Memory and the Infrared

Résumé : Memory effects in scattering processes are described in terms of the asymptotic retarded fields. These fields are completely determined by the scattering data and the zero mode part is set by the soft photon theorem. The dressed asymptotic states defining an infrared finite S- matrix for charged particles can be defined as quantum coherent states using the corpuscular resolution of the asymptotic retarded fields. Im- posing that the net radiated energy in the scattering is zero leads to the new set of conservation laws for the scattering S-matrix which are equivalent to the decoupling of the soft modes. The actual observabil- ity of the memory requires a non vanishing radiated energy and could be described using the infrared part of the differential cross section that only depends on the scattering data and the radiated energy. This is the IR safe cross section with any number of emitted pho- tons carrying total energy equal to the energy involved in the actual memory detection.