View source for The Physics of Quantum Chromodynamics in Particle Physics

==Introduction==

Quantum Chromodynamics (QCD) is a theory in [[Theoretical Physics|theoretical physics]] that describes the interactions of quarks and gluons, which are the fundamental constituents of matter. QCD is a component of the [[Standard Model|Standard Model]] of particle physics, which describes the electromagnetic, weak, and strong nuclear forces. 

[[Image:Detail-21766.jpg|thumb|center|Close-up of a quark, the fundamental particle in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21767.jpg|thumb|center|Close-up of a quark, the fundamental particle in Quantum Chromodynamics.|class=only_on_desktop]]

==Fundamental Concepts==

===Quarks and Gluons===

Quarks are elementary particles that combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Gluons are elementary particles that mediate the strong force between quarks. In QCD, quarks and gluons are considered point-like particles, meaning they are treated as if they have no size. 

[[Image:Detail-21768.jpg|thumb|center|Representation of a gluon, the mediator of the strong force in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21769.jpg|thumb|center|Representation of a gluon, the mediator of the strong force in Quantum Chromodynamics.|class=only_on_desktop]]

===Color Charge===

In QCD, the term "color" does not refer to the colors of the visible spectrum, but is a type of charge that quarks and gluons carry. There are three types of color charges: red, green, and blue. Anti-quarks carry anti-colors. The strong force acts between particles that carry color charge, and the exchange of gluons mediates this force. 

[[Image:Detail-21770.jpg|thumb|center|Representation of color charge in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21771.jpg|thumb|center|Representation of color charge in Quantum Chromodynamics.|class=only_on_desktop]]

===Confinement and Asymptotic Freedom===

Confinement refers to the phenomenon that quarks and gluons are never observed in isolation. They are always found within hadrons. This is because the strong force increases with distance, unlike other forces. Asymptotic freedom, on the other hand, refers to the fact that quarks and gluons interact weakly at high energies or short distances. 

[[Image:Detail-21772.jpg|thumb|center|Representation of the confinement phenomenon in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21773.jpg|thumb|center|Representation of the confinement phenomenon in Quantum Chromodynamics.|class=only_on_desktop]]

==Mathematical Framework==

QCD is a non-Abelian gauge theory based on the SU(3) group. The Lagrangian of QCD describes the dynamics of quarks and gluons. The equations of motion derived from this Lagrangian are nonlinear differential equations, which are difficult to solve. 

[[Image:Detail-21774.jpg|thumb|center|Representation of the Lagrangian in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21775.jpg|thumb|center|Representation of the Lagrangian in Quantum Chromodynamics.|class=only_on_desktop]]

==Experimental Evidence==

There is a wealth of experimental evidence supporting QCD, including measurements of the strong coupling constant, observations of jet production in high-energy collisions, and the discovery of asymptotic freedom. 

[[Image:Detail-21776.jpg|thumb|center|Representation of an experiment providing evidence for Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21777.jpg|thumb|center|Representation of an experiment providing evidence for Quantum Chromodynamics.|class=only_on_desktop]]

==Applications and Implications==

QCD has many applications in particle physics and cosmology. It is crucial for understanding the properties of the proton, neutron, and other hadrons. It also plays a vital role in the early universe and in the interiors of neutron stars.

[[Image:Detail-21778.jpg|thumb|center|Representation of a neutron star, where Quantum Chromodynamics plays a vital role.|class=only_on_mobile]]
[[Image:Detail-21779.jpg|thumb|center|Representation of a neutron star, where Quantum Chromodynamics plays a vital role.|class=only_on_desktop]]

==Challenges and Open Questions==

Despite its successes, QCD also presents several challenges. One of the main challenges is the confinement problem, which is the question of why quarks and gluons are confined inside hadrons. Another challenge is the strong CP problem, which is the question of why the strong force does not violate CP symmetry. 

[[Image:Detail-21780.jpg|thumb|center|Representation of a challenge in Quantum Chromodynamics.|class=only_on_mobile]]
[[Image:Detail-21781.jpg|thumb|center|Representation of a challenge in Quantum Chromodynamics.|class=only_on_desktop]]

==See Also==

* [[Quantum Field Theory]]
* [[Standard Model]]
* [[Particle Physics]]
* [[Strong Interaction]]
* [[Hadron]]

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