arXiv subjects

Mu-Chun Chen

Publications and source records attributed to Mu-Chun Chen.

At least 55 records · Page 3Linked to original sources

arXiv2012

Dirac Leptogenesis with a Non-anomalous $U(1)^{\prime}$ Family Symmetry

We propose a model for Dirac leptogenesis based on a non-anomalous $U(1)^{\prime}$ gauged family symmetry. The anomaly cancellation conditions are satisfied with no new chiral fermions other than the three right-handed neutrinos, giving rise to stringent constraints among the charges. Realistic masses and mixing angles are obtained for all fermions. The model predicts neutrinos of the Dirac type with naturally suppressed masses. Dirac leptogenesis is achieved through the decay of the flavon fields. The cascade decays of the vector-like heavy fermions in the Froggatt-Nielsen mechanism play a crucial role in the separation of the primodial lepton numbers. We find that a large region of parameter space of the model gives rise to a sufficient cosmological baryon number asymmetry through Dirac leptogenesis.

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arXiv2012

The mu term and neutrino masses

The well-known Giudice-Masiero mechanism explains the presence of a mu term of the order of the gravitino mass, but does not explain why the holomorphic mass term is absent in the superpotential. We discuss anomaly-free discrete symmetries which are both compatible with SU(5) unification of matter and the Giudice-Masiero mechanism, i.e. forbid the mu term in the superpotential while allowing the necessary Kaehler potential term. We find that these are Z_M^R symmetries with the following properties: (i) M is a multiple of four; (ii) the Higgs bilinear H_u H_d transforms trivially; (iii) the superspace coordinate theta has charge M/4 and, accordingly, the superpotential has charge M/2; (iv) dimension five proton decay operators are automatically absent. All Z_M^R symmetries are anomaly-free due to a non-trivial transformation of a Green-Schwarz axion, and, as a consequence, a holomorphic mu term appears at the non-perturbative level. There is a unique symmetry that is consistent with the Weinberg operator while there is a class of Z_M^R symmetries which explain suppressed Dirac neutrino masses.

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arXiv2011

Lifting Slepton Masses with a Non-universal, Non-anomalous U(1)'_{NAF} in Anomaly Mediated SUSY breaking

We extend the Minimum Supersymmetry Standard Model by a non-anomalous family (NAF) U(1)'_{NAF} gauge symmetry. All gauge anomalies are cancelled with no additional exotics other than the three right-handed neutrinos. The FI D-terms associated with the U(1)'_{NAF} symmetry lead to additional positive contributions to slepton squared masses. In a RG invariant way, this thus solves the tachyonic slepton mass problem in Anomaly Mediated Supersymmetry Breaking. In addition, the U (1)'_{NAF} symmetry naturally gives rise to the fermion mass hierarchy and mixing angles, and determines the mass spectrum of the sparticles.

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arXiv2011

Predictions for Neutrino Masses, $ββ_{0ν}$-Decay and Lepton Flavor Violation in a SUSY $SU(5) \times T^{\prime}$ Model of Flavour

We obtain predictions for the neutrino masses, the effective Majorana mass in neutrinoless double beta decay and for the rates of the lepton flavor violating processes $μ\rightarrow e + γ$, $τ\rightarrow e + γ$ and $ τ\rightarrow μ+ γ$ in a SUSY $SU(5) \times T^{\prime}$ Model of flavour, which gives rise to realistic masses and mixing patterns for quarks and leptons.

hep-ph↗

arXiv2011

Leptogenesis in a SUSY SU(5) x T' Model with Geometrical CP Violation

The complex Clebsch-Gordon coefficients in the double tetrahedral group T' offers the possibility that CP violation can be entirely geometrical in origin, as pointed out by us recently. In this note, we investigate leptogenesis in a SUSY SU(5) x T' model, which gives rise to realistic quark masses and CKM matrix elements, a near tri-bimaximal neutrino mixing pattern, as well as CP violating measures for all quarks and leptons. In terms of 9 independent parameters in the Yukawa sector, the predicted values of the 22 observables agree with current experimental values, including the recent T2K and MINOS results. The correction to the tri-bimaximal mixing pattern is related to the Cabibbo angle, leading to interesting quark-lepton complementarity sum rules. Our predicted value for the leptonic Dirac CP phase is 227 degrees, which gives rise to sufficient amount of lepton number asymmetry, in the presence of the flavor effect. As this is the only non-vanishing CP violating phase in the lepton sector, there is a direct connection between leptogenesis and CP violation in neutrino oscillation in our model.

hep-ph↗

arXiv2011

TeV Scale Models of Neutrino Masses and Their Phenomenology

We review various TeV scale models for neutrino masses utilizing different types of seesaw mechanisms, higher dimensional operators, expanded gauge symmetries, or low scale extra dimensions. In addition, we discuss the implications of these models for the collider experiments and low energy lepton flavor violation searches.

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arXiv2010

Models and Phenomenology of Neutrino Masses circa 2010

We review recent developments in models of neutrino masses and mixing. Emphases are given to models based on finite group family symmetries from which the tri-bimaximal neutrino mixing can arise. In particular, we describe one recent model based on SUSY SU(5) combined with a family symmetry based on the double tetrahedral group, T'. All 22 observable fermion masses and mixing angles and CP violating measures are fitted with only 9 parameters. In this model, a near tri-bimaximal MNS matrix and a realistic CKM matrix are simultaneously generated; the MNS matrix gets slightly modified by virtue of having the Georgi-Jarlskog relations. Due to the presence of complex Clebsch-Gordan coefficients in T', CP violation in this model is entirely geometrical in origin. The prediction of the model for the leptonic Dirac CP phase is 227 degrees, which turns out to be very close to the current best fit value of 220 degrees from SuperK.

hep-ph↗

arXiv2010

Geometrical Origin of CP Violation and CKM and MNS Matrices in SU(5) x T'

We propose the complex group theoretical Clebsch-Gordon coefficients as a novel origin of CP violation. This is manifest in our model based on SUSY SU(5) combined with the double tetrahedral group, T', as a family symmetry. Due to the presence of the doublet representations in T', there exist complex CG coefficients, leading to explicit CP violation in the model, while the Yukawa couplings and the vacuum expectation values of the scalar fields remain real. The tri-bimaximal neutrino mixing matrix arises from the CG coefficients of T'. In addition to the prediction for theta_{13} ~ (1/ (3\sqrt{2}) theta_{c}, the model gives rise to a sum rule, tan^{2} theta_{sol} ~ tan^{2} theta_{sol, TBM}} + (1/2) theta_{c} cos(delta_{lep}), which is a consequence of the Georgi-Jarlskog relations in the charged fermion sector. The predicted leptonic Dirac CP phase, delta_{lep}, gives the correct value of the solar mixing angle, and the predicted CP violation measures in the quark sector are consistent with the current experimental data. Since the Dirac CP phase is the only non-vanishing phase predicted in the lepton sector, there is a connection between leptogenesis and low energy leptonic CP violating processes in our model.

hep-ph↗

arXiv2010

Non-universal, Non-anomalous U(1)' in a Model with Anomaly Mediated SUSY Breaking

We construct a Minimum Supersymmetry Standard Model expanded by a non-anomalous family (NAF) U(1)'_{NAF} gauge symmetry. All gauge anomalies are cancelled with no additional exotics other than the three right-handed neutrinos. The FI D-terms associated with the U(1)'_{NAF} symmetry lead to additional positive contributions to slepton squared masses. In a RG invariant way, this thus solves the tachyonic slepton mass problem in Anomaly Mediated Supersymmetry Breaking. In addition, the U(1)'_{NAF} symmetry naturally gives rise to the fermion mass hierarchy and mixing angles, and determines the mass spectrum of the sparticles. Our model also provides a counter example to the previous claim that the only U(1)' that can give rise to realistic fermion mass hierarchy and mixing pattern must be anomalous.

hep-ph↗

arXiv2010

Low Scale Non-universal, Non-anomalous U(1)'_F in a Minimal Supersymmetric Standard Model

We propose a non-universal U(1)'_F symmetry combined with the Minimal Supersymmetric Standard Model. All anomaly cancellation conditions are satisfied without exotic fields other than three right-handed neutrinos. Because our model allows all three generations of chiral superfields to have different U(1)'_F charges, upon the breaking of the U(1)'_F symmetry at a low scale, realistic masses and mixing angles in both the quark and lepton sectors are obtained. In our model, neutrinos are predicted to be Dirac fermions and their mass ordering is of the inverted hierarchy type. The U(1)'_F charges of the chiral super-fields also naturally suppress the mu term and automatically forbid baryon number and lepton number violating operators. While all flavor-changing neutral current constraints in the down quark and charged lepton sectors can be satisfied, we find that constraint from D0-D0bar turns out to be much more stringent than the constraints from the precision electroweak data.

hep-ph↗

arXiv2010

TeV Scale Seesaw and a flavorful Z' at the LHC

Small neutrino masses and their large mixing angles can be generated at the TeV scale by augmenting the Standard Model with an additional generation dependent, anomaly-free U(1)_{nu} symmetry, in the presence of three right-handed neutrinos. The Z' gauge boson associated with the breaking of the U(1)_{nu} symmetry can be produced at the LHC. The flavorful nature of the Z' can be established by measuring its non-universal couplings to the charged leptons as determined by the lepton's U(1)_{nu} charges, which also govern the neutrino flavor structure. While the LHC has the potential of discovering the Z' up to M_{Z'} = 4.5 TeV with 100 fb^(-1) data at the center of mass energy sqrt{s} = 14 TeV, to establish the flavorful nature of the Z' requires much higher integrated luminosity. For our bench mark parameters that are consistent with neutrino oscillation data, at sqrt{s} = 14 TeV, a 5 sigma distinction between the dielectron and dimuon channels for M_{Z'} = 3 TeV requires 500 fb^(-1) of data. We find that the forward backward asymmetry distributions can also be useful in distinguishing the dielectron and dimuon channels in the low invariant mass and transverse momentum regions.

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arXiv2010

A Viable Randall-Sundrum Model for Quarks and Leptons with T' Family Symmetry

We propose a Randall-Sundrum model with a bulk family symmetry based on the double tetrahedral group, T', which generates the tri-bimaximal neutrino mixing pattern and a realistic CKM matrix, including CP violation. Unlike 4D models where the generation of mass hierarchy requires additional symmetry, the warped geometry naturally gives rise to the fermion mass hierarchy through wavefunction localization. The T' symmetry forbids tree-level flavor changing neutral currents in both the quark and lepton sectors, as different generations of fermions are unified into multiplets of T'. This results in a low first KK mass scale and thus the model can be tested at collider experiments.

hep-ph↗

arXiv2009

Neutrinos versus Quarks, MNS versus CKM

We review recent developments in models of fermion masses and mixing for both quark and lepton sectors. Emphases are given to models based on finite group family symmetries. In particular, we describe one recent model based on SU(5) combined with a family symmetry based on the double tetrahedral group, T'. In this model, a near tri-bimaximal MNS matrix and a realistic CKM matrix are simultaneously generated; the MNS matrix gets slightly modified by virtue of having the Georgi-Jarlskog relations. Due to the presence of complex Clebsch-Gordon coefficients in T', CP violation in this model is entirely geometrical in origin.

hep-ph↗

arXiv2009

Group Theoretical Origin of CP Violation

We propose the complex group theoretical Clebsch-Gordon coefficients as a novel origin of CP violation. This is manifest in our model based on SU(5) combined with the T' group as the family symmetry. The complex CG coefficients in T' lead to explicit CP violation which is thus geometrical in origin. The predicted CP violation measures in the quark sector are consistent with the current experimental data. The corrections due to leptonic Dirac CP violating phase gives the experimental best fit value for the solar mixing angle, and we also gets the right amount of the baryonic asymmetry.

hep-ph↗

arXiv2009

A Viable Flavor Model for Quarks and Leptons in RS with T' Family Symmetry

We propose a Randall-Sundrum model with a bulk family symmetry based on the double tetrahedral group, T', which generates the tri-bimaximal neutrino mixing pattern and a realistic CKM matrix. The T' symmetry forbids tree-level flavor-changing-neutral-currents in both the quark and lepton sectors, as different generations of fermions are unified into multiplets of T'. This results in a low first KK mass scale and thus the model can be tested at collider experiments.

hep-ph↗

arXiv2009

A Novel Origin of CP Violation

We propose the complex group theoretical Clebsch-Gordon coefficients as a novel origin of CP violation. This is manifest in our model based on SU(5) combined with the double tetrahedral group, T'. Due to the presence of the doublet representations in $T^{\prime}$, there exist complex CG coefficients, leading to explicit CP violation in the model, while the Yukawa couplings and the vacuum expectation of the scalar fields remain real. The tri-bimaximal neutrino mixing matrix arises from the CG coefficients of the T'. In addition to the prediction for theta_{13} ~ theta_{c}/(3 sqrt{2}), the model gives rise to a sum rule, tan^2 theta_{sol} ~ \tan^2 theta_{sol,TBM} + (1/2) theta_{c} cos(delta), which is a consequence of the Georgi-Jarlskog relations in the charged fermion sector. The leptonic Dirac CP violating phase, delta_{ell}, is predicted to be ~227^{o}, which turns out to be the value needed to account for the difference between the experimental best fit value for the solar mixing angle and the TBM prediction. The predicted CP violation measures in the quark sector are also consistent with the current experimental data.

hep-ph↗

arXiv2009

A4 See-Saw Models and Form Dominance

We introduce the idea of Form Dominance in the (type I) see-saw mechanism, according to which a particular right-handed neutrino mass eigenstate is associated with a particular physical neutrino mass eigenstate, leading to a form diagonalizable effective neutrino mass matrix. Form Dominance, which allows an arbitrary neutrino mass spectrum, may be regarded as a generalization of Constrained Sequential Dominance which only allows strongly hierarchical neutrino masses. We consider alternative implementations of the see-saw mechanism in minimal A4 see-saw models and show that such models satisfy Form Dominance, leading to neutrino mass sum rules which predict closely spaced neutrino masses with a normal or inverted neutrino mass ordering. To avoid the partial cancellations inherent in such models we propose Natural Form Dominance, in which a different flavon is associated with each physical neutrino mass eigenstate.

hep-ph↗

arXiv2009

Minimal Flavor Violation in the Lepton Sector of the Randall-Sundrum Model

We propose a realization of Minimal Flavor Violation in the lepton sector of the Randall-Sundrum model. With the MFV assumption, the only source of flavor violation are the 5D Yukawa couplings, and the usual two independent sources of flavor violation are related. In the limit of massless neutrinos, the bulk mass matrices and 5D Yukawa matrices are simultaneously diagonalized, and hence the absence of FCNCs. In the case of massive neutrinos, the contributions to FCNCs in the charged lepton sector are highly suppressed, due to the smallness of neutrino masses. In addition, the MFV assumption also allows suppressing one-loop charged current contributions to flavor changing processes by reducing the size of the Yukawa couplings, which is not possible in the generic anarchical case. We found that the first KK mass scale as low as ~ 3 TeV can be allowed. In both cases, we present a set of numerical results that give rise to realistic lepton masses and mixing angles. Mild hierarchy in the 5D Yukawa matrix of O(25) in our numerical example is required to be consistent with two large and one small mixing angles. This tuning could be improved by having a more thorough search of the parameter space.

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