PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

Chapter 3
Two Higgs Doublet Type-III Seesaw
3.1 Introduction
In the previous chapter we have discussed the different possibilities to generate the
dimension-5 [1] operator Ô( LLHH ), which give rise to the Majorana neutrino masses
M
after the electroweak symmetry breaking. In this scheme which is the novel seesaw mechanism
[2], the neutrino masses turn out to be naturally small as they are suppressed by
the heavy mass scale M of the integrated-out heavy modes. We have also discussed the
different seesaw mechanisms which are referred in the literature as type-I, type-II [3] and
type-III [4–12]. Distinct from the type-I, the type-III seesaw has the following crucial feature.
Since the additional heavy fermions belong to the adjoint representation of SU(2),
they have gauge interactions. This makes it easier to produce them in collider experiments.
With the LHC all set to take data, it is pertinent to check the viability of testing
the seesaw models at colliders. The implications of the type-III seesaw at LHC was first
studied in [13] and [14] in the context of a SU(5) GUT model. In the SU(5) model it is
possible to naturally have the adjoint fermions in the 100 GeVto 1 TeV mass range, opening
up the possibility of observing them at LHC. The authors of these papers identified
the dilepton channel with 4 jets as the signature of the triplet fermions. Subsequently, a
lot of work has followed on testing type-III seesaw at LHC [15–17].
In the usual type-III (and also type-I) version of the seesaw model with one Higgs
doublet, the neutrino mass matrix is,
m ν = −v 2 Y T Σ
1
M Y Σ. (3.1)
where v is the vacuum expectation value of the Higgs field, M is the mass matrix of the
triplet fermions and Y Σ is the Yukawa couplings of the triplet fermions with the standard
modelleptondoubletsandHiggs. Topredictneutrinomasses∼ 0.1eVwithoutfinetuning
the Yukawas, one requires that M ∼ 10 14 GeV. On the other hand, an essential requisite
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