasma. For example, they have discovered that it behaves more like a perfect fluid with zero viscosity than a gas.

By recreating the quark-gluon plasma in particle accelerators, scientists can gain a glimpse into the early moments of the universe and the immediate aftermath of the Big Bang, when matter first came into being.

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