Pseudo-Nambu–Goldstone Dark Matter―Reconciliation between Direct Detection Experiments and Thermal Relic Abundance

Abstract

Thermal production mechanism for dark matter is attractive. Such thermal dark matter is being explored by various ways. Direct detection experiments utilize scattering events between dark matter particle and nuclei under the ground. However, no dark matter signal is found so far despite its high experimental sensitivity. As a result, the direct detection experiments impose a severe upper bound on the interaction strength between thermal dark matter and ordinary matter particles. In this article, we show that a pseudo-Nambu–Goldstone boson as a thermal dark matter candidate can naturally evade this severe bound. This is because the scattering amplitude is suppressed by small momentum transfer while the annihilation cross section determining the relic abundance is not suppressed. Furthermore, we also point out that early kinetic decoupling of dark matter is induced if the dark matter mass is close to resonances. This effect can give a correction to the calculation of coupling strength between dark matter and ordinary matter particles. Therefore, it will be testable via precise measurements of the Higgs coupling.