Karim Ghorbani, PhD
Arak University,
Physics Department
P O.Box 879
Email: K-Ghorbani@araku.ac.ir


Quantum Chromo Dynamics (QCD):

Quantum Chromo Dynamics (QCD) is the theory behind the strong interactions or the strong force. These interactions occur among quarks and gluons. Quarks and gluons play the same role as electrons and photons in electromagnetisem respectively. Nevertheless, the strong interactions exhibit a richer and exquisite tapestry of involvement and nonlinearity. This stems from the fact that the Non-Abelian gauge symmetry of QCD allows self interaction of gluons whereas there is no such interaction in electromagnetism which is governed by an Abelian gauge symmetry. Two or even three phenomena in QCD are rather stunning. Asymptotic freedom and confinement are the first two and and we may also contemplate taking the spontaneous symmetry breaking as prominent a phenomenon as the first two. These extremely interesting phenomena stand out of the other properties of QCD, those which may be regarded as the byproduct of the most fundamental ones. Asymptotic freedom was discovered almost thirty years ago by David Gross, Frank Wilczek and David Politzer who received the Nobel prize in physics in 2004 for their discovery.

Asymptotically free theories are weakly interacting theories at very high energy. In principle, this type of theories are easier to deal with at high energies. In the other extreme, by which I mean at very low energies, the strong interaction becomes really strong and this may be the reason behind the yet-to-be-understood confinement phenomenon. Low energy QCD is what we call Hadron physics. Confinement prevents quarks and gluons from being observed as free entities. At very high energy, however, jet production alludes to the existence of free quarks and gluons.

Lattice QCD is the brute force method of numerical computations of the observables of QCD in this energy domain where the standard perturbative technology stops working. Lattice results, however, need at the end to be extrapolated to the continuum. This may be accomplished by means of effective field theories.