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 condensate: <??> ? (235 MeV)3
- gluon condensate: <asG??G??> ? (500 MeV)4
as(q2) ~1/ln(q2 / L2), L~200 MeV