Theoretical Overview of Spectator Tagging in Lepton-Nucleon DIS

Spectator tagging denotes processes on (light) ions where a nucleon or nuclear fragment is detected in the target fragmentation region and the spectator has a slow (0 - few 100 MeV) momentum compared to the ion center-of-mass. Detection of the spectator results in additional control over the initial nuclear configuration of the ion target compared to inclusive scattering where one averages over all possible configurations. In this talk, I discuss the theoretical framework used to describe these reactions, which is based on light-front quantization and results in a natural separation of nuclear and nucleon structure. I give an overview of several applications of spectator tagging in the context of fixed target (Jefferson Lab 12 GeV) and collider (future electron-ion collider) programs . These include free neutron structure, nuclear configuration dependence of medium modifications (EMC effect) and measurements with polarized light ions (deuteron, ³He). I also comment on how final-state interactions with the spectator can modify measured observables.