arXiv:1003.1468v2 [hep-ph] 27 Oct 2010
Neutrino Masses and a TeV Scale Seesaw Mechanism
Center for High Energy Physics, Peking university, Beijing 100871, China
A simple extension of the Standard Model providing TeV scale seesaw mechanism is presented.
Beside the Standard Model particles and right-handed Majorana neutrinos, the model contains a
singly charged scalar, an extra Higgs doublet and three vector like singly charged fermions. In our
model, Dirac neutrino mass matrix raises only at the loop level. Small but non-zero Majorana
neutrino masses come from integrating out heavy Majorana neutrinos, which can be at the TeV
scale. The phenomenologies of the model are investigated, including scalar mass spectrum, neutrino
masses and mixings, lepton flavor violations, heavy neutrino magnetic moments as well as possible
collider signatures of the model at the LHC.
∗Electronic address: email@example.com
The observation of neutrino oscillations [1–4] has revealed that neutrinos have small
but non-zero masses and lepton flavors are mixed, which can not be accommodated in the
Standard Model (SM) without introducing extra ingredients. As such, neutrino physics
offer an exciting window into new physics beyond the SM. Perhaps the most attractive
approach towards understanding the origin of small neutrino masses is using the dimension-
five weinberg operator :
Lbεbaφa+ h.c. , (1)
which comes from integrating out new superheavy particles.
A simple way to obtain the operator in Eq. (1) is through the Type-I seesaw mechanism
, in which three right-handed neutrinos with large Majorana masses are introduced to
the SM. Then three active neutrinos may acquire tiny Majorana masses through the Type-
I seesaw formula, i.e., the mass matrix of light neutrinos is given by Mν= −MDM−1
where MDis the Dirac mass matrix linking left-handed light neutrinos to right-handed heavy
neutrinos and MRis the mass matrix of heavy Majorana neutrinos. Actually, there are three
tree-level seesaw scenarios (namely type-I, Type-II  and Type-III  seesaw mechanisms)
and one loop-level seesaw scenario (namely Ma  model), which may lead to the effective
operator in Eq. (1).
Although seesaw mechanisms can work naturally to generate Majorana neutrino masses,
they lose direct testability on the experimental side. A direct test of seesaw mechanism would
involve the detection of these heavy seesaw particles at a collider and the measurement of
their Yukawa couplings with the electroweak doublets. In the canonical seesaw mechanism,
heavy seesaw particles turn out to be too heavy, i.e., 1014∼16GeV, to be experimentally
accessible. One straightforward way out is to lower the seesaw scale “by hand” down to the
TeV scale, an energy frontier to be explored by the Large Hadron Collider (LHC). However
this requires the structural cancellation between the Yukawa coupling texture and the heavy
Majorana mass matrix, i.e. MDM−1
D≈ 0 [10–15] at the tree level, and is thus unnatural!
To solve this unnaturalness problem, we propose a novel TeV-scale seesaw mechanism in
this paper. The model includes, in addition to the SM fields and right-handed Majorana
neutrinos, a charged scalar singlet, an extra Higgs doublet and three vector like singly
charged fermions. Due to Z2discrete flavor symmetry, right-handed Majorana neutrinos
don’t couple to left-handed lepton doublets, such that Dirac mass matrix only raises at the
loop level and is comparable with the charged lepton mass matrix. This drives down the
seesaw scale to the TeV, and thus the model is detectable at the LHC.
The paper is organized as follows: In section II, we describe our model. Section III is
devoted to investigate the phenomenologies of the model, including neutrino masses and
mixings, lepton flavor violations, transition magnetic moments of heavy Majorana neutrinos
as well as possible collider signatures. We conclude in Section IV. An alternative settings to
the model is presented in appendix A.
II. THE MODEL
In our model, we extend the SM by introducing three right-handed Majorana neutrinos
NR, three singly charged vector-like fermion SL,SR, an extra Higgs doublet Hn, a singly
charged scalar Φ as well as discrete Z2flavor symmetry. The Z2charges for these fields is
given in table I. Due to Z2symmetry, right-handed neutrinos don’t couple to SM Higgs.
TABLE I: Z2charges of particles.
fields ℓLeRNRSLSRH HnΦ
+1 +1 -1 -1 +1 +1 +1 +1
As a result the new lagrangian can be written as
LN= V (H,Hn,Φ) − ℓLYSHSR− SLMSSR− YNSLΦNR−1
RMRNR+ h.c. ,(2)
where YSand YNare new Yukawa couplings, MSand MRare mass matrices of S and NR,
respectively. Z2symmetry is explicitly broken by SLMSSRterm. It can be recovered by
adding an extra scalar singlet η, with Z2charge −1 and Yukawa coupling SLηSR. We will
not consider Yukawa couplings ℓLHneRand ℓLHnSR, which can be forbidden by another Z
symmetry. The following is the full Higgs potential:
V = −m2
1H†H − m2
3Φ†Φ + λ1(H†H)2+ λ2(H†
nHn) +?λnΦ(HTiσ2Hn) + h.c.?
+λ6(Φ†Φ)(H†H) + λ7(Φ†Φ)(H†
We define ?H? = v1/√2 and ?Hn? = v2/√2.
minimum, one finds that
After imposing the conditions of global
where λo= 1/2(λ3+ λ4).
In the basis (h−,h−
n,S−), we can derive the mass matrix for charged scalars:
2. MCcan be diagonalized by the 3×3 unitary transformation
V : V†MCV∗= diag(MG+,MH+,MS+). The mass eigenvalues for these charged scalars are
where ρ ≡ −2m2
MG+ = 0 ;
where B ≡ ρ − λ4(v2
matrix for CP-even scalars in the basis (h,hn)Tand CP-odd scalars in the basis (G,Gn)T:
2). Here G+is the SM goldstone boson. We also derive the mass
where λs= 1/2(λ4− λ5).
We also derive the masses for gauge bosons, which are M2
W = g2(v2
2)/4cos2θw, separately. Such that electroweak precision observable ρ ≡
Zcos2θw= 1 in our theory. Our scalar field sector is similar to that in Zee modelM2
. We present in appendix A a different setting to the particle contents, by replacing
scalar singlet with triplet.
In this section, we devote to investigate some phenomenological implications of our model.
We focus on (A) neutrino masses and mixings; (B)lepton flavor violations; (C) electromag-
netic properties and (D) collider signatures of heavy majorana neutrinos, which will be
deployed in the following:
A.Neutrino masses and lepton mixing martrix
In our model, there is no Dirac neutrino mass term at the tree level. However we can
derive a small Dirac neutrino mass matrix at the loop level. The relevant feynman diagram
is shown in Fig. 1.
FIG. 1: One-loop correction to the Dirac neutrino mass matrix.
A direct calculation results in
where the one loop function appearing in upper equation is given by
with βi= m2/m2
i. When m1= m2= m, F reduces to 1/2m2.
is the Dirac neutrino mass matrix linking the left and right hand neutrinos,Here Mloop
which only raises at the loop level in our model. If neutrinos are Dirac particles, then
Eq. (8) is just neutrino mass formula, whose predication must be consistent with present
neutrino oscillation data. In this paper, we assume that neutrinos are Majorana particles,
i.e., left-handed and right-handed neutrinos have different mass eigenvalues. Then three
active neutrino masses can be generated from seesaw mechanism. In this case, we can write
down the 6 × 6 neutrino mass matrix:
which can be diagonalized by the unitary transformation U†MU∗=?
M; or explicitly,
≪MR, the light Majorana neutrino mass formula is then Mν
T. Notice that Dirac neutrino mass matrix is suppressed by loop fac-
tor, we assume Mloop
∼ O(MeV), which will not cause any fine-tune problem. Then, to
generate electron-volt scale active neutrino masses, heavy Majorana neutrinos would be of
the order of several hundred GeV.
We also obtain the charged lepton mass matrix in the basis (EL,SL)T,
where ME= v/√2YEand MC= v/√2YS.
According to Eqs. (11) and (12), we may derive the lepton mixing matrix (MNS), which
comes from the mismatch between the diagonalizations of the neutrino mass matrix and
charged lepton mass matrix, i.e., U = V†
U ≈ (1 −1
As a result, the effective charged and neutral current interactions for charged leptons can
be written as
√2eαγµPLUαiνiWµ+ h.c. ;
2+ sin2θw)PL+ δαβsin2θwPR
The MNS matrix in Eq. (13) is non-unitary, which is mainly because the large mixing
between charged leptons and heavy vector like fermions. To a better degree of accuracy,
we have UU†≈ 1 − |MCM−1
precision electroweak data (e.g., on the invisible width of the Z0boson, universality tests
S|2. A global analysis of current neutrino oscillation data and
and rare decays) has yield quite strong constraints on the unitarity of U. Translating the
numerical results of Refs. [17–20] into the restriction on |MCM−1
at the 90% confidence level. In addition, interactions in Eqs. (14) and (15) will lead to tree
S|2, we have
< 1.6 · 10−2
< 1.0 · 10−2
< 1.0 · 10−2
< 1.1 · 10−2
< 7.0 · 10−5
< 1.6 · 10−2
< 7.0 · 10−5
< 1.0 · 10−2
< 1.0 · 10−2
level lepton flavor violations (as can be seen in Eq. (15)) and “ zero distance” effects  in
neutrino oscillations, which can be verified in the future long baseline neutrino oscillation
B. Lepton flavor violations (LFV)
Notice that the emergence of big unitary violation of MNS matrix can lead to observable
LFV effects. In this subsection, we investigate constraints on parameter space from LFV
In our model, ℓi→ 3ℓjmay occur at the tree level, just like the case in type III seesaw
mechanism. The branching ratios for the µ → 3e can be given by
?|UU†|ee(−1 + 2sin2θw)2+ 4sin4θw
Here Ω is the final states phase space integration
where zd= 4z0, zu= (1−√z0)2, z0= m2
Radiative decays, i.e., ℓi→ ℓj+γ occur at one-loop level. The branching ratios for these
processes can be written as
√zαzβ(1 − zh) − 2zβ(1 − zh) + 2√zαzβ
(1 − zh)2
where zα= m2
BR(µ → e−e+e−) = |UU†|2
−2z2+ (1 + 3z0)z − 4z0(1 + z0) +2z0(1 − z0)2
2(1,z0,z)dz , (18)
µand λ(x,y,z) = x2+y2+z2−2xy−2xz−2yz.
BR(ℓβ→ ℓαγ) =
BR(ℓβ→ νβℓα¯ να) ,(19)
−zβ(1 − zh) −√zαzβ
(1 − zh)3
Si; zβ= m2
Si; zh= M2
In Fig. 2 (a), we plot BR(µ → 3e) as function of |UU†|2
current experimental constraints. Our result shows that, to meet the experimental data,
eµ. The horizon line stands for
like fermion, we plot, in Fig. 2 (b), BR(µ → eγ)/|YSY†
Mh= 120 GeV. We find that, to get big Yukawa coupling YS, MSmust lie around 270 GeV
eµmust lie below 8.5 × 10−6. Assuming that there is only one generation vector
eµas function of MSby setting
or be heavier than several TeV.
C.Electromagnetic properties of heavy Majorana neutrinos
The electromagnetic properties of Majorana neutrinos show up, in a quantum field the-
ory, as its interaction with the photon, and is described by the following effective interaction
BR(µ → 3e)
BR(µ → eγ)/|(YSY†
FIG. 2: Branching ratios for (a) µ → 3e and (b) µ → eγ.
vertex: Leff=¯ψΓµψAµ. The most general matrix element of Leffbetween two one-particle
states, i.e., ?p′,s′|Jµ(0)|p,s? = ¯ us′(p′)Γµus(p), which is consistent with the Lorentz invari-
ance, can be written as
¯ u(p2,s2)Γµu(p1,s1) = ¯ u(p2,s2)?E(q2)γµ− M(q2)iσµνqν+ H(q2)qµ?u(p1,s1)
+¯ u(p2,s2)?G(q2)γµγ5− T (q2)iσµνqνγ5+ S(q2)qµγ5?u(p1,s1)
where q = p2− p1. 2MM(0) and 2MT (0) correspond to the magnetic moment and electro
dipole moment of heavy neutrinos, respectively.
Due to the Majorana nature, the magnetic moment of heavy Majorana neutrinos is zero.
There is only transition magnetic moment for them. In the model considered, we have four
diagrams contributing to the transition magnetic moment, which are depicted in Fig. 3.
The Yukawa interactions of heavy Majorana neutrinos with Φ and S can be rewritten in the
through which we can derive relevant feynman rules. Assuming that heavy Majorana neu-
trinos are nearly degenerate, i.e., Mα≈ Mβ≈ M, we derive the transition magnetic moment
for heavy Majorana neutrinos
i) − I(M2
x(1 − x)2
(1 − x)A + x(x − 1)B + xC,
FIG. 3: Feynman diagrams contributing to heavy Majorana neutrino transition magnetic moment.
where Miand MΦare the mass eigenvalues of heavy vector-like fermion S and scalar Φ,
Now, we turn to some numerical analysis.As shown in Eq. (2), YNSLΦNRis to-
tally the interaction of new fields beyond the SM, so that there is no experimental con-
√4π (to satisfy the perturbation theory). We plot, in Fig.
straint on YNexcept O(YN) <
iα]| as function of MN. Assuming O([(Y†
iα]) ∼ 1, We can find that the transition magnetic moment of heavy Majorana
neutrinos can be of O(10−2) for special parameter settings. Our result in Eq. (23) is different
from that in Ref.  for not considering the Yukawa coupling NC
RΦSR, which is forbidden
by the Z2symmetry in our model. Given the large electromagnetic form factors, heavy
Majorana neutrinos can be produced at the LHC through the electromagnetic interaction.
We switch to comment on the collider signatures of our model. All of the new particles
introduced in the model lie around several hundred GeV. Singly charged scalar and vector
like fermions can be produced through the electromagnetic interaction at the LHC. The
most promising production channel may be pp → S+S−→ ℓ+ℓ−jjjj for heavy charged
FIG. 4: Numerical illustration for the transition magnetic moment of heavy Majorana neutrinos
as function of MN. We assume there is only one generation vector-like fermion S and choose
(MΦ, Mi) = (350, 150) (GeV) for solid line, (300, 200) (GeV) for dashed line and (300, 250) (GeV)
for dotted line.
fermions and pp → Φ+Φ−→ jjjj for heavy charged scalar. The production cross sections
for these charged particles at the LHC (with√s = 14TeV) are about several fb when heavy
particle masses lie around 300 GeV [21, 22]. The large transition magnetic moment can help
to produce the heavy Majorana neutrinos at the LHC. Its signatures are similar to that in
Type-III seesaw model [23–25]. The only distinguish is that, heavy neutrinos can not be
produced through weak interactions and must be produced in pair in our model.
IV. CONCLUDING REMARKS
In this paper, we have proposed a novel TeV-scale seesaw mechanism. One salient feature
of our model is that Dirac neutrino mass matrix raises only at the loop level. As a result,
the heavy Majorana neutrinos can be several hundred GeV. Another salient feature is that
heavy Majorana neutrinos can get large electromagnetic form factors, through which they
can be produced and detected at the LHC. We have derived light Majorana neutrino mass
formula and calculated constraints on parameter space from LFV processes. At last we
have discussed the signatures of heavy fermions (vector-like fermions and heavy Majorana
neutrinos) and scalar at the LHC.
The author thanks to Tong Li, Yi Liao and Shu Luo for useful discussion. This work was
supported in part by the National Natural Science Foundation of China.
Appendix A: An alternative setting to the model
Beside the model presented in section II, we can extending the SM with different particle
contents, which may lead to the same TeV seesaw mechanism. For example, we can sub-
stitute SL,SR, Φ, with vector like fermion triplets ΣL,ΣRand scalar triplet ∆. In this case
the lagrangian can be written as
L = LSM− ℓLYΨ˜HΣR− ΣLMΣΣR− YNTr[ΨL∆]NR+ V (H,∆) .(A1)
Here the weak hypercharge of the Ψ and Σ are zero. ΨL, Φ and NRare odd , while the
other fields are even under Z2transformation.
The Higgs potential can be written as
4λ(H†H)2+ λXH†Σiσ2H + ··· , (A2)
where dots denote Higgs potential terms we don’t concern.
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