Abstract
We review a class of models in which the Standard Model (SM) is augmented by vector-like leptons: one doublet and a singlet, which are odd under an unbroken discrete Z_2 symmetry. As a result, the neutral component of these additional vector-like leptons are stable and behave as dark matter. We study the phenomenological constraints on the model parameters and elucidate the parameter space for relic density, direct detection and collider signatures of dark matter. In such models, we further add a scalar triplet of hypercharge two and study the consequences. In particular, after electro weak symmetry breaking (EWSB), the triplet scalar gets an induced vacuum expectation value (vev), which yield Majorana masses not only to the light neutrinos but also to vector-like leptonic doublet DM. Due to the Majorana mass of DM, the Z mediated elastic scattering with nucleon is forbidden and hence allowing the model to survive from stringent direct search bound. The DM without scalar triplet lives in a small singlet-doublet leptonic mixing region (sin θ ≤ 0.1) due to large contribution from singlet component and have small mass difference (∆m ∼ 10 GeV) with charged companion, the NLSP (next to lightest stable particle), to aid co-annihilation for yielding correct relic density. Both these observations change to certain extent in presence of scalar triplet to aid observability of hadronically quiet leptonic final states at LHC, while one may also confirm/rule-out the model through displaced vertex signal of NLSP, a characteristic signature of the model in relic density and direct search allowed parameter space.
Highlights
The existence of dark matter (DM) in a large scale (> a few kpc) has been proven irrefutably by various astrophysical observations
Let us assume that the dark sector is composed of a vector-like lepton doublet: N = (N0 N− )T, which is odd under an extended Z2 symmetry [ called inert lepton doublet (ILD)], while all the Standard Model (SM) fields are even
The relic density allowed parameter space provide a wide class of phenomenological implications to be explored in DM direct search experiments and in collider searches through signal excess or displaced vertex
Summary
The existence of dark matter (DM) in a large scale (> a few kpc) has been proven irrefutably by various astrophysical observations. In absence of scalar triplet, the singlet-doublet DM is allowed to have only a tiny fraction of doublet component (sin θ ≤ 0.1) to evade direct search bound In this limit, due to large contribution from the singlet component, the annihilation cross section of the DM becomes smaller than what it requires to achieve correct relic density. In presence of the scalar triplet, we show that both the singletdoublet mixing (sin θ ) and the mass difference with NLSP ( m) can be relaxed and larger parameter space is available for correct relic density and being compatible with the latest direct detection bounds. The scalar triplet aid observability of hadronically quiet leptonic final states at LHC while one may confirm/rule-out the model through displaced vertex of NLSP, a characteristic signature of the model in relic density and direct search allowed parameter space. We provide some vertices of inert lepton doublet (ILD) DM in presence of scalar triplet in Appendix A
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