Finite time path field theory perturbative methods for local quantum spin chain quenches

We discuss local magnetic field quenches using perturbative methods of finite time path field theory in the following spin chains: Ising and XY in a transverse magnetic field. Their common characteristics are: i) they are integrable via mapping to second quantized noninteracting fermion problem; ii) when the ground state is nondegenerate (true for finite chains except in special cases) it can be represented as a vacuum of Bogoliubov fermions. By switching on a local magnetic field perturbation at finite time, the problem becomes nonintegrable and must be approached via numeric or perturbative methods. Using the formalism of finite time path field theory based on Wigner transforms of projected functions, we show how to: i) calculate the basic "bubble" diagram in the Loschmidt echo of a quenched chain to any order in the perturbation; ii) resum the generalized Schwinger-Dyson equation for the fermion two point retarded functions in the "bubble" diagram, hence achieving the resummation of perturbative expansion of Loschmidt echo for a wide range of perturbation strengths under certain analiticity assumptions. Limitations of the assumptions and possible generalizations beyond it and also for other spin chains are further discussed.