QCD properties

• The QCD Lagrangian:

  • Non-abelian gauge theory
  • Int. symmetry group: SU(3)color

• Properties of the strong interaction:

  • Confinement: Quarks do not occur isolated in nature: only in hadronic (colorless) bound-states as mesons [qq] and baryons [qqq]. as(q2) strong at large distances (low q2).
  • Asymptotic freedom: At short distances (high q2), as(q2) decreases logarithmically: quarks & gluons weakly coupled. Perturbative methods applicable (pQCD).
  • 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 (& gluon-gluon) virtual pairs :
    • quark & gluon condensates: <YY>»(235 MeV)3,<asGmnGmn>»(500 MeV)4