Abstract
In Higgs portal models of fermion dark matter, scalar couplings are unavoidably suppressed by strong bounds from direct detection experiments. As a consequence, thermal dark matter relics must coexist with mediators in a compressed spectrum of dark particles. Small couplings and small mass splittings lead to slow mediator decays, leaving signatures with displaced vertices or disappearing tracks at colliders. We perform a comprehensive analysis of long-lived mediators at the LHC in the context of a minimal dark matter model with a naturally small Higgs portal, also known as the wino-bino scenario in supersymmetry. Existing searches for disappearing charged tracks and displaced hard leptons already exclude tiny portal couplings that cannot be probed by current direct and indirect detection experiments. For larger portal couplings, we predict new signatures with displaced soft leptons, which are accessible with run-II data. Searches for displaced particles are sensitive to weakly coupling mediators with masses up to the TeV scale, well beyond the reach of prompt signals.
Highlights
Couplings, which are naturally suppressed by a high cutoff scale [13]
We have investigated singlet-triplet fermion dark matter interacting with the standard model through a scalar or pseudo-scalar Higgs portal
Besides the different Lorentz structure of the Higgs and W -boson couplings, the fermion mixing in the pseudo-scalar scenario is generically smaller than in the scalar scenario. This leads to different lifetimes of the mediators, with observable consequences for the dark matter and collider phenomenology
Summary
Singlet-triplet mixing induces scalar Higgs couplings and charged vector currents of the lightest state χ. In the limit of small mixing, the lightest state is mostly a weak singlet with suppressed couplings to the standard model. It is instructive to study the features of our model in the limit of small singlet-triplet mixing In this limit, the couplings of the lightest state χ are approximated by. Gauge and diagonal Higgs couplings of the lightest state are weaker in the pseudo-scalar scenario. The lightest state χ is mostly a gauge singlet, while the heavier states χ+ and χh approximately correspond to the charged and neutral components of a weak triplet In both scenarios, the mass splitting between the heavier states is given by. The mass hierarchy plays a crucial role for the collider phenomenology of the dark fermions (see section 6)
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