Direct Detection of Light Dark Matter in the Scotogenic Model

Abstract

The scotogenic model, also known as the radiative seesaw model, provides an elegant theoretical framework that simultaneously accounts for the generation of active neutrino masses and the existence of dark matter. Current realisations of the model typically focus on weakly interacting massive particle (WIMP) dark matter candidates with masses above the GeV scale. However, despite decades of direct detection searches, no conclusive evidence for WIMP dark matter has yet been found. One possible explanation for this null result is that dark matter may lie below the GeV scale, where conventional nuclear-recoil experiments lose sensitivity due to kinematic thresholds. This possibility motivates the study of sub-GeV realisations of the scotogenic model. This thesis carries out that study for Majorana dark matter in the minimal scotogenic model. Assuming a 𝜏-philic flavour structure, the leading-order electromagnetic interaction is a loop-induced anapole moment. Non-relativistic effective field theory then yields the inelastic scattering cross-section against bound atomic electrons in liquid xenon. Confronting these rates with XENON1T data places upper limits on the anapole coupling, and identifies a steep loss of sensitivity below a dark matter mass 𝑀1 ≈ 30 MeV, driven by the quantisation of the ionisation signal. The pipeline reproduces the published XENON1T anapole constraint to within 3%. Enforcing the LEP bound on the scalar mediator mass (𝑚𝜂± ≳ 90 GeV), the thermal relic density requires non-perturbative couplings (𝑦 > √4𝜋) below 𝑀1 ≈ 480 MeV, while XENON1T only excludes unphysical values (𝑦 ≳ 265). Thermal freeze-out therefore survives only for 𝑀1 ≳ 480 MeV, with the mediator confined to a 37 GeV window above the LEP bound and a coupling close to the perturbative ceiling. Even there the thermal target lies two orders of magnitude below the XENON1T reach in coupling. Below that window the model requires non-thermal freeze-in production, suppressing the scattering rate by roughly 45 orders of magnitude. In neither case is direct detection the discovery channel, and verification shifts to collider searches for the long-lived charged mediator.