WIMP Shadows: Phenomenology of Secluded Dark Matter in Three Minimal BSM Scenarios

Abstract: We present a comprehensive study of secluded dark matter (DM) $\chi$, where the relic abundance is set by annihilations into lighter dark mediators $\phi$ that couple only feebly to the Standard Model (SM). In contrast to canonical WIMPs, which are now strongly constrained by direct and indirect searches, secluded models still achieve the observed relic abundance via thermal freeze-out into hidden-sector mediators, while predicting highly suppressed present-day signals. We analyze three minimal models: (i) a $U(1)_X$ gauge boson ($A'$) with kinetic mixing; (ii) a scalar DM candidate $S$ with a scalar mediator $K$ that has a trilinear vertex; and (iii) a Dirac fermion $\chi$ whose mass arises from a Higgs-mixed singlet $H_p$. For each model we derive annihilation and scattering rates in both WIMP-like and secluded regimes, and solve the Boltzmann equations: a single-species equation for the WIMP case and a coupled $\chi$-$\phi$ system for the secluded case to account for possible early departure of the mediator from thermal equilibrium with the SM bath. In this regard, we provide explicit lower limits on the portal coupling $\epsilon$ required to keep the mediator in thermal equilibrium with the SM bath and to ensure mediator decay before BBN. We show that for portal couplings $\epsilon \ll 10^{-3}$ the relic density is dominantly controlled by DM annihilation into mediator pairs, while spin-independent scattering lies well below current limits and remains viable even for future experiments approaching the irreducible neutrino background floor. Indirect constraints are typically weak due to $p$-wave suppression, off-resonance $s$-channels, and cascade spectra controlled by $\epsilon^2$. Finally, we highlight the most promising collider tests, which remain sensitive despite tiny portal couplings.