Quantum chromodynamics (QCD)

In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks and gluons, the fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of quantum field theory called a non-abelian gauge theory, with symmetry group SU(3). The QCD analog of electric charge is a property called color. Gluons are the force carrier of the theory, just as photons are for the electromagnetic force in quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A large body of experimental evidence for QCD has been gathered over the years. In simpler words, quantum chromodynamics is the process of quarks that possess a color charge, exchanging the strong interaction via trading gluons to form nucleons (i. e. protons and neutrons).

2 thoughts on “Quantum chromodynamics (QCD)

  1. shinichi Post author

    Quantum chromodynamics



    QCD exhibits three salient properties:

    • Color confinement. Due to the force between two color charges remaining constant as they are separated, the energy grows until a quark–antiquark pair is spontaneously produced, turning the initial hadron into a pair of hadrons instead of isolating a color charge. Although analytically unproven, color confinement is well established from lattice QCD calculations and decades of experiments.
    • Asymptotic freedom, a steady reduction in the strength of interactions between quarks and gluons as the energy scale of those interactions increases (and the corresponding length scale decreases). The asymptotic freedom of QCD was discovered in 1973 by David Gross and Frank Wilczek, and independently by David Politzer in the same year. For this work, all three shared the 2004 Nobel Prize in Physics.
    • Chiral symmetry breaking, the spontaneous symmetry breaking of an important global symmetry of quarks, detailed below, with the result of generating masses for hadrons far above the masses of the quarks, and making pseudoscalar mesons exceptionally light. Yoichiro Nambu was awarded the 2008 Nobel Prize in Physics for elucidating the phenomenon, a dozen years before the advent of QCD. Lattice simulations have confirmed all his generic predictions.
  2. shinichi Post author

    ーTheory for Theoryー

    福嶋 健二


    自然界には 4 つの基本的な相互作用が知られている。電磁相互作用、重力相互作用は感覚的にも認知され、古典物理学で古くから研究されてきたが、弱い相互作用、強い相互作用は現代的な量子論なくして、その意味をとらえることはできまい。これら 4 つの中でも強い相互作用の基本理論である量子色力学あるいは QCD (Quantum Chromodynamics の短縮形) は、理論としての完成度が傑出している。電磁相互作用を記述する場の量子論である量子電気力学あるいは QED は、高エネルギー(連続極限)で破綻してしまう。この破綻は弱い相互作用との統合により、電弱相互作用という形で解決するのだが、ヒッグスセクターに階層性問題など不自然さが残る。従って電弱相互作用は、未知のより完全な理論の低エネルギー有効理論に過ぎないと考えられている。そういう意味で言えば、実は QCD も有効理論なのだろうが、QCD そのものには、理論としての明らかな破綻や物理的スケールの不整合など全くない。そこで、QCD をひとつの指導原理と設定して、QCD から導かれるべき既知現象の解明、あるいは QCD に隠れている未知現象の探索を目指すのが、QCD 物性物理研究の特色である。


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