Overcoming Velocity Suppression in Dark-Matter Direct-Detection Experiments

Understand

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.