Some qualitative features of QCD

One of the most interesting and distinctive features of the narrow resonance spectroscopy as compared with the spectroscopy of the older particles, where only light u, d, s quarks come into play, i.e. up to masses below 3 GeV/c, is the highly satisfactory predictive power of the phenomenological models used to describe the particle spectrum in this new sector. This is mainly due to the fact that the large mass of the quarks involved allows one to make use of non-relativistic dynamics (i.e. Schrodinger equation) which would not have been a sensible approximation in the old sector with light quarks. 

To describe the second ingredient that makes the non-relativistic approach a practical tool for numerical computation in the heavy quark sector, we have to briefly review here some of the basic characteristics of QCD, the candidate theory of strong interactions which will be used to describe the interaction potential between quarks. QCD will be discussed in detail in Chapters 20-23. 

In such a theory, one requires eight massless coloured vector bosons (called gluons) to mediate the strong interaction. With colour as an exact S5unmetry one cannot expect to see free gluons, but their existence has 

Colour, like charge, cannot be destroyed but, contrary to charge, physical states (hadrons) must be colourless (i.e. colour singlets). Although no one has so far been able to prove that confinement is a property of QCD, this is usually assumed to be the case. 

As an example of a practical consequence of the above assumptions, the formula (9.5.25) for R (Fig. 11.1) is altered, and to second order in as becomes (below the region of electroweak interference) 

---