The possibility of additional quarks and leptons beyond the three generations already established is discussed. The make-up of this Report is (I) Introduction

If neutrino masses are realized through the see-saw mechanism, can the right-handed neutrinos be produced and detected at present and future colliders? The answer is negative in the most popular see-saw scenarios for the simple reason that they are too heavy in these models. However, a simple extension of the Standard Model (SM) particle content, including mirror fermions, two SU (2)L triplet and one singlet Higgs fields, leads to a scenario in which the see-saw mechanism is realized with the Majorana mass MR of the right-handed neutrino being of the order of the electroweak scale or smaller. A custodial SU (2) symmetry arising from the two triplet Higgs fields ensures that ρ = 1 at tree level even when their vacuum expectation values (VEV) which determine the value of MR, can be as large as the electroweak scale. MR is found to obey the bound

No abstract

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.