THE EXTENDED CHIRAL QUARK-MODEL AND QCD

We consider the low energy effective action of QCD below the chiral sy mmetry breaking scale, including, in Wilson's spirit, all operators of dimensionality less or equal to 6 which can be built with quark and c hiral fields. The effect of gluon interactions is contained in a numbe r of coupling constants, whose running is studied. The resulting model is an extension of both the chiral quark model and the Nambu-Jona-Las inio one. Constraints on the coefficients of the effective Lagrangian are derived from the requirement of chiral symmetry restoration at ene rgies above the chiral symmetry breaking scale, from matching to QCD a t intermediate scales, and by fitting some hadronic observables. In th is model two types of pseudoscalar states (massless pions and massive I;Pi-mesons), as well as one scalar one arise as a consequence of dyna mical chiral symmetry breaking. Their masses and coupling constants ar e studied. We also predict a number of low energy structural constants . We find out that QCD favors a low energy effective theory which is l argely dominated by the simplest chiral quark model, whereas higher di mensional operators (such as those of the Nambu-Jona-Lasinio type) can be assumed to be small. (C) 1998 Elsevier Science B.V.