Latest revision as of 12:26, 20 May 2026

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chromodynamics (QCD) quantum chromodynamics (QCD) is the quantum field theory that describes the strong interaction between quarks and gluons, based on a non-Abelian SU(3) gauge symmetry. It explains how quarks are bound together to form hadrons such as protons and neutrons. Gluon-mediated interaction between quarks in quantum chromodynamics, illustrating color charge exchange Quantum chromodynamics (QCD) is the quantum field theory that describes the strong interaction between quarks and gluons, based on a non-Abelian SU(3) gauge symmetry. It explains how quarks are bound together to form hadrons such as protons and neutrons. Quarks – matter fields carrying color charge Gluons – gauge bosons mediating the strong force Quarks come in different “colors” (analogous to charge), while gluons carry combinations of color and anticolor. QCD involves two types of fundamental particles:

Fundamental particles

QCD involves two types of fundamental particles:

Quarks – matter fields carrying color charge
Gluons – gauge bosons mediating the strong force

Quarks come in different “colors” (analogous to charge), while gluons carry combinations of color and anticolor.^[1]

Gauge symmetry

The symmetry group of QCD is $S U (3)$ , which is non-Abelian. The generators satisfy: $[T_{a}, T_{b}] = i f_{a b c} T_{c}$

This non-commuting structure leads to self-interactions of the gauge fields (gluons).^[2]

QCD Lagrangian

The QCD Lagrangian is: $ℒ = {\bar{ψ}}_{i} (i γ^{μ} D_{μ} - m) ψ_{i} - \frac{1}{4} F_{μ ν}^{a} F^{μ ν a}$

where:

$D_{μ} = \partial_{μ} + i g A_{μ}^{a} T_{a}$
$F_{μ ν}^{a}$ is the non-Abelian field strength tensor
$g$ is the strong coupling constant

This describes both quark dynamics and gluon interactions.^[3]

Gluon self-interaction

Unlike photons in QED, gluons carry color charge and interact with each other.

This leads to:

nonlinear dynamics
complex field configurations
strong coupling behavior at low energies

Gluon self-interactions are a defining feature of QCD.^[1]

Confinement

Quarks and gluons are never observed in isolation. Instead, they are confined within composite particles called hadrons.

As quarks are separated, the force between them does not decrease but remains strong, effectively preventing their isolation.

This phenomenon is known as confinement and is a key prediction of QCD.^[4]

Asymptotic freedom

At very high energies (short distances), the strong coupling becomes weaker. This property is known as asymptotic freedom.

It is described by the running coupling: $α_{s} (μ)$

which decreases as the energy scale $μ$ increases.

This behavior was a major theoretical breakthrough and confirmed experimentally.^[3]

Hadrons and bound states

Quarks combine to form:

baryons (three quarks, e.g., proton, neutron)
mesons (quark–antiquark pairs)

These composite particles are the observable states of QCD.

The internal structure of hadrons is governed by the dynamics of quarks and gluons.

Role in the Standard Model

QCD is one of the three fundamental interactions in the Standard Model, alongside:

electroweak interaction
(and gravity outside the model)

It is responsible for binding quarks into nucleons and nucleons into atomic nuclei.

Conceptual importance

Quantum chromodynamics demonstrates how non-Abelian gauge symmetry leads to rich and complex physical phenomena such as confinement and asymptotic freedom.

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 {{Short description|Quantum field theory describing the strong interaction between quarks and gluons based on SU(3) gauge symmetry}}
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Physics:Quantum chromodynamics (QCD): Difference between revisions

Latest revision as of 12:26, 20 May 2026

Fundamental particles

Gauge symmetry

QCD Lagrangian

Gluon self-interaction

Confinement

Asymptotic freedom

Hadrons and bound states

Role in the Standard Model

Conceptual importance

See also

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