We catalogue and study three-Higgs-doublet models in terms of all possible allowed symmetries (continuous and discrete, Abelian and non-Abelian), where such symmetries may be identified as flavour symmetries of quarks and leptons. We analyse the potential in each case, and derive the conditions under which the vacuum alignments (0, 0, v), (0, v, v) and (v, v, v) are minima of the potential. For the alignment (0, 0, v), relevant for dark matter models, we calculate the corresponding physical Higgs boson mass spectrum. Motivated by supersymmetry, we extend the analysis to the case of three up-type Higgs doublets and three down-type Higgs doublets (six doublets in total). Many of the results are also applicable to flavon models where the three Higgs doublets are replaced by three electroweak singlets.

The Higgs boson of the Standard Model is being searched for at the LHC in the channel $H\to \gamma \gamma$. In the Higgs Triplet Model (HTM), there are contributions to this decay from loops of doubly charged scalars ($H^{\pm \pm }$) and singly charged scalars ($H^{\pm }$) which are not present in the Standard Model. These additional contributions are mediated by the trilinear couplings $H_1{H}^{++}{H}^{--}$ and $H_1{H}^{+}{H}^{-}$, where $H_1$ is the lightest $CP$-even Higgs boson. We point out the possibility of constructive interference of the $H^{\pm \pm }$ contribution with that of the $W^{\pm }$ contribution, which enables a substantial enhancement of the branching ratio of $H_1\to \gamma \gamma$ in the HTM. The magnitude of the contribution of $H^{\pm \pm }$ is essentially determined by the mass of $H^{\pm \pm }$ ($m_{H^{\pm \pm }}$) and a quartic scalar coupling (${\lambda }_1$), with constructive interference arising for ${\lambda }_1<0$. Consequently, the ongoing searches for $H\to \gamma \gamma$ restrict the parameter space of $[m_{H^{\pm \pm }},{\lambda }_1]$ more stringently for ${\lambda }_1<0$ than for the recently studied case of destructive interference with ${\lambda }_1>0$. Moreover, if the excess of $\gamma \gamma$ events around a mass of 125 GeV in the LHC searches for $H\to \gamma \gamma$ is substantiated with larger data samples, and the branching ratio is measured to be somewhat larger than that for the SM Higgs boson, then such an enhancement could be readily accommodated by $H_1$ in the HTM with ${\lambda }_1<0$. DOI: http://dx.doi.org/10.1103/PhysRevD.86.035015 © 2012 American Physical Society

We examine the observability of heavy neutrino (ν_h) signatures of a U(1)′ enlarged Standard Model (SM) encompassing three heavy Majorana neutrinos alongside the known light neutrino states at the the Large Hadron Collider (LHC). We show that heavy neutrinos can be rather long-lived particles producing distinctive displaced vertices that can be accessed in the CERN LHC detectors. We concentrate here on the gluon fusion production mechanism gg → H_1,2 → ν_hν_h, where H₁ is the discovered SM-like Higgs and H₂ is a heavier state, yielding displaced leptons following ν_h decays into weak gauge bosons. Using data collected by the end of the LHC Run 2, these signatures would prove to be accessible with negligibly small background.

We explore the phenomenology of models containing one Vector-Like Quark (VLQ), t′, which can decay into the Standard Model (SM) top quark, t, and a new spin-0 neutral boson, S, the latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the t′ can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming t′ pair production via strong interactions, we design signal regions optimised for one t′ → S t transition (while being inclusive on the other $$ \overline{t} $$ t ¯ ′ decay, and vice versa), followed by the decay of S into the two very clean experimental signatures S → γ γ and S → Z (→ ℓ ⁺ ℓ ⁻ )γ. We perform a dedicated signal- to-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR(t′ → S t) = 100%, we are therefore able to realistically quantify the sensitivity of the LHC to both the t′ and S masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on t′ decays into SM bosons (W ^± , Z , h).

We study a class of non-exotic minimal U(1)^′ extensions of the Standard Model, which includes all scenarios that are anomaly-free with the ordinary fermion content augmented by one Right-Handed neutrino per generation, wherein the new Abelian gauge group is spontaneously broken by the non-zero Vacuum Expectation Value of an additional Higgs singlet field, in turn providing mass to a Z ^′ state. By adopting the B − L example, whose results can be recast into those pertaining to the whole aforementioned class, and allowing for both scalar and gauge mixing, we first extract the surviving parameter space in presence of up-to-date theoretical and experimental constraints. Over the corresponding parameter configurations, we then delineate the high energy behaviour of such constructs in terms of their stability and perturbativity. Finally, we highlight key production and decay channels of the new states entering the spectra of this class of models, i.e., heavy neutrinos, a second Higgs state and the Z ^′, which are amenable to experimental investigation at the Large Hadron Collider. We therefore set the stage to establish a direct link between measurements obtainable at the Electro-Weak scale and the dynamics of the underlying model up to those where a Grand Unification Theory embedding a U(1)^′ can be realised.