Rich and complex dark sectors are abundant in particle physics theories. Here
we propose performing spectroscopy of the mass structure of dark sectors via
mono-photon searches at lepton colliders. The energy of the mono-photon tracks
the invariant mass of the invisible system it recoils against, which enables
studying the resonance structure of the dark sector. We demonstrate this idea
with several well-motivated models of dark sectors. Such spectroscopy
measurements could potentially be performed at Belle II, BES-III and future
low-energy lepton colliders.Comment: 6 pages, 5 figure
