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 nonrelativistic limit. However, at the nucleon level, such couplings are effectively enhanced by factors of order O(m(N)/m(q)) similar to 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.