Physics:Quantum chromodynamics (QCD)
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Quantum chromodynamics (QCD) is the quantum field theory that describes the strong interaction between quarks and gluons, based on a non-Abelian gauge symmetry.[1] It explains how quarks are bound together to form hadrons such as protons and neutrons.
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.[2]
Gauge symmetry
The symmetry group of QCD is , which is non-Abelian. The generators satisfy:
This non-commuting structure leads to self-interactions of the gauge fields (gluons).[3]
QCD Lagrangian
The QCD Lagrangian is:
where:
- is the non-Abelian field strength tensor
- is the strong coupling constant
This describes both quark dynamics and gluon interactions.[1]
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.[2]
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:
which decreases as the energy scale increases.
This behavior was a major theoretical breakthrough and confirmed experimentally.[1]
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.
It is a cornerstone of modern particle physics and essential for understanding the structure of matter.
See also
Table of content (89 articles)
Index
- Foundations
- Conceptual and interpretations
- Mathematical structure and systems
- Atomic and spectroscopy
- Wavefunctions and modes
- Quantum information and computing
- Quantum optics and experiments
- Open quantum systems
- Quantum field theory
- Statistical mechanics and kinetic theory
- Plasma and fusion physics
- Timeline
- Advanced and frontier topics
Full contents
- Physics:Quantum Density matrix
- Physics:Quantum Exactly solvable quantum systems
- Physics:Quantum Formulas Collection
- Physics:Quantum A Matter Of Size
- Physics:Quantum Symmetry in quantum mechanics
- Physics:Quantum Angular momentum operator
- Physics:Runge–Lenz vector
- Physics:Quantum Approximation Methods
- Physics:Quantum Matter Elements and Particles
- Physics:Quantum Wavefunction
- Physics:Quantum Superposition principle
- Physics:Quantum Eigenstates and eigenvalues
- Physics:Quantum Boundary conditions and quantization
- Physics:Quantum Standing waves and modes
- Physics:Quantum Normal modes and field quantization
- Physics:Number of independent spatial modes in a spherical volume
- Physics:Quantum Density of states
- Physics:Quantum field theory (QFT) basics
- Physics:Quantum field theory (QFT) core
- Physics:Quantum Fields and Particles
- Physics:Quantum Second quantization
- Physics:Quantum Harmonic Oscillator field modes
- Physics:Quantum Creation and annihilation operators
- Physics:Quantum vacuum fluctuations
- Physics:Quantum Propagators in quantum field theory
- Physics:Quantum Feynman diagrams
- Physics:Quantum Path integral formulation
- Physics:Quantum Renormalization in field theory
- Physics:Quantum Renormalization group
- Physics:Quantum Field Theory Gauge symmetry
- Physics:Quantum Non-Abelian gauge theory
- Physics:Quantum Electrodynamics (QED)
- Physics:Quantum chromodynamics (QCD)
- Physics:Quantum Electroweak theory
- Physics:Quantum Standard Model
- Physics:Quantum Statistical mechanics
- Physics:Quantum Partition function
- Physics:Quantum Distribution functions
- Physics:Quantum Liouville equation
- Physics:Quantum Kinetic theory
- Physics:Quantum Boltzmann equation
- Physics:Quantum BBGKY hierarchy
- Physics:Quantum Transport theory
- Physics:Quantum Relaxation and thermalization
- Physics:Plasma physics (fusion context)
- Physics:Tokamak physics
- Physics:Tokamak edge physics and recycling asymmetries
- Hierarchy of modern physics models showing the progression from quantum statistical mechanics to kinetic theory and plasma physics, culminating in tokamak edge transport and recycling asymmetries.
References
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