MASSIVE FERMIONS IN LATTICE GAUGE-THEORY

This paper presents a formulation of lattice fermions applicable to al l quark masses, large and small. We incorporate interactions from prev ious light-fermion and heavy-fermion methods, and thus ensure a smooth connection to these limiting cases. The couplings in improved actions are obtained for arbitrary fermion mass m(q), without expansions arou nd small- or large-mass limits. We treat both the action and external currents. By interpreting on-shell improvement criteria through the la ttice theory's Hamiltonian, one finds that cutoff artifacts factorize into the form b(n)(m(q)a)[pa](sn) where p is a momentum characteristic of the system under study, s(n) is related to the dimension of the nt h interaction, and b(n)(m(q)a) is a bounded function, numerically alwa ys of order 1 or less. In heavy-quark systems p is typically rather sm aller than the fermion mass m(q). Therefore, artifacts of order (m(q)a )(s) do not arise, even when m(q)a greater than or similar to 1. An im portant by-product of our analysis is an interpretation of the Wilson and Sheikholeslami-Wohlert actions applied to nonrelativistic fermions .