I will describe the Deep Underground Neutrino Experiment (DUNE) and its prototype experiment at CERN: ProtoDUNE. ProtoDUNE is a smaller version of a DUNE far detector module that we use to improve our understanding of liquid argon physics in preparation for DUNE. In particular I will describe our measurement of electron-ion recombination in liquid argon using test beam data. This measurement constrains one of the largest systematics in our neutrino energy reconstruction and enables our search for a matter-antimatter asymmetry in neutrino oscillations.
