Upgrading sterile neutrino dark matter to FImP using scale invariance

In this article we propose a class of extremely light feebly interacting massive particle, FImPs. They are combination of feebly interacting massive particle with scale invariance, by which DM stability, mass origin and relic density are inherently related. In the scale invariant version of the Standard Model (SM) with three right-handed neutrinos (nu SISM), the lightest N-1 realizes the FImP scenario. In this example scalar singlets, which are intrinsic to the nu SISM, generate mass and relic density for this FImP simultaneously. Moreover, they are badly needed for electroweak symmetry spontaneously breaking. Interestingly, a 7.1 keV N-1 with correct relic density, which can explain the recent 3.55 keV X-ray line, lies in the bulk parameter space of our model.