Description
Affleck et al used worldline instantons to obtain an all-orders-in-$\alpha$ result for Schwinger pair production by a constant electric field.
Dunne and Schubert et al showed how to use instantons to obtain the probability for non-constant fields.
In both cases, and in most subsequent papers, the instantons are closed loops, which give probabilities integrated over particle momenta (and summed over spins).
We have developed an instanton formalism with open worldlines to obtain the momentum spectrum for Schwinger pair production in realistic 4D fields.
We have also shown how to use open worldlines for other strong-field processes such as nonlinear Compton emission of hard photons ($e^{-}\to e^{-}\gamma$), nonlinear Breit-Wheeler ($\gamma\to e^{+}e^{-}$) and trident ($e^{-}\to e^{-}e^{-}e^{+}$).
