Abstract
A reliable comparison of different dark matter (DM) searches requires models that satisfy certain consistency requirements like gauge invariance and perturbative unitarity. As a well-motivated example, we study two-mediator DM (2MDM). The model is based on a spontaneously broken $U(1)'$ gauge symmetry and contains a Majorana DM particle as well as two $s$-channel mediators, one vector (the $Z'$) and one scalar (the dark Higgs). We perform a global scan over the parameters of the model assuming that the DM relic density is obtained by thermal freeze-out in the early Universe and imposing a large set of constraints: direct and indirect DM searches, monojet, dijet and dilepton searches at colliders, Higgs observables, electroweak precision tests and perturbative unitarity. We conclude that thermal DM is only allowed either close to an $s$-channel resonance or if at least one mediator is lighter than the DM particle. In these cases a thermal DM abundance can be obtained although DM couplings to the Standard Model are tiny. Interestingly, we find that vector-mediated DM-nucleon scattering leads to relevant constraints despite the velocity-suppressed cross section, and that indirect detection can be important if DM annihilations into both mediators are kinematically allowed.
Highlights
The idea that dark matter (DM) communicates with the Standard Model (SM) via the exchange of additional new particles has recently received large amounts of interest [1,2,3,4,5,6,7,8,9,10]
We perform a global scan over the parameters of the model assuming that the DM relic density is obtained by thermal freeze-out in the early Universe and imposing a large set of constraints: direct and indirect DM searches, monojet, dijet and dilepton searches at colliders, Higgs observables, electroweak precision tests and perturbative unitarity
For mχ > ms, the DM particles can annihilate into pairs of dark Higgs bosons, which subsequently decay into other SM particles
Summary
The idea that dark matter (DM) communicates with the Standard Model (SM) via the exchange of additional new particles (so-called dark mediators) has recently received large amounts of interest [1,2,3,4,5,6,7,8,9,10]. For a model with a single s-channel mediator the typical conclusion is that the couplings of the mediator to SM particles have to be rather small in order to be in agreement with experimental bounds and as a result DM overproduction can only be avoided in rather special corners of parameter space [4, 6] It has been pointed out, that for a meaningful comparison of different constraints the dark mediator model needs to be consistent with gauge invariance and perturbative unitarity [16]. As a straight-forward way to satisfy these requirements we consider a DM model containing a new U(1) gauge group and a dark Higgs that breaks the U(1) and generates the mass of the fermionic DM particle as well as the mass of the Z gauge boson Both the Z and the dark Higgs can couple to SM particles and thereby mediate the interactions of DM. For the Z such a coupling can arise if some or all of the SM fermions are charged under the U(1) gauge group, while the dark Higgs can couple to SM states via mixing with the SM Higgs
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