Kramers-Kr\"onig approach to the electric permittivity of the vacuum in a strong constant electric field

We study the electric permittivity of the QED vacuum in the presence of a strong constant electric field, motivated by the analogy between the dynamically-assisted Schwinger effect in strong-field QED and the Franz-Keldysh effect in semiconductor physics. We develop a linear-response theory based on the non-equilibrium in-in formalism and the Furry-picture perturbation theory, with which and also utilizing the Kramers-Kr\"onig relation, we calculate the electric permittivity without assuming weak fields and low-frequency probes. We discover that the electric permittivity exhibits characteristic oscillating dependence on the probe frequency, which directly reflects the change of the QED-vacuum structure by the strong field. We also establish a quantitative correspondence between the electric permittivity and the number of electron-positron pairs produced by the dynamically-assisted Schwinger effect.