Quark–gluon plasma

Quantum chromodynamics (QCD) is the theory that describes the strong interaction, a fundamental force describing the interactions between quarks and gluons. Quarks are the fundamental particles that come together to form protons and neutrons, while gluons are the force carriers that mediate the strong force that holds quarks together within protons, neutrons, and other hadrons.

A fascinating state of matter related to QCD is quark-gluon plasma (QGP). Quark-gluon plasma is a state in which quarks and gluons, which are normally bound inside hadrons, move around freely in a thermal medium of high energy density and temperature. Understanding this state helps physicists explore the early universe moments after the Big Bang when similar conditions existed.

Let's take a deeper look at the concept of quark–gluon plasma, learn how it arises, what its significance is, and what it teaches us about the universe and matter.

Creation of quark–gluon plasma

Under normal conditions, quarks are confined within hadrons due to a property called "color confinement". However, at extremely high temperatures and energy densities, such as in heavy-ion collisions, quarks and gluons are no longer confined and form a plasma. This situation is described by the theory of QCD in that the color degrees of freedom become asymptotically free, a phenomenon called "asymptotic freedom".

To visualize quark–gluon plasma, consider how matter changes between solid, liquid, and gaseous states under increasing temperature:

Solid → Liquid → Gas → Plasma

In the field of particles:

hadrons → quark–gluon plasma

This transition to quark–gluon plasma involves reaching temperatures in excess of two trillion Kelvin, which is far higher than any temperature found in nature except the early universe.

Experimental creation of quark–gluon plasma

Quark–gluon plasma is tested in laboratories using heavy-ion collisions. Facilities such as the Large Hadron Collider (LHC) at CERN in Switzerland and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the US smash heavy ions such as gold or lead at nearly the speed of light to recreate these extreme conditions.

Here's a simplified illustration of how heavy ion collisions occur:

  Gold Ion → ← Gold Ion
  _Confrontation_/
     _Creation of quark-gluon plasma_/

When these collisions occur, the kinetic energy is converted into heat, creating a tiny fireball in which the quarks and gluons exist independently for only a fraction of a second, after which they cool down and recompose into particles detectable by the collider.

Importance of quark–gluon plasma

The study of quark–gluon plasma provides information about:

• Conditions of the early universe: By recreating the extreme temperature and density conditions that occurred just a few microseconds after the Big Bang, physicists can study the properties and evolution of the early universe.
• Understanding the strong force: Studying how quarks and gluons behave when not confined in hadrons gives scientists insights into the nature of the strong force and confinement.
• Phase transitions: Understanding the transition from hadronic matter to quark–gluon plasma and vice versa increases our knowledge of phase transitions related to QCD.

Features of quark–gluon plasma

QGP exhibits several unique properties:

• Perfect fluidity: Despite being composed of free quarks and gluons, quark–gluon plasma behaves like a nearly perfect fluid, with extremely low viscosity.
• Mass flow: The plasma produced exhibits anisotropic flow, which forms patterns similar to those seen in fluid dynamics.
• Jet quenching: The high-energy particles produced in the collision, called jets, lose their energy while passing through the QGP, which can be observed as jet quenching.

The equation describing the relation between pressure (P), energy density ((epsilon)), and temperature (T) in the QGP can be expressed as:

P = c_s^2 times epsilon

where c_s is the speed of sound in quark-gluon plasma, and (c_s approx 1/sqrt{3}).

Visualizations and examples

Here's a simplified illustration of how quarks and gluons move within QGP:

  ,
  |Q | |Q' | |G |
   g | g | q' | → free movement
    +--+ +--+ +--+ with gluons
  Normal hadron quark–gluon plasma

Quarks in hadrons are tightly bound to gluons and exchange forces. In QGP, they are free to move around without any binding.

Conclusion

The study of quark-gluon plasma is a crucial component of modern physics and cosmology. Exploring QGP helps scientists recreate the conditions of the Big Bang and better understand the behavior of matter on the smallest scales. With experiments ongoing at colliders around the world, the quest to understand QGP continues, promising greater insights into profound questions about the beginning of the universe and the forces that govern subatomic particles.

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