Implications of $Tr{\gamma }_5=0$ in Lattice Gauge Theory

We analyze the implications of the relation $Tr{\gamma }_5=0$, which is customarily assumed in practical lattice calculations. On the basis of the finite dimensional representations of the Ginsparg-Wilson algebra, it is shown that this relation reflects the species doubling in lattice theory; topological excitations associated with species doublers, which have eigenvalue $2\mathrm{/}a$, contribute to $Tr{\gamma }_5$ without any suppression. In this sense, the relation $Tr{\gamma }_5=0$ is valid only when we allow the presence of unphysical states in the Hilbert space; this statement is also valid in the Pauli-Villars regularization. If one eliminates the contributions of the unphysical states, the trace $Tr{\gamma }_5$ is replaced by $Tr\Gamma_{5}\equiv Tr \gamma_{5}(1 -{1/2}aD)$ which gives rise to the Pontryagin index, to be consistent with the continuum analysis.