Streaking At High Energies With Electrons And Positrons
  State-of-the-art attosecond metrology deals with the detection and characterization of photon pulses with typical energies up to the hundreds of eV and time resolution of several tens of attoseconds. Such short pulses are used for example to control the motion of electrons on the atomic scale or to measure inner-shell atomic dynamics. The next challenge of time-resolving the inner-nuclear dynamics, transient meson states and resonances requires photon pulses below attosecond duration and with energies exceeding the MeV scale.   Here we discuss a detection scheme for time-resolving high-energy gamma ray pulses down to the zeptosecond timescale. The scheme is based on the concept of attosecond streak imaging, but instead of conversion of photons into electrons in a nonlinear medium, the high-energy process of electron-positron pair creation is utilized. These pairs are produced in vacuum through the collision of a test pulse to be characterized with an intense laser pulse, and they acquire additional energy and momentum depending on their phase in the streaking pulse at the moment of production. A coincidence measurement of the electron and positron momenta after the interaction provides information on the pair production phase within the streaking pulse. We examine the limitations imposed by quantum radiation reaction in multiphoton Compton scattering on this detection scheme, and discuss other necessary conditions to render the scheme feasible in the upcoming Extreme Light Infrastructure (ELI) laser facility. 
Andreas Ipp, J\"org Evers, Christoph H. Keitel, Karen Z. Hatsagortsyan
physics.ins-det hep-ph physics.acc-ph