PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

Γ 2HDM (Σ 0 m → ν m H 0 ) ≃ Y Σ 2 sin2 αM Σ
(1− M2 H
2
64π M )2 , (3.77)
Σ
Γ 2HDM (Σ ± m → l mH ± 0 ) ≃ Y Σ 2 sin2 αM Σ
(1− M2 H
2
32π M )2 , (3.78)
Σ
where the first two expressions are for decays to h 0 or A 0 and the last two for decays to
H 0 . Again, for the same value of M Σ ∼ 100 GeV in both models, one requires λ ∼ 10 −5 -
10 −6 for the one Higgs doublet model in order to produce m ν ∼ 0.1 eV, while Y Σ ∼ 1 for
our two Higgs doublet model. Therefore, clearly
Γ 2HDM (Σ 0 m → ν m h 0 /A 0 ) ∼ 10 11 ×Γ 1HDM (Σ 0 m → ν m H 0 ),
Γ 2HDM (Σ ± m → l± m h0 /A 0 ) ∼ 10 11 ×Γ 1HDM (Σ ± m → l± m H0 ).
Hence, the the exotic fermions decay about 10 11 times faster in our model compared to
the one Higgs doublet model. This could lead to observational consequences at LHC.
In particular, authors of [15] talk about using “displaced vertices” as a signature of the
type-III seesaw mechanism. In our model the lifetime of the exotic fermions is a factor
of 10 11 shorter and so will be the gap between their primary production vertex and the
decay vertex. This model therefore predicts no displaced vertex for the heavy fermion
decays. The other decay modes such as Σ 0 m → ν mH 0 and Σ ± m → l± m H0 are suppressed
by the sin 2 α ∼ 10 −12 factor and hence turn out to be comparable to the decay rates in
the one Higgs doublet model. As a result, the branching ratio to this mode is negligible
and can be neglected. In our model, decay of triplet fermions into h 0 , A 0 and H ± are
predominant. We discuss the decay modes of the different Higgs fields h 0 ,H 0 , A 0 and H ±
in section 3.6. Among the different Higgs fields, the h 0 decay predominantly into b¯b pairs,
but with a very long lifetime, as we will discuss in section 3.6.
3.5.5 Flavor Structure and the Decay Branching Ratios
In this section we present the branching fractions of the heavy fermion decays. Table 3.2
shows the branching fractions for the Σ ± m , while Table 3.3 gives the branching fraction
for Σ 0 m decays. For the channels with neutrino in the final state, we give the sum of the
branching fraction into all the three generations, as observationally it will be impossible
to see the neutrino generations at LHC. We do not show decays to gauge bosons and
H 0 as they are suppressed by a factor of 10 11 with respect to the decays into h 0 , A 0 and
H ± . As a result of the inherent µ-τ symmetry in the model, Σ ±/0
m 3 decays to electrons is
strictly forbidden and branching ratios of their decay into µ m and τ m leptons are equal.
We find that due to the form of U 22 , S 22 and T 22 given in Eqs. (3.49), (3.50) and (3.51),
the probability of Σ m ±/0
2 to decay into µ m and τ m leptons is equal. We also find that the
branching fractions of Σ ± m 2
is almost equal to the branching fractions of Σ ± m 3
, and similarly
for the neutral heavy fermions. The difference between the branching fraction to h 0 , A 0
60