Can, and under which conditions, the 125-GeV SM-like scalar with the signal strengths for its decays into W + W − , ZZ, γ γ , bb and ττ being consistent with experiments be accommodated in models that go beyond the Standard Model? Is it truly what it appears to be, namely the SM Higgs boson, or could it be quite different? A minimal extension of the original electroweak-scale right-handed neutrino model, in which right-handed neutrinos naturally obtain electroweak-scale masses, shows a scalar spectrum which includes either the 125-GeV SM-like scalar or a scalar which is quite unlike that of the Standard Model, both of which possess signal strengths compatible with experiment. In other words, the 125-GeV scalar could be an impostor.

A model of electroweak-scale right-handed neutrino (EWνR ) model was constructed five years ago in which the right-handed neutrinos are members of mirror fermion weak doublets and where the Majorana masses of the right-handed neutrinos are found to be naturally of the order of the electroweak scale. These features facilitate their searches at the LHC through signals such as likesign dilepton events. This model contains, in addition to the mirror quarks and leptons, extra scalars transforming as weak triplets. In this paper, we study the constraints imposed on these additional particles by the electroweak precision parameters S, T, and U. These constraints are crucial in determining the viability of the electroweak νR model and the allowed parameter space needed for a detailed phenomenology of the model. *

Observation of non-zero neutrino masses at a scale ∼ 10 −1 − 10 −2 eV is a major problem in the otherwise highly successful Standard Model. The most elegant mechanism to explain such tiny neutrino masses is the seesaw mechanism with right handed neutrinos. However, the required seesaw scale is so high, ∼ 10 14 GeV, it will not have any collider implications. Recently, an explicit model has been constructed to realize the seesaw mechanism with the right handed neutrinos at the electroweak scale. The model has a mirror symmetry having both the left and right lepton and quark doublets and singlets for the same SU (2) W gauge symmetry. Additional Higgs multiplets have been introduced to realize this scenario. It turns out that these extra Higgs fields also help to satisfy the precision electroweak tests, and other observables. Because the scale of the symmetry breaking is electroweak, both the mirror quark and mirror leptons have masses in the electroweak scale in the range ∼ 150 − 800 GeV. The mirror quarks / leptons decay to ordinary quarks /leptons plus very light neutral scalars. In this work, we calculate the final state signals arising from the pair productions of these mirror quarks and their subsequent decays. We find that these signals are well observable over the Standard Model background for 13 TeV LHC. Depending on the associated Yukawa couplings, these decays can also give rise displaced vertices with long decay length, very different from the usual displaced vertices associated with b decays.

The existence of tiny neutrino masses at a scale more than a million times smaller than the lightest charged fermion mass, namely the electron, and their mixings can not be explained within the framework of the exceptionally successful Standard Model(SM).Several mechanisms were proposed to explain the tiny neutrino masses, most prominent among which is the so-called seesaw mechanism. Many models were built around this Additional signals will be the presence of displaced vertices depending on the smallness of the Yukawa couplings of the mirror leptons with the ordinary leptons and the singlet Higgs present in the model. Of particular importance to the EW-scale ν R model is the production of ν R which will be a direct test of the seesaw mechanism at collider energies.

In this thesis, we present the model of right-handed neutrinos at electroweak scale (EW⌫ R ) and its extended version under two crucial tests by the electroweak precision measurements and the discovery of the 125 GeV SM-like Higgs boson.The key feature of the model is the existence of non-sterile electroweak scale righthanded neutrinos together with scalar triplets. We show that the EW⌫ R model with the new particle content satisfies well the constraint of the electroweak precision measurements through the contribution to the oblique parameters, S, T, U .Moreover, the 125 GeV SM-like Higgs discovery necessitates extending the minimal EW⌫ R model by adding one more complex scalar doublet and imposing a new symmetry. We present two very distinct scenarios in which the 125 GeV Higgs boson candidate of the model behaves like and unlike the SM one. In both cases, the signal strength of the 125 GeV candidate satisfies the experimental results of the LHC. The phenomenology of the heavy spin-zero states is also taken into account.-i-