Less structure on $8$ Mpc scales from decaying sterile neutrino dark matter

Late decays of dark matter to a lighter, warm dark matter component are a known way to reduce the amplitude of the matter power spectrum on scales of $8$ Mpc. However, only very few particle physics models have been put forward that exhibit the required properties and allow to relate them to other observables. In this work, we investigate a model based on two interacting sterile neutrinos and a scalar singlet. The heavier of the neutrinos is produced in the early Universe by the interplay of oscillations and the new interactions in the dark sector and constitutes the dominant component of dark matter. If the Yukawa matrix that describes the interactions of the steriles with the scalar is not diagonal, the heavier state can decay to three light sterile neutrinos. In contrast to the usual scenario, this leads to an all massive final state without radiation-like particles. We identify the part of the parameter space where these decays can lead to a reduction of S$_8$ at a level that matches observations. We then confront this region with the requirements of reproducing the observed relic density, as well as existing constraints from X-ray searches and Lyman-$\alpha$ forest data.