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Field Theory in the Imaginary-time Formulation

The imaginary-time formulation is investigated in the field theory. The Dirac and photon's Lagrangians are treated in the imaginary-time and space and are invariant under scale transformations, which are motivated by the Tolman-Ehrenfest relation and Wilson's approach in the renormalization group. With the functional approaches, the propagators of leptons and photons can be derived both for the real- and the imaginary-time. The theory is free of UV divergence in the integration range of the imaginary-time, $(0,β\,]$, which is required by the KMS condition. The results of one-loop radiative corrections are proved to be consistent with those in QED. The renormalization group equations with respect to vacuum's temperature give no Landau pole and show identical renormalization coefficients as those from the $\overline{MS}$ scheme of the renormalization in the limit of zero temperature.