Quarks and the light hadrons

The evidence for point-like quarks and their quantum numbers is reviewed, together with the arguments which suggest a 'colour' degree of freedom for them, corresponding to an SU(3)sub(C) colour symmetry. Gauging the colour current leads directly to the theory known as quantum chromodynamics, a theory which is renormalizable, has asymptotic freedom, and may have quite naturally both chiral symmetry and SU(N) flavour symmetry, the observed situation resulting from a spontaneous breaking of these last two symmetries. The need for a relativistic treatment of quark and antiquark systems which include light quarks is emphasized and illustrated by a discussion of the vector quark-antiquark systems using a Bethe-Salpeter equation as a starting point; neglecting spin-orbit splitting, these vector states may be fitted with a flavour-independent confining potential. The baryonic spectrum is discussed briefly; the states correspond more closely to independent excitations of light quark and heavy quark oscillators, than to definite permutation symmetries. Some spin-orbit couplings of a fundamental origin lead to some discrepancies between data and theory. (author)