Dark matter direct detection of a fermionic singlet at one loop

Preprint number: CP3-Origins-2018-9 DNRF90
Authors: Juan Herrero-García (University of Adelaide), Emiliano Molinaro (University of Aarhus & CP3-Origins), and Michael A. Schmidt (University of Sydney)

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The strong direct detection limits could be pointing to dark matter – nucleus scattering at loop level. We study in detail the prototype example of an electroweak singlet (both Dirac or Majorana) dark matter fermion coupled to an extended dark sector, composed of a new fermion and a new scalar. Given the strong limits on colored particles from direct and indirect searches, we assume that the fields of the new dark sector are color singlets. We outline the possible simplified models, including the well-motivated case in which the extra scalar or fermion is a Standard Model particle, in which case we study the synergy between direct detection, relic abundance, lepton flavor violation and neutrino masses. We compute the contributions from the photon, the Z and the Higgs penguins to the differential cross section for arbitrary quantum numbers of the dark sector. Furthermore, we derive compact expressions in certain limits, i.e., when all new particles are heavier than the dark matter mass and when the fermion running in the loop is light, like a Standard Model lepton. We study in detail the predicted direct detection rate and how current and future direct detection limits constrain the model parameters. We find that there is an interesting interplay between lepton flavor violation, direct detection and the observed relic abundance.