Axion Dark Matter Detection with Cold Molecules
  Current techniques cannot detect axion dark matter over much of its parameter
space, particularly in the theoretically well-motivated region where the axion
decay constant f_a lies near the GUT and Planck scales. We suggest a novel
experimental method to search for QCD axion dark matter in this region. The
axion field oscillates at a frequency equal to its mass when it is a component
of dark matter. These oscillations induce time varying CP-odd nuclear moments,
such as electric dipole and Schiff moments. The coupling between internal
atomic fields and these nuclear moments gives rise to time varying shifts to
atomic energy levels. These effects can be enhanced by using elements with
large Schiff moments such as the light Actinides, and states with large
spontaneous parity violation, such as molecules in a background electric field.
The energy level shift in such a molecule can be ~ 10^-24 eV or larger. While
challenging, this energy shift may be observable in a molecular clock
configuration with technology presently under development. The detectability of
this energy shift is enhanced by the fact that it is a time varying shift whose
oscillation frequency is set by fundamental physics and is therefore
independent of the details of the experiment. This signal is most easily
observed in the sub-MHz range, allowing detection when f_a is > 10^16 GeV, and
possibly as low as 10^15 GeV. A discovery in such an experiment would not only
reveal the nature of dark matter and confirm the axion as the solution to the
strong CP problem, it would also provide a glimpse of physics at the highest
energy scales, far beyond what can be directly probed in the laboratory.
