  A general framework is proposed to tackle analytically local quantum quenches
in integrable impurity systems, combining a mapping onto a boundary problem
with the form factor approach to boundary-condition-changing operators
introduced in Phys. Rev. Lett. 80, 4370 (1998). We discuss how to compute
exactly two central quantities of interest: the Loschmidt echo and the
distribution of the work done during the quantum quench. Our results display an
interesting crossover physics characterized by the energy scale T_b of the
impurity corresponding to the Kondo temperature. We discuss in detail the
non-interacting case as a paradigm and benchmark for more complicated
integrable impurity models, and check our results using numerical methods.
