Review of QCD properties

• The QCD Lagrangian:

  • - Degrees of freedom: quarks and gluons
  • - Non-abelian gauge theory
  • - Internal symmetry group: SU(3)color

• Properties of the strong interaction:

  • 1. Confinement: Quarks do not occur isolated in nature: only in hadronic (colorless) bound-states as mesons [qq] & baryons [qqq]. as(Q2) strong at large distances (low Q2).
  • 2. Asymptotic freedom: At short distances (high Q2), as(Q2) decreases logarithmically: quarks & gluons weakly coupled. Perturbative methods applicable (pQCD).
  • 3. Chiral symmetry breaking:
    • Quarks acquire dynamically a mass in a confined medium:
    • Current mq (5-10 MeV) << Constituent mq (~300 MeV)
    • Non-zero QCD ground-state (non-perturbative vacuum, "sea") filled with quark-antiquark and gluon-gluon virtual pairs :
    • quark & gluon condensates: <YY>»-(235 MeV)3, <asGmnGmn>»-(500 MeV)4