Non-relativistic Effective Interactions of Spin 1 Dark Matter
  We investigate the non-relativistic reduction of simplified models for spin 1
dark matter (DM) with the aim of identifying features in the phenomenology of
DM-quark interactions which are specific to vector DM. In the case of DM-quark
interactions mediated by a spin 1 particle, we find two DM-nucleon interaction
operators arising from the non-relativistic reduction of simplified models for
spin 1 DM that are specific to spin 1 DM, and which were not considered in
previous studies. They are quadratic in the momentum transfer, linear in a
symmetric combination of polarisation vectors for the DM particle, and arise
from simplified models which do not generate momentum transfer independent
operators as leading interactions in the non-relativistic expansion of
DM-nucleon scattering amplitudes. Within these simplified models, the new
operators cannot be neglected when computing DM signals at direct detection
experiments. For example, we find that nuclear recoil energy spectra computed
by including or neglecting the new operators can differ by up to one order of
magnitude for nuclear recoil energies larger than about 20 keV and DM masses
below $50$ GeV. Furthermore, the shape of the expected nuclear recoil spectra
depends significantly on whether the new operators are taken into account or
not. Finally, neglecting the contribution to DM direct detection signals from
the new operators leads to inaccurate conclusions when assessing the
compatibility of a future direct detection signal with CMB constraints on the
DM relic density, especially when the number of signal events is small, e.g.
$\mathcal{O}(1)$.
