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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] and 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
Notes:
The solutions of the theory must exhibit 2 “properties” observed in Nature (but not in the Lagrangian):
Confinement: quarks and gluons do not exist as isolated particles The elementary degrees of freedom are not observed free in Nature. the color electric field energy generated by an isolated quark is calculated to be truly infinite, due to the energy it creates in distant fields
Chiral symmetry breaking: The constituent quark masses are different of the current masses (~0 MeV).