Dark matter phenomenology of SM and enlarged Higgs sectors extended with vector-like leptons

Eur. Phys. J. C (2017) 77:456 DOI 10.1140/epjc/s10052-017-5015-2 Regular Article - Theoretical Physics Dark matter phenomenology of SM and enlarged Higgs sectors extended with vector-like leptons Andrei Angelescu1, Giorgio Arcadi2,a 1 Laboratoire de Physique Théorique, Université Paris-Saclay, CNRS, 91405 Orsay, France 2 Max Planck Institüt für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany Received: 27 January 2017 / Accepted: 23 June 2017 / Published online: 7 July 2017 © The Author(s) 2017. This article is an open access publication Abstract We will investigate the scenario in which the Standard Model (SM) Higgs sector and its two-doublet extension (called the Two Higgs Doublet Model or 2HDM) are the “portal” for the interactions between the Standard Model and a fermionic Dark Matter (DM) candidate. The latter is the lightest stable neutral particle of a family of vectorlike leptons (VLLs). We will provide an extensive overview of this scenario combining the constraints coming purely from DM phenomenology with more general constraints like Electroweak Precision Test (EWPT) as well as with collider searches. In the case that the new fermionic sector interacts with the SM Higgs sector, constraints from DM phenomenology force the new states to lie above the TeV scale. This requirement is relaxed in the case of 2HDM. Nevertheless, strong constraints coming from EWPTs and the Renormalization Group Equations (RGEs) limit the impact of VLFs on collider phenomenology. Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . 1 2 Vector-like extensions of the Standard Model . . . . 3 3 Two Higgs doublet models . . . . . . . . . . . . . . 7 4 Conclusions . . . . . . . . . . . . . . . . . . . . . 26 References . . . . . . . . . . . . . . . . . . . . . . . . 27 1 Introduction Weakly Interacting Massive Particles (WIMPs) represent probably the most popular class of Dark Matter (DM) candidates. Among the features which make this kind of candidates so attractive, it is for sure worth mentioning the production mechanism. WIMP DM was indeed part of the primordial thermal bath at early stages of the history of the Universe a e-mail: and decoupled (freeze-out) at later stages, when the temperature was below their mass (i.e. non-relativistic decoupling), since the interactions with the SM particles were not efficient anymore with respect to the Hubble expansion rates. Under the assumption of standard cosmological history, the comoving abundance of the DM is set by a single particle physics input, namely the thermally averaged pair annihilation cross section. The experimentally favored value of DM abundance, expressed by the quantity h 2 ≈ 0.12 [1], corresponds to a thermally averaged cross section σ v ∼ 10−26 cm3 s−1 . Interactions of this size are potentially accessible to a broad variety of search strategies, ranging from direct/indirect detection to production at colliders, making the WIMP paradigm highly testable. From the point of view of model building, WIMP frameworks feature interactions between pairs of DM particles (in order to guarantee the cosmological stability of the DM, operators with a single DM field are in general forbidden, e.g. through a symmetry) and pair of SM states, induced by suitable mediator fields. The simplest option, in this sense, is probably represented by s-channel electrically neutral mediators, dubbed “portals”, which can couple the DM with SM fermions (see e.g. [2–4]), although couplings with the SM gauge bosons might also be feasible [5–8]. The DM relic density is thus determined via s-channel exchange of the mediator states. By simple crossing symmetry arguments these processes can be, for example, related to the rate of DM Direct Detection, induced by the t-channel interaction between the DM and the SM quarks, and to the ones of DM pair production at colliders, which can be probed mostly through mono-jet events [9–12]. Interestingly, the SM features two potential s-channel mediators, namely the Z and the Higgs bosons. One possible result concerns “Z-portal” DM [13] scenarios. However, they are rather contrived, since, because of gauge invariance, interactions between a SM singlet DM and the Z can arise only at the non-renormalizable level [14,15]. “Higgs portal” models 123 456 Page 2 of 30 are instead very popular, although rather constrained [16– 20], since a DM spin-0 (1), even if it is a singlet with respect to the SM gauge group, can interact with the SM Higgs doublet H via four-field operators connecting the bilinear H H † with a DM pair and giving rise, after electroweak (EW) symmetry breaking, to an effective vertex between a DM pair and the physical Higgs field h. The fermionic “Higgs portal” is instead a dimension-5 operator. Furthermore this is strongly constrained, also with respect to the scalar and vector DM cases, because of the strong direct detection rates accompanied by a velocity suppressed annihilation cross section [18,19]. In order to couple at the renormalizable level with the Z and/or Higgs bosons, the fermionic DM should feature a (small) hyper- and SU (2) charged component. This could be realized through the mixing of a pure SM singlet and extra states with non-trivial quantum numbers under SU (2)×U (1) (see e.g. [21–24] for some constructions). The DM should then be a stable neutral state belonging to a new, non-trivial particle sector. New chiral fermions, with mass originating from EWSB, are strongly disfavored experimentally [25]. More suitable options are instead represented by fermions belonging to a real representation or forming vector-like pairs. In this work we will consider this last option and then extend the fermionic content of the SM with a “family” of new fields, with analogous quantum numbers as the SM leptons and the right-handed neutrinos, and with bare mass terms, which are allowed by gauge symmetry, since the new fermions are vector-like under the SM gauge group. Therefore, these fields are dubbed “vector-like leptons” (VLLs). In the absence of mixing with SM leptons, the lightest new fermionic state, if electrically neutral, constitutes a DM candidate. In this setup the DM is coupled, through Yukawa interactions, with the SM Higgs and with the Z and W bosons, featuring, in general, non-zero components charged under hypercharge and weak isospin. This kind of scenario is, unfortunately, very strongly constrained since the Higgs and Z-boson mediate Spin Independent (SI) interactions between the DM and the nucleons, which are in increasing tension with experimental constraints. Similarly to the Higgs and Z-portal models it is possible to comply with these limits and achieve, at the same time, the correct relic density only for rather heavy DM masses or, possibly, in the presence of coannihilation processes, thus implying mass degeneracies in the new fermionic sector. A more interesting option would consist in enlarging the mediator sector by considering two Higgs doublets ( (...truncated)