Paper detail

Comment on the subtlety of defining real-time path integral in lattice gauge theories

Recently, Hoshina, Fujii, and Kikukawa pointed out that the naive lattice gauge theory action in Minkowski signature does not result in a unitary theory in the continuum limit, and Kanwar and Wagman proposed alternative lattice actions to the Wilson action without divergences. We here show that the subtlety can be understood from the asymptotic expansion of the modified Bessel function, which has been discussed for path integral of compact variables in nonrelativistic quantum mechanics. The essential ingredient for defining the appropriate continuum theory is the $i\varepsilon$ prescription, and with the proper implementation of the $i\varepsilon$ we show that the Wilson action can be used for the real-time path integrals. It is here important that the $i\varepsilon$ should be implemented for both timelike and spacelike plaquettes. We also argue the reason why the $i\varepsilon$ becomes required for the Wilson action from the Hamiltonian formalism. The $i\varepsilon$ is needed to manifestly suppress the contributions from singular paths, for which the Wilson action can give different values from those of the actual continuum action.