Loop Effects in Direct Detection

We consider loop level contributions to dark matter scattering off nucleons in cases where the spin independent scattering cross section is absent or suppressed at tree level. In the case of a pseudoscalar interaction, for which the tree level cross section is both spin-dependent and suppressed by 4 powers of the exchanged momentum, we show that loop diagrams give rise to a non- zero spin independent cross section. Importantly, if the pseudoscalar interaction is formulated using a gauge invariant framework, loop effects generate an effective $\bar{\chi}\chi h$ vertex and result in a scattering cross section that is within reach of current or forthcoming experiments. We also consider the case of inelastic dark matter, for which the tree-level direct detection cross section is negligible when the inelastic $\chi_1 N \to \chi_2 N$ process is kinematically suppressed. In this case, loop diagrams generate an interaction with both initial and final $\chi_1$ states and hence permit measurable, spin independent, $\chi_1 N \to \chi_1 N$ elastic scattering. As such, we are able to probe parameter space that was previously considered inaccessible to direct detection