An inverse seesaw model with global U(1 ) H symmetry
aa r X i v : . [ h e p - ph ] O c t APCTP Pre2019- 005, KIAS-P19010
An inverse seesaw model with global U (1) H symmetry Ujjal Kumar Dey, ∗ Takaaki Nomura, † and Hiroshi Okada
3, 4, ‡ Asia Pacific Center for Theoretical Physics (APCTP) - Headquarters San 31,Hyoja-dong, Nam-gu, Pohang 790-784, Korea School of Physics, KIAS, Seoul 02455, Republic of Korea Asia Pacific Center for Theoretical Physics, Pohang 37673, Republic of Korea Department of Physics, Pohang University of Scienceand Technology, Pohang 37673, Republic of Korea (Dated: October 15, 2019)
Abstract
We propose an inverse seesaw model based on hidden global symmetry U (1) H in which werealize tiny neutrino masses with rather natural manner taking into account relevant experimentalbounds. The small Majorana mass for inverse seesaw mechanism is induced via small vacuumexpectation value of a triplet scalar field whose Yukawa interactions with standard model fermionsare controlled by U (1) H . We discuss the phenomenology of the exotic particles present in themodel including the Goldstone boson coming from breaking of the global symmetry, and exploretestability at the Large Hadron Collider experiments. ∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected] . INTRODUCTION Inverse seesaw mechanism [1, 2] is one of the promising candidates to induce neutrinomasses and their mixing that are typically understood by an additional symmetry beyond thestandard model (SM). The mechanism requires both chiralities of neutral fermions ( N L/R )that couple to the SM fermions with different manner, and their mass structures are alsodifferent. That is why an additional symmetry is needed [3–6]. Furthermore, it demandshierarchies among mass parameters associated with neutral fermions including active neutri-nos where especially Majorana mass of N L should be tiny. Indeed, realizing such hierarchiesin a natural way is rather challenging from model building perspective [4, 5].In this paper, we propose an inverse seesaw model under a hidden global U (1) H symme-try [7] in which we try to realize natural hierarchies among neutral fermion mass matrix,taking advantage of experimental constraints of electroweak precision test to our model.In our model, we introduce SU (2) L triplet scalar and exotic lepton doublets with non-zero U (1) H charges and scalar singlets. The Yukawa interaction among the triplet and exoticlepton doublet induces small Majorana mass term required for inverse seesaw mechanismdue to small vacuum expectation value (VEV) of the triplet while Yukawa interaction be-tween the SM lepton doublet and the scalar triplet is forbidden by the U (1) H symmetry. Inaddition, as a result of the global symmetry that accommodates a physical Goldstone boson(GB) [13, 14], our model can be well-tested at future colliders since the GB couples to theSM fermions through their kinetic terms. This letter is organized as follows. In Sec. II, we introduce our model and formulatethe scalar sector, charged-lepton sector, and neutral fermion sector, and briefly discusselectroweak precision test. In the scalar sector, we show hierarchies among VEVs thatconnect to our desired hierarchies among neutral fermions. Using electroweak precisiondata, we show the constraint on the VEV of an isospin triplet scalar at tree level, andthis bound is directly related to the hierarchies of neutral fermion mass matrices. Thenwe formulate the charged-lepton mass matrix that encapsulates with the mixing betweenSM charged-leptons and newly introduced heavier ones. From this we show that their mass There are several studies utilizing hidden gauged symmetries to explain neutrino masses [8–12]. Even in the case of a gauged symmetry, GB is induced one introduces two or more two bosons that breaksthe additional U (1) symmetry; see, e.g., ref. [9]. aL e aR L ′ aL L ′ aR H ∆ ϕ ϕ SU (2) L U (1) Y − − − −
12 12 U (1) H ℓ ℓ ℓ ℓ ℓ TABLE I: Charge assignments of the our lepton and scalar fields under local SU (2) L × U (1) Y andglobal U (1) H symmetries, where the upper index a is the number of family that runs over 1-3 andall of them are singlet under SU (3) C . hierarchies could be naturally realized, while maintaining the experimental bound on thenew heavier leptons. Note that these hierarchies are also related to our neutral fermionmass matrix. In the last part of this section, we discuss the neutral fermion mass matrixand show how to realize these hierarchies. Also we discuss the bounds on non-unitarityeffect originated from the inverse seesaw model. In Sec. III we discuss the phenomenologyof our model. We mainly consider the decays of the exotic charged leptons and they canleave their footprints in the multi-lepton signatures of the collider experiments, like LHC.Finally we summarize our results and conclude. II. MODEL
In this section we formulate our model in which we introduce a global U (1) H symme-try. The fermionic sector is augmented by three families of vector-like fermions L ′ L/R ≡ [ N L/R , E
L/R ] T with charge ( , − /
2) under the SU (2) L × U (1) Y gauge symmetry, whileunder the global U (1) H right- and left-handed ones are assigned charges 2 ℓ and ℓ respec-tively. As for the scalar sector, we add an isospin triplet scalar ∆ with hypercharge 1, andtwo singlet scalars ϕ , with zero hypercharge, while (2 ℓ, ℓ, ℓ ) are respectively assigned to(∆ , ϕ , ϕ ) under the global U (1) H symmetry. The SM-like Higgs field is denoted by H .The vacuum expectation values (VEVs) of ( H, ϕ , ϕ , ∆) are ( v/ √ , v ′ / √ , v ′ / √ , v ∆ / √ −L ℓ = y ℓ aa ¯ L aL He aR + f ab ¯ L ′ aL L ′ bR ϕ ∗ + g L ab ( ¯ L ′ c ) aL ˜∆ L ′ bL + y D ab ¯ L aL L ′ bR ϕ ∗ + h . c ., (1)3here the indices a, b (= 1 −
3) represent the number of families, ˜∆ ≡ iσ ∆ † , and y ℓ isassumed to be diagonal matrix without loss of generality. After spontaneous symmetrybreaking, one finds the charged-lepton mass matrix m ℓ = y ℓ v/ √ Scalar potential and VEVs :First of all, we define each scalar field as follows: H ≡ h + v + h + ia √ , ∆ ≡ δ + √ δ ++ v ∆ + δ R + iδ I √ − δ + √ , ϕ / ≡ v ′ / + ϕ R / + iϕ I / √ . (2)The scalar potential in our model is given by, V = − µ h H † H + M Tr[∆ † ∆] − µ | ϕ | + M | ϕ | − ( µ ϕ ϕ ∗ ϕ ϕ + h.c. ) − [ λ H T ˜∆ Hϕ + h.c. ] + λ H ( H † H ) + λ ϕ | ϕ | + λ ϕ | ϕ | + λ Hϕ ( H † H ) | ϕ | + λ Hϕ ( H † H ) | ϕ | + λ ϕ ϕ | ϕ | | ϕ | + V trivial , (3)where V trivial indicates trivial quartic terms containing scalar triplet. For simplicity weassume all the couplings to be real. The quartic coupling λ plays a role in reducing thescale of VEV of ∆ to O (1) GeV. Applying the minimization condition ∂ V /∂v = ∂ V /∂v ′ , = ∂ V /∂v ∆ = 0, we obtain the VEVs approximately as v ≃ s µ h λ H , v ′ ≃ s µ λ ϕ , v ′ ≃ √ µ ϕ v ′ M + λ ϕ ϕ v ′ , v ∆ ≃ λ v v ′ M , (4)where we assume v ′ ∼ v ∆ ≪ { v ′ , v } and λ Hϕ to be small. The above hierarchy of VEVsare motivated from the mass hierarchy in neutral fermions as we discuss below. To obtainthe VEV hierarchy and electroweak vacuum consistently we need to choose appropriateparameters. When we impose v ′ & × GeV as required by constraints from masslessGoldstone boson interactions as discussed below, and taking v ′ ∼ v ∆ = O (1) GeV and M ∼ λ ϕ ϕ v ′ ∼ M ∼ v , we should choose µ ϕ . − GeV and λ ϕ ϕ . − . Furthermore λ Hϕ should be very small as O (10 − ) or less when v ′ & × GeV since p λ Hϕ v ′ . O (100)GeV in order to realize electroweak vacuum. Also one loop contribution to λ Hϕ is roughlyobtained as ∼ λ Hϕ λ ϕ ϕ / (4 π ) , and the condition λ Hϕ . − requires λ Hϕ λ ϕ ϕ . − .Notice that smallness of µ ϕ would be natural in the ’t Hooft sense since we can restorea “lepton number” symmetry for µ ϕ → { L L , e R , L ′ R , ϕ } and 0 for { L ′ L , ϕ , ∆ } .4ince we assume small λ Hϕ coupling, the mass matrix for CP-even scalars from the SMsinglets ϕ , is derived approximately12 ϕ R ϕ R T λ ϕ v ′ λ ϕ ϕ v ′ v ′ − √ µ ϕ v ′ λ ϕ ϕ v ′ v ′ − √ µ ϕ v ′ M + λ ϕ ϕ v ′ ϕ R ϕ R . (5)Note that mixing between ϕ R and ϕ R would be small since off-diagonal elements aresuppressed by v ′ and µ ϕ unless values of diagonal elements are not too close. Thus weapproximate ϕ R and ϕ R as mass eigenstates and corresponding mass eigenvalues are m ϕ R ≃ p λ ϕ v ′ , m ϕ R ≃ r M + 12 λ ϕ ϕ v ′ . (6)In CP-odd scalar sector from SM singlet ϕ , , we have one massless Goldstone bosonassociated with breaking of global U (1) H symmetry. Applying our assumption of v ′ ≪ v ′ ,we can approximately identify ϕ I as Goldstone boson while ϕ I is massive scalar bosonwith mass value m ϕ I ≃ p M + λ ϕ ϕ v ′ /
2. Thus the ϕ R and ϕ I have approximatelydegenerate masses. In our following analysis, we consider massive scalars from ϕ and ϕ have electroweak scale masses expecting collider phenomenology of these scalar bosons. Wethen impose that λ ϕ . − since v ′ should be larger than ∼ × GeV from constraints ofGoldstone boson having axion-like interaction as discussed below. In addition λ ϕ ϕ . − is required since ϕ loop correction to λ ϕ is roughly estimated to be ∼ λ ϕ ϕ / (4 π ) . Clearlywith these choices of quartic couplings one can have the massive scalars in the electroweakscale relevant for the collider phenomenology.The SM Higgs and scalar triplet sector are mostly same as the usual scalar triplet modelsince we assume the mixing among SM singlet scalar to be sufficiently small. The mass scaleof scalar bosons in the triplet is given by M ∆ . ρ parameter :The VEV of ∆ is restricted by the ρ -parameter at tree level that is given by [15] ρ ≈ v + 2 v v + 4 v , (7)where the experimental value is given by ρ = 1 . +0 . − . at 2 σ confidence level [16]. Onthe other hand, we have v SM = p v + 2 v ≈
246 GeV. Therefore the upper bound on v ∆ isof the order O (1) GeV . Theoretical origins are studied by refs. [17–19]. harged-lepton sector :The charged-lepton mass matrix consists of the component of the SM mass matrix andheavier one, after the spontaneous symmetry breaking. We define the mass matrix M to be f v ′ / √
2. Furthermore, we assume the mass matrices m D and M to be diagonal for simplicity.Then, for each generation indicated by ” a ”, we can write the charged-lepton fermion massmatrix as ¯ e aL ¯ E aL T M E a e aR E aR = ¯ e aL ¯ E aL T m ℓ a m D a M a e aR E aR , (8) M E a M † E a = m ℓ a + m D a m D a M a m D a M a M a , M † E a M E a = m ℓ a m D a m ℓ a m D a m ℓ a M a + m D a . (9)The mass matrix is diagonalized by the transformation ( e aL ( R ) , E aL ( R ) ) → V † L a ( R a ) ( e aL ( R ) , E aL ( R ) ). Thus we can obtain diagonalization matrices V L a and V R a whichrespectively diagonalize M E a M † E a and M † E a M E a as V L a M E a M † E a V † L = V R a M † E a M E a V † R a =diag( m e a , M E a ), such that V L a = cos θ L a − sin θ L a sin θ L a cos θ L a , V R a = cos θ R a − sin θ R a sin θ R a cos θ R a , (10)tan 2 θ L a = 2 m D a M a M a − m ℓ a − m D a ≃ m D a M a , tan 2 θ R a = 2 m D a m ℓ a M a + m D a − m ℓ a ≃ m D a m ℓ a M a , (11)where we have assumed m ℓ a , m D a ≪ M a . Then the mass eigenvalues for e a and E a aresimply given by m ℓ a and M a . Also their mixing angles θ R a and θ L a are very small andsatisfy θ R a ≪ θ L a , since the lower bound on the mass of the heavier leptons is about 100GeV [16] that is suggested by the current experimental data at LHC and LEP. This isbecause hierarchies in mass parameter m ℓ a , m D a ≪ M a is comparatively natural. Note alsothat the constraints from electroweak precision measurements ( S, T, U -parameters) can beavoided when the components in exotic lepton doublet have degenerate mass. In fact weconsider such a case where mass of E and N are dominantly given by Dirac mass M . Neutrino sector :After the spontaneous symmetry breaking, neutral fermion mass matrix in the basis of6 ν cL , N R , N cL ) T is given by M N = m D m D M M µ ∗ , (12)where m D , M are diagonal while µ ≡ g L v ∆ / √ × m ν ≈ µ ∗ (cid:16) m D M (cid:17) . (13)Once we fix m D /M ∼ O (0 . µ ∼ O (10 − ) GeV in order to satisfy the observed neu-trino mass squared differences. It suggests that g L ∼ O (10 − ) when we fix v ∆ ∼ O (1) GeV .The neutrino mass matrix is diagonalized by unitary matrix U MNS ; D ν = U TMNS m ν U MNS ,where D ν ≡ diag( m , m , m ). Constraint from non-unitarity can simply be obtained by con-sidering the hermitian matrix F ≡ M − m D . Combining several experimental results [24],the upper bounds are given by [25]: | F F † | ≤ . × − . × − . × − . × − . × − . × − . × − . × − . × − . (14)Since F is assumed to be diagonal, the stringent constraint originates from 2-2 componentof | F F † | . It suggests that | F | ∼ m D /M . − that is always safe in our model thanks tothe small VEV v ′ . III. PHENOMENOLOGY
In this section, we discuss the phenomenology of the model focusing on the decay andproduction of exotic particles. One specific property of our model is the existence of physicalGoldstone boson (GB) as a result of global U (1) H symmetry breaking . Here we firstderive interactions associated with the Goldstone boson which is denoted by α G identified These hierarchies could be explained by several mechanisms such as radiative models [20–22] and effectivemodels with higher order terms [23]. Physical GB would obtain tiny mass due to breaking of global U (1) symmetry by gravitational effects [26].In our analysis we simply consider it as a massless particle. ϕ = e i αGv ′ ( v ′ + ϕ R ) / √ v ′ ≪ v ′ . Then fields with U (1) H charge arerewritten as ϕ → e i αGv ′ ϕ , ∆ → e i αGv ′ ∆, L ′ aL → e i αGv ′ L ′ aL and L ′ aR → e i αGv ′ L ′ aR . Then ϕ and ϕ interact with α G as follows L ⊃ v ′ ϕ R ∂ µ α G ∂ µ α G + 12 v ′ ϕ R ϕ R ∂ µ α G ∂ µ α G + i v ′ ∂ µ α G ( ∂ µ ϕ R ϕ I − ϕ R ∂ µ ϕ I )+ 2 v ′ v ′ ϕ R ∂ µ α G ∂ µ α G + 1 v ′ ( ϕ R ϕ R + ϕ I ϕ I ) ∂ µ α G ∂ µ α G . (15)The covariant derivative of ∆ is rewritten including Goldstone boson as, D µ ∆ = ∂ µ ∆ + i v ′ ∂ µ α G ∆ − i g √ (cid:0) W + µ [ T + , ∆] + W − µ [ T − , ∆] (cid:1) − i g cos θ W Z µ (cid:16)h σ , ∆ i − sin θ W ˆ Q ∆ (cid:17) − ieA µ ˆ Q ∆ , (16)where g is SU (2) L gauge coupling, T ± = ( σ ± iσ ) / θ W is Weinberg angle, and ˆ Q is electriccharge operator acting on each component of the multiplet. Then we can obtain interactionsof α G and ∆ from kinetic term Tr[( D µ ∆) † ( D µ ∆)]. The triplet Higgs decay modes with α G are suppressed by v ∆ /v ′ factor so that components in ∆ decay via gauge or scalar potentialinteractions. Also exotic lepton L ′ a interaction with α G is derived from kinetic term as L ⊃ − ¯ L ′ a γ µ (cid:18) v ′ ∂ µ α G P L + 2 v ′ ∂ µ α G P R (cid:19) L ′ a + ¯ L ′ a γ µ (cid:18) g √ W + µ T + + W − µ T − ) + g cos θ W (cid:16) σ − sin θ W ˆ Q (cid:17) Z µ + e ˆ QA µ (cid:19) L ′ a . (17)Here we briefly discuss constraints on our physical GB. The GB can have axion-like photoncoupling since assignment of U (1) H charge for extra leptons L ′ depends on its chirality. Thenwe roughly obtain the effective interaction as [27] L α G γγ ∼ e π v ′ α G F µν ˜ F µν , (18)where F µν is photon field strength. Then cooling of stars provide a constraint for thiscoupling such that [28, 29] e π v ′ . . × − GeV − → v ′ & . × GeV . (19)We thus consider v ′ to be very high scale as 5 × GeV. In addition, experiments whichsearch for new force mediated by axion-like particles provide another constraint [30, 31] butit is less severe compared to the star cooling one. If we take large v ′ the existence of our GB8oes not cause serious problem in cosmology since it does not couple to the SM particlesvia some direct coupling except to the Higgs boson whose couplings are well suppressedby 1 /v ′ factor and controlled by the parameters in the potential which we assume to besmall. Thus the GB decouples from thermal bath in sufficiently early Universe since allhidden particles are heavier than O (100) GeV scale, and it almost does not affect numberof relativistic degrees of freedom in the current Universe. Even if the GB interacts with SMparticle via Higgs portal, cosmological constraint can be avoided when the GB decouplesfrom the thermal bath at the scale larger than muon mass scale [13]. A. Decay of exotic particles
In this subsection, we discuss the decays of exotic particles in the model. At first, wewrite the Lagrangian relevant to decay of exotic charged lepton E a as follows L ⊃ v ′ m D a M a ¯ ℓ a γ µ P L E a ∂ µ α G + f aa m D a M a ¯ ℓ a P R E a ϕ R + y D aa √ ℓ a P R E a ( ϕ R + iϕ I )+ y D aa √ m D a m ℓ a M a ¯ ℓ a P R ℓ a ( ϕ R + iϕ I ) + h.c. , (20)where we have omitted subdominant terms. The partial decay widths are computed asΓ E a − → ℓ a − ϕ R ≃ f aa πM a y D aa v ′ M a ( M a − m ϕ ) , (21)Γ E a − → ℓ a − ϕ R = Γ E a → ℓ a ϕ I ≃ y D aa πM a ( M a − m ϕ ) , (22)Γ E a − → ℓ a − α G ≃ y D aa v ′ πv ′ M a , (23)where we have used m D a = y D aa v ′ / √
2. Thus E a ± dominantly decay into ℓ a ± ϕ R ( I ) since theother modes are suppressed by small VEV v ′ . Then we also estimate partial decay widthsof ϕ R ( ϕ I ) as, Γ ϕ R → α G α G ≃ m ϕ v ′ πv ′ , (24)Γ ϕ R → ℓ a − ℓ a + = Γ ϕ I → ℓ a ¯ ℓ a ≃ y D aa π v ′ m ℓ a M a m ϕ . (25)Thus we find that ϕ R and ϕ I dominantly decays into ℓ a + ℓ a − when we take v ′ = 5 × GeV. Therefore dominant decay chains of E a − are E a − → ℓ a − ϕ I → ℓ a − ℓ b − ℓ b + and E a − → ℓ a − ϕ R → ℓ a − ℓ b − ℓ b + with branching ratio (BR) of 0 . p ® E + E -
14 TeV8 TeV
200 400 600 80010 - M E @ GeV D Σ @ pb D pp ® E - Ν pp ® E + Ν
200 400 600 80010 - - - - M E @ GeV D Σ @ pb D FIG. 1: Left: E + E − pair production cross section as a function of its mass. Right: E ± singleproduction cross section as a function of its mass where we fix m D /M = 0 . we took coupling among ϕ R and SM Higgs to be zero but ϕ R can decay SM particles throughsuch a coupling. B. Collider physics
In this subsection we discuss collider signatures of our model. The exotic charged scalarbosons from Higgs triplet can be produced by electroweak production and they dominantlydecay into gauge bosons as the triplet components have degenerate mass. This phenomenol-ogy is the same as scalar triplet model with relatively large triplet VEV case ( v ∆ ∼ { δ ± , δ ±± } decays into SM gauge bosons. Phenomenologyof such case can be found, for example, in refs. [32–36].Then we focus on exotic charged lepton production at the LHC. The exotic charged leptonpair can be produced by pp → Z/γ → E a + E a − via gauge interaction depicted in Eq.(16).Furthermore single production process can be induced through mixing between E a and SMcharged leptons. We obtain relevant interaction by L ⊃ g √ m D a M a ¯ E a γ µ P L ν a W − µ + h.c. , (26)where mixing effect in neutral current is canceled. Thus E a can be singly produced as pp → W − (+) → E a − ¯ ν a ( E a + ν a ) at the LHC. For these processes cross sections are estimated10y using CalcHEP [37] with the CTEQ6 parton distribution functions (PDFs) [38]. We showpair and single production cross sections as a function of exotic lepton mass in left and rightpanels in Fig. 1, where we fixed m D a /M a = 0 .
01 in our calculation and only the lightestexotic charged lepton E ± is considered. Then we find that single production cross sectionis much smaller than pair production one due to the suppression by mixing factor m D a /M a .The pair production cross section is larger than ∼ M a . √ s = 14 TeV. Since the exotic charged lepton decays into multi-lepton final state,the cross section can be constrained by multi-lepton search at the LHC where inclusivesearch indicates cross section producing more than three electron/muon is required to be σ · BR . M a &
500 GeV.One specific signal from E + E − pair production is multi-lepton process given by E + E − → ℓ + ℓ − ϕ I ,R ϕ I ,R → ℓ + ℓ − ℓ + ℓ − ℓ + ℓ − . Notice that we require scalar bosons ϕ I ,R to havemasses of O (100) GeV so that the decay process is dominant; if these scalar bosons aremuch heavier, the dominant decay mode of E ± is W ± ν mode through mixing betweenheavy extra lepton and the SM leptons. Here, signal events are generated by employing theevent generator MADGRAPH/MADEVENT 5 [40], where the necessary Feynman rules and relevantparameters of the model are implemented by use of
FeynRules 2.0 [41] and the
NNPDF23LO1
PDF [42] is adopted. Then the
PYTHIA 6 [43] is applied to deal with hadronization effects,the initial-state radiation (ISR) and final-state radiation (FSR) effects, and the generatedevents are also run though the
PGS 4 for detector level simulation [44]. Then we select eventswith 6 charged leptons and impose cuts for lepton transverse momentum as p T ( ℓ ) >
15 GeV.In left panel of Fig. 2, we show number of events distribution for invariant mass for e + e + e − assuming E ± and ϕ I ,R dominantly decay into mode including electron and m ϕ I ,R = 100GeV with integrated luminosity 300 fb − at the LHC 14 TeV . We see the clear peak at theexotic lepton mass and distributions around the peak as we can not always choose e + e + e − combination coming from E + decay chain. In addition we show distribution for invariantmass for e + e − in right panel of Fig. 2 which shows the peak at the mass of ϕ I ,R . The signalis clean and number background (BG) events can be suppressed requiring multiple chargedlepton final state. In fact number of BG events is negligibly small requiring 6 charged lepton Invariant mass for e − e − e + provides similar distribution.
00 GeV14 TeV300 fb - M a =
500 GeV600 GeV m j =
100 GeV M e + e + e - @ GeV D ð o f e v e n t s (cid:144) b i n
14 TeV300 fb - M a =
500 GeV m j =
100 GeV M e + e - @ GeV D ð o f e v e n t s (cid:144) b i n FIG. 2: Left:Number of events for invariant mass for e + e + e − with m ϕ ≡ m ϕ I ,R = 100 GeV andintegrated luminosity 300 fb − at the LHC 14 TeV Right: Number of events for invariant mass for e + e − . in final state where the largest cross section from SM processes is σ ( pp → ZZZ, Z → ℓ + ℓ − )and it is around 10 − fb magnitude. Thus the signal can be tested well at the future LHCexperiments if exotic lepton mass is less than 1 TeV. More detailed simulation includingsophisticated cut analysis is beyond the scope of this paper and it is left for future work. IV. SUMMARY AND CONCLUSIONS
We have constructed an inverse seesaw model based on U (1) H global symmetry in whichwe introduced exotic lepton doublets L ′ and new scalar fields including scalar triplet with U (1) H charge. The exotic lepton doublets are vector-like under gauge symmetry but chiralunder U (1) H and Majorana mass of neutral components are zero before U (1) H symmetrybreaking. Then U (1) H is spontaneously broken by VEVs of scalar fields and only neutralcomponent of L ′ L obtains Majorana mass term via triplet VEV due to appropriate U (1) H charge assignment. We thus realize neutral fermion mass matrix which has the structuresimilar to inverse seesaw mechanism. Our advantage of this model is to realize the neutrinomass matrix with natural mass parameters in the framework of inverse seesaw scenario.Therefore, the lepton number is broken by exotic fermions and its violation is restricted12y the triplet VEV, that is of the order 1 GeV, originated from the constraint of obliqueparameters. Also m D /M , which is also proportional to the neutrino mass matrix, is naturallysuppressed, when we appropriately assign the lepton number to the fields. This is because m D is proportional to µ ϕ ∼ O (1) GeV, which is the source of lepton number violation, thatis expected to be small due to the ’t Hooft sense, while M should be greater than 100 GeVwhich is required by the current experimental data such as LHC and LEP. We thus havenaturally achieved that the neutrino mass matrix be less than the order of 10 − GeV.We have also discussed the phenomenology of the model focusing on decay and pro-duction processes of the exotic particles. The decay widths of exotic charged leptons E ± are estimated taking into account interactions with Goldstone boson from U (1) H symmetrybreaking. We have found E ± will provide multi-lepton final states from decay chain. Then E ± production cross section has been estimated which is induced by electroweak interac-tions. Testable number of multi-lepton events could be obtained if mass of E ± is less than O (1) TeV scale. Acknowledgments
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