  The proposed LDMX experiment would provide roughly a meter-long region of
instrumented tracking and calorimetry that acts as a beam stop for multi-GeV
electrons in which each electron is tagged and its evolution measured. This
would offer an unprecedented opportunity to access both collider-invisible and
ultra-short lifetime decays of new particles produced in electron (or
muon)-nuclear fixed-target collisions. In this paper, we show that the missing
momentum channel and displaced decay signals in such an experiment could
provide world-leading sensitivity to sub-GeV dark matter, millicharged
particles, and visibly or invisibly decaying axions, scalars, dark photons, and
a range of other new physics scenarios.
