Abstract
Dirac fermion dark matter models with heavy Z′ mediators are subject to stringent constraints from spin-independent direct searches and from LHC bounds, cornering them to live near the Z′ resonance. Such constraints can be relaxed, however, by turning off the vector coupling to Standard Model fermions, thus weakening direct detection bounds, or by resorting to light Z′ masses, below the Z pole, to escape heavy resonance searches at the LHC. In this work we investigate both cases, as well as the applicability of our findings to Majorana dark matter. We derive collider bounds for light Z′ gauge bosons using the CLS method, spin-dependent scattering limits, as well as the spin-independent scattering rate arising from the evolution of couplings between the energy scale of the mediator mass and the nuclear energy scale, and indirect detection limits. We show that such scenarios are still rather constrained by data, and that near resonance they could accommodate the gamma-ray GeV excess in the Galactic center.
Highlights
Collider searches hinge on the fact that high-energy proton-proton collisions at the LHC can generate dark matter particles in association with other exotic particles
We derive collider bounds for light Z gauge bosons using the CLS method, spin-dependent scattering limits, as well as the spin-independent scattering rate arising from the evolution of couplings between the energy scale of the mediator mass and the nuclear energy scale, and indirect detection limits
Dirac fermion dark matter models in the context of heavy vector mediators are forced to live near the Z resonance due to the a combination of spin-independent and LHC bounds
Summary
Searches for high- and low-mass dilepton resonances at the LHC have been an excellent probe of models containing new neutral vector bosons [65, 66]. In the case where the new vector boson mediates the interaction between the SM and the dark sector, constraints from dijets and monojet searches for the Z are complementary in the mass versus coupling plane [36] These are the most stringent constraints for leptophobic dark Z models. When couplings to leptons are sizable, though, dileptons searches have the potential to exclude larger portions of the models’ parameter space [67,68,69] compared do dijets. This can be understood in view of the relative size of the production cross section for dijets and dileptons and their correspondent irreducible backgrounds: first, both production mechanisms are electroweak processes; second the dominant backgrounds for dijets and dileptons are the QCD jet pair production and the Drell-Yan processes, respectively. At LO, for the dominant backgrounds we have σ(pp → Z → + −)/σ(pp → jj) ∼ O(10−4) at the 13 TeV LHC [70], and a similar ratio should be expected at 7 and 8 TeV center-of-mass energies
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