Effects of dark atom excitations
  New stable quarks and charged leptons may exist and be hidden from detection,
as they are bound by Coulomb interaction in neutral dark atoms of composite
dark matter. This possibility leads to fundamentally new types of indirect
effects related to the excitation of such dark atoms followed by their
electromagnetic de-excitation. Stable -2 charged particles, bound to primordial
helium in O-helium (OHe) atoms, represent the simplest model of dark atoms.
Here we consider the structure of OHe atomic levels which is a necessary input
for the indirect tests of such composite dark matter scenarios, and we give the
spectrum of electromagnetic transitions from the levels excited in OHe
collisions.
