Operator mixing in the ϵ -expansion: Scheme and evanescent-operator independence

We consider theories with fermionic degrees of freedom that have a fixed point of Wilson-Fisher type in noninteger dimension $d = 4 - 2ϵ$. Due to the presence of evanescent operators, i.e., operators that vanish in integer dimensions, these theories contain families of infinitely many operators that can mix with each other under renormalization. We clarify the dependence of the corresponding anomalous-dimension matrix on the choice of renormalization scheme beyond leading order in ϵ-expansion. In standard choices of scheme, we find that eigenvalues at the fixed point cannot be extracted from a finite-dimensional block. We illustrate in examples a truncation approach to compute the eigenvalues. These are observable scaling dimensions, and, indeed, we find that the dependence on the choice of scheme cancels. As an application, we obtain the IR scaling dimension of four-fermion operators in QED in $d = 4 - 2ϵ$ at order $O(ϵ^2)$.