Overcoming Velocity Suppression in Dark-Matter Direct-Detection
  Experiments
  Pseudoscalar couplings between Standard-Model quarks and dark matter are
normally not considered relevant for dark-matter direct-detection experiments
because they lead to velocity-suppressed scattering cross-sections in the
non-relativistic limit. However, at the nucleon level, such couplings are
effectively enhanced by factors of order ${\cal O}(m_N/m_q)\sim 10^3$, where
$m_N$ and $m_q$ are appropriate nucleon and quark masses respectively. This
enhancement can thus be sufficient to overcome the corresponding velocity
suppression, implying --- contrary to common lore --- that direct-detection
experiments can indeed be sensitive to pseudoscalar couplings. In this work, we
explain how this enhancement arises, and present a model-independent analysis
of pseudoscalar interactions at direct-detection experiments. We also identify
those portions of the corresponding dark-matter parameter space which can be
probed at current and future experiments of this type, and discuss the role of
isospin violation in enhancing the corresponding experimental reach.
