Finite-range Lattice Momentum Operators for Quantum Field Theory
We propose a Z-transform framework for the analysis and synthesis of finite range lattice momentum operators in quantum field theory. In this formulation, translation-invariant lattice operators are represented as functions of the complex variable $z$ in the unit circle, allowing their spectral properties to be analyzed using tools from digital signal processing and rational approximation theory. Within this framework, the fermion doubling problem is reinterpreted as the appearance of unwanted zeros of the discrete momentum operator on the unit circle --- an aliasing phenomenon in the sense of the Nyquist sampling theorem --- and the conditions for ghost suppression are expressed as precise constraints on the zero structure of the operator's transfer function. It is proven that no rational function can satisfy all required conditions simultaneously, motivating the finite impulse response approach developed here. This reframing naturally suggests a class of finite-range momentum operators, constructed by solving a least-squares approximation problem in the frequency domain. The resulting finite impulse response (FIR) operator approximates the continuum derivative across the full Brillouin zone, with ghost suppression achieved through the accuracy of the spectral approximation rather than through the addition of a symmetry-breaking Wilson term or the infinite-range nonlocal SLAC derivative. Numerical investigation confirms that near $θ= π$ only plane waves propagate coherently, and these exhibit group velocities far exceeding the speed of light, further distinguishing them from physical low-energy excitations. No ghost wave packet solutions exist near $θ= π$.