It is worth generalizing the previous discussion to the case with two new singlet fermions χ
_{1} and χ
_{2}. The Majorana mass matrix for these two fermions has three parameters, and the dimensionfive Higgs portal operators form a matrix
(1) 
After electroweak symmetry breaking, the BSM fermions form two mass eigenstates χ
_{1} and χ
_{2}, with mass
m_{2} >
m_{1}. If we take relatively light fermions
m_{h} > 2
m_{2}, the decays
h→ χ
_{2}χ
_{2},
h→χ
_{1}χ
_{2} and
h→χ
_{1}χ
_{1} are all possible. This kind of interaction appears in, for instance, the NMSSM, where χ
_{2} and χ
_{1} are mostly bino and singlinolike, respectively, and the higherdimension Higgs portal coupling of Eq. (
1) results after integrating out the charged Higgsinos.
Let us first consider the case where there is a
Z_{2} symmetry which takes χ
_{i}→ −χ
_{i}. In this case, χ
_{1} is stable, but the heavier new state decays as χ
_{2} → χ
_{1}+
X. If the Higgs portal coupling of Eq. (
1) is the only coupling of the χ
_{i}, then the decay will proceed through an offshell Higgs, χ
_{2}→
h^{*}χ
_{1}→
ff (
gg, γγ) χ
_{1}. In this case, branching fractions into different SM partons will be determined by the Higgs couplings, and will typically result in Higgs decays to
E/
_{T} plus one or two nonresonant quarkantiquark, leptonantilepton, or gluon pairs, depending on the available phase space.
If the χ
_{i} have additional interactions besides their coupling to the Higgs, such as a dipole coupling to the hypercharge field strength,
(2) 
or a coupling to the
Z boson induced by mixing with states transforming under
SU(2)
_{L},

(3) 
then other decay patterns are possible. The dipole operator allows the decays χ
_{2}→γχ
_{1}, as well as χ
_{2}→χ
_{1} Z if
m_{2}−
m_{1} >
m_{Z} (phase space suppression renders decays through an offshell
Z largely irrelevant when
m_{2}−
m_{1} <
m_{Z}). The operator of Eq. (
3) also yields χ
_{2}→ χ
_{1} Z when phase space allows, or if
m_{2}−
m_{1} <
m_{Z}, will mediate the threebody decays χ
_{2}→
ff χ
_{1} with branching ratios set by the
Z branching fractions.
Note that a common feature of all these decays is that the pairs of SM partons have a kinematic endpoint at
m_{ff, gg,γγ} <
m_{2}−
m_{1}, and that the transverse mass of the visible partons and the
E/
_{T} is bounded from above.
The
Z boson coupling can arise in NMSSMlike models, see e.g. NMSSMS, or in models with additional RH neutrinos [
1,
2] that mix with the SM neutrinos. In the latter case, the couplings
h_{ij} in (
3) are usually small and therefore the neutrino decay lengths are macroscopic. In the former case, the couplings can instead be larger, and the Majorana fermions can have a prompt decay into SM fermions. Additional examples are models with a fourth generation of fermions where the two fourth generation neutrinos do not mix with the SM neutrinos [
3,
4,
5]. In these models, the mass range
M_{1} >~30 GeV,
M_{2}−
M_{1} <~20 GeV is allowed by LEP measurements of the
Z width and LEP bounds on
e^{+}e^{−}→ χ
_{1} χ
_{2}, χ
_{2} χ
_{2} [
3]. In this region of parameter space,
h→ χ
_{2} χ
_{2}, as well as
h→χ
_{1} χ
_{1}, can have a sizable branching ratio [
4]. Furthermore, the heavier neutrino χ
_{2} can decay promptly via χ
_{2}→
Z^{*} χ
_{1}, while the lighter neutrino χ
_{1} is longlived.
If the
Z_{2} parity is violated, allowing χ
_{1} to decay, Higgs decays to as many as ten partons may result. We will not consider such complex decays in this work, but one should bear in mind that they can occur.
Many models with new fermion species also contain new bosonic degrees of freedom, which, if light, open new possibilities for the decays of the χ
_{i}. We will see examples of this in
NMSSM+F.
References

[1]M. L. Graesser,
Experimental Constraints on Higgs Boson Decays to TeVscale RightHanded Neutrinos, [
arXiv:0705.2190].

[2]M. L. Graesser,
Broadening the Higgs boson with righthanded neutrinos and a higher dimension operator at the electroweak scale,
Phys.Rev. D76 (2007) 075006, [
arXiv:0704.0438].

[3]L. M. Carpenter,
Fourth Generation Lepton Sectors with Stable Majorana Neutrinos: From LEP to LHC, [
arXiv:1010.5502].

[4]W.Y. Keung and P. Schwaller,
Long Lived Fourth Generation and the Higgs,
JHEP 1106 (2011) 054, [
arXiv:1103.3765].
[5]L. M. Carpenter and D. Whiteson,
Higgs Decays to Unstable Neutrinos: Collider Constraints from Inclusive LikeSign Dilepton Searches, [
arXiv:1107.2123].
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