By İdil Doğa Türkmen


In this study, the Quark-Gluon Plasma, which is the high-energy state of matter, that is a dense substance found at a high nuclear level and temperature, formed by the separation of hadrons into quarks and gluons, and the areas where it can be found and created are mentioned.

1. What is Quark-Gluon Plasma?

Quark-gluon plasma (QGP) is a novel state of nuclear matter that is present at an extremely high level. Hadrons (protons, neutrons, pions, etc.) lose their identity and dissolve in a soup of constituents, quarks, and gluons. (Roman Pasechnik, 2017) For the past decade, physicists around the world have been trying to recreate this soup known as quark-gluon plasma (QGP) by combining atomic nuclei with enough energy to produce temperatures of trillions of degrees.

Although quarks and gluons make up protons and neutrons, they behave very differently from these heavy particles. Their interactions are governed in part by a theory known as quantum color dynamics, developed by MIT professors Jerome Friedman and Frank Wilczek, who have won Nobel prizes for their work. However, the actual behavior of quarks and gluons is difficult to study because they are trapped in heavier particles. The only place in the universe where QGP (quark gluon particles) exists is inside high-speed accelerators, even if only for very short periods of time. (Trafton, 2010)

(Image 1: Quark-gluon plasma, a state of matter where quarks are not bound. Brookhaven National Laboratory.)


2. Building Blocks of Quark-Gluon Plasma

Plasma is made up of quarks, the particles that make up nucleons and some other fundamental particles, and gluons, which are massless particles that "carry" the force between quarks. Gluons are particles that quarks exchange when they interact, or in modern physics, gluons mediate the strong force between quarks. Because quarks form protons and neutrons, this leads to the force that holds protons and neutrons together in a nucleus.

2.1. Quark

Quark is one of the fundamental particles in physics. They form hadrons like protons and neutrons. Protons and neutrons are complementary parts of atomic nuclei. The antiparticles of quarks are called antiquarks. Quarks and antiquarks are the only particles in nature that interact with the four fundamental forces: gravity, electromagnetism, the weak (nuclear) force, and the strong (nuclear) force. (Cagatay, 2020)

2.2. Gluon

In quantum electrodynamic theory, interactions between particles are described by the emission and absorption of photons. In other words, according to quantum electrodynamics, the photon is the particle that carries and transmits the force. Quantum color dynamics, which was developed based on this idea, predicted that instead of a photon, there is a gluon particle carrying the force. Gluons carry the force, i.e. "color", between particles. "Color" is the equivalent of electrical charges in electrodynamics in color dynamics; They are not the colors that our eyes see in the true sense. (Turaçlar, 2021)

3. Properties of Quark-Gluon Plasma

Plasma is an ionized gas, like the substance in sparks or lightning. But unlike these plasmas, as the name suggests, the quark-gluon plasma is composed of different particles than other plasmas.

(Image 2: Meson and baryons, which are known states from quark bonding patterns, and newly discovered dibaryon and tetraquark in quark gluon plasma. Green particles are quarks, red particles are antiquarks. (Juelich, 2014) )


Moreover, the way quarks and gluons interact is different from other particles. At low temperatures and densities, they attract each other more strongly as they separate from each other, which explains why free quarks are not found in nature. (Quark-Gluon Plasma, no date) In addition, it has different properties from normal materials, such as loss of energy in elastic collisions (normally, kinetic energy is conserved in elastic collisions).

4. Quark-Gluon Plasma Formation Process

For a few millionths of a second after the Big Bang, the universe consisted of a hot soup of elementary particles called quarks and gluons. After a few microseconds, these particles began to cool to form protons and neutrons, the building blocks of matter. (Trafton, 2010) Scientists can obtain this information only by examining in detail the transition from free quarks and gluons to confined quarks in larger particles such as protons and neutrons. However, the only place where quarks and gluons exist as free particles is inside the quark-gluon plasma. This means that the only way to understand the formation process is to study the plasma as it cools to form composite particles. Although physicists would like to study the quark-gluon plasma, obtaining it is a whole separate endeavor. Most scientists argue that quark soup can only exist in the cores of neutron stars, which are city-sized stellar objects that are naturally about 1.5 times larger than the sun. But the nearest neutron star is about 400 light-years away and penetrating its billion-degree environment would be technically difficult, if not impossible. Therefore, in order for quark-gluon plasma to be studied experimentally, it must be produced on Earth in accelerators. (Badea, 2020)

(Image 3: QGP phase diagram. Adapted from original by R.S. Bhalerao. (Quark–gluon plasma, 2021))


As can be observed in Figure 3, in the temperature and baryon chemical potential graph, the quark gluon plasma is located in regions where the temperature and chemical baryon potential are high. These regions are close to the critical point, that is, the phase in the first formation process of the universe. As a result, both the temperature and the chemical baryon potential of the particles must reach the highest point for the formation of quark gluon plasma. When it reaches this point, arids become exotic (dibaryon or tetraquark) and QGP occurs.

5. The First Discovery of the Quark-Gluon Plasma

"If you're interested in the properties of the microsecond old universe, the best way to study it is not to build a telescope, but to build an accelerator," says Krishna Rajagopal, an MIT theoretical physicist working on QGP. (Trafton, 2010)

Because of the enormous energies required for the discovery of quark-gluon plasma, the lab would need to be a particle accelerator with the plasma produced as a result of collisions between particles accelerating towards each other in counter-rotating beams, rather than the particles hitting a stationary target. In the case of a stationary target, not all incoming particle energy is available for the reaction, as most of it must go into the kinetic energy of the products to conserve momentum. Even in colliding nuclei, most of the energy is surrounded by large chunks of nuclei and therefore cannot be used to produce plasma. Their nuclei are chosen for their large atomic weight to make the collision energy as large as possible.

First, a team at the Center for Nuclear Research (CERN) in Geneva conducted a preliminary experiment with a lead core beam and stationary targets of various materials, but the results, while encouraging, were inconclusive.

The Relative Heavy Ion Collider (RHIC) at Brookhaven National Laboratory was built specifically to produce quark-gluon plasma. In the 2.4-mile-long RHIC ring, fully ionized gold ions move in both directions simultaneously and can meet in six places around the ring for collisions.

Earlier RHIC results suggested that when gold nuclei collided head-on, their kinetic energies split many nucleons and formed a hot, dense plasma of quarks and gluons that should immediately begin to expand and cool. The hot plasma lasts only 10-23 seconds, and only when the plasma has cooled enough that the quarks and gluons freeze, leaving behind thousands of sprays of elementary particles that bear the signature of hot, dense plasma. (Image 4) (Quark-Gluon Plasma, no date)

(Image 4: Spray of thousands of particles produced from the collisions of colliding gold nuclei. Brookhaven National Laboratory.)


(Image 5: Simulation of Gold-Gold Collision in RHIC)


Due to relativistic effects, the colliding nuclei appear as flat plates as they approach each other. Shortly after their collision, a quark-gluon plasma (purple) forms, but quickly cools and condenses into elementary particles (green), which themselves decay into other particles shown in the last frame to the right. (Brookhaven National Laboratory)

6. Current State of Quark-Gluon Plasma Research

In 2005, scientists at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reported that gold atoms were brought together at nearly the speed of light to form QGP (quark-gluon plasma). These collisions can produce temperatures of up to 4 trillion degrees. This is 250,000 times hotter than the interior of the Sun and hot enough to melt protons and neutrons into quarks and gluons. The resulting super-hot, super-dense matter, about a trillionth of a centimeter, could give scientists new insights into the properties of the very early universe. By making higher-energy collisions today, scientists hope to learn more about the properties of the quark-gluon plasma and whether it becomes gas-like at higher temperatures. They also want to delve further into the surprising similarities seen between QGP and ultracold gases (near absolute zero) created in the lab by scientists working on this topic. Both substances are nearly frictionless, he says, and theoretical physicists suspect that string theory could explain both phenomena, offering a glimpse into an even earlier stage of the universe's formation. (Trafton, 2010) Quark-Gluon Plasma has reached a very interesting point in its life today and studies continue.

7. Conclusion

As a result of the study, it was understood that the formation of the quark-gluon plasma mentioned in the hypothesis was correct. In addition, the fact that quark-gluon plasma is a substance seen when the universe was forming in the Big Bang and can be studied by being created in accelerators today may be an important source for the beginning of the universe. For example, by examining the conditions under which the quark-gluon plasma is formed, more detailed information can be obtained about the conditions that took place when the universe was formed. Studies of scientists on this subject may lead to important discoveries in the future. As Dr Bellwied explains, two lines of research in this area are particularly important. “First, we characterize the new short-lived state of matter and its properties to learn about the fundamentals of free quarks and gluons,” he says. "Secondly, we are trying to understand the formation of matter and new forms of matter by studying the transition back from Quark Gluon Plasma." This first aspect of Quark Gluon Plasma research may provide perhaps the most immediate benefits to our understanding of particle physics. As fundamental particles, quarks and gluons are fundamental elements of the Standard Model, which, according to our current understanding, describes the nature of all matter in the universe. With the ability to examine the physical properties of individual particles independently of the influence of others, researchers can improve the predictions of the Standard Model and perhaps even reveal some of its limitations. The second aspect is that Quark Gluon Plasma examines the re-entrapment process and subsequent products in more detail, providing new insights into aspects of the nature of matter that cannot be proven by more conventional experiments. Three specific areas of his research are of particular interest, including particles containing "strange" quarks, the enigmatic contents of ultra-dense neutron stars, and the mysterious effect of quantum mechanics on plasma dynamics. (Bellwied, 2021)


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