EVIDENCE FOR PRONOUNCED QUARK LOOP EFFECTS IN QCD

We have measured the hadron spectrum in lattice QCD, using staggered f ermions, for 0 (the quenched approximation), 2 and 4 light degenerate dynamical quarks. In addition to earlier results involving extrapolati ons in valence quark masses for fixed dynamical mass, we also report r esults where we extrapolate in the dynamical mass for 4 flavors. We se e a marked difference in the hadron spectrum for 2 and 4 flavors; the hadron spectrum is nearly parity doubled for 4 flavors, indicating sma ller effects of chiral symmetry breaking. This pronounced effect in th e hadron spectrum cannot be removed by a simple change in scale as the number of light quark flavors is changed. Further simulations at larg er volume are needed to rule out finite volume effects.