arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 343 records · Page 19Linked to original sources

arXiv2003

Structure of Cosmological CP Violation via Neutrino Seesaw

The cosmological matter-antimatter asymmetry can originate from CP-violating interactions of seesaw Majorana neutrinos via leptogenesis in the thermal phase of the early universe. Having the cosmological CP-phase for leptogenesis requires at least two right-handed Majorana neutrinos. Using only the low energy neutrino observables we quantitatively reconstruct a minimal neutrino seesaw. We establish a general criterion for minimal seesaw schemes in which the cosmological CP-phase is {\it completely} reconstructed from the low energy CP-phases measured by neutrino oscillation and neutrinoless double-beta decay experiments. We reveal and analyze two distinct classes of such minimal schemes that are shown to be highly predictive. Extension of our reconstruction formalism to a three-heavy-neutrino seesaw is discussed.

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arXiv2003

Neutrino Bilarge Mixing and Flavor Physics in the Flipped SU(5) Model

We have constructed a specific supersymmetric flipped SU(5) GUT model in which bilarge neutrino mixing is incorporated. Because the up-type and down-type quarks in the model are flipped in the representations ten and five with respect to the usual SU(5), the radiatively generated flavor mixing in squark mass matrices due to the large neutrino mixing has a pattern different from those in the conventional SU(5) and SO(10) supersymmetric GUTs. This leads to phenomenological consequences quite different from SU(5) or SO(10) supersymmetric GUT models. That is, it has almost no impact on B physics. On the contrary, the model has effects in top and charm physics as well as lepton physics. In particular, it gives promising prediction on the mass difference, $ΔM_D$, of the $D-{\bar D}$ mixing which for some ranges of the parameter space with large $\tanβ$ can be at the order of $10^9 ~\hbar ~s^{-1}$, one order of magnitude smaller than the experimental upper bound. In some regions of the parameter space $ΔM_D$ can saturate the present bound. For these ranges of parameter space, $t \to u,c+h^0$ can reach $10^{-5}-10^{-6}$ which would be observed at the LHC and future $γ-γ$ colliders.

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arXiv2003

Quasi-Supersymmetric $G^3$ Unification from Intersecting D6-Branes on Type IIA Orientifolds

We construct three quasi-supersymmetric $G^3$ GUT models with $S_3$ symmetry and gauge coupling unification from intersecting D6-branes on Type IIA orientifolds. The Standard Model fermions and Higgs doublets can be embedded into the bifundamental representations in these models, and there is no any other unnecessary massless representation. Especially in Model I with gauge group $U(4)^3$, we just have three-family SM fermions and three pairs of Higgs particles. The $G^3$ gauge symmetry in these models can be broken down to the Standard Model gauge symmetry by introducing light open string states. And 1 TeV scale supersymmetry breaking soft masses imply the reasonable intermediate string scale.

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arXiv2003

Deconstruction of Gauge Symmetry Breaking by Discrete Symmetry and $G^N$ Unification

We deconstruct the non-supersymmetric SU(5) breaking by discrete symmetry on the space-time $M^4\times S^1$ and $M^4\times S^1/(Z_2\times Z_2')$ in the Higgs mechanism deconstruction scenario. And we explain the subtle point on how to exactly match the continuum results with the latticized results on the quotient space $S^1/Z_2$ and $S^1/(Z_2\times Z_2')$. We also propose an effective deconstruction scenario and discuss the gauge symmetry breaking by the discrete symmetry on theory space in this approach. As an application, we suggest the $G^N$ unification where $G^N$ is broken down to $SU(3)\times SU(2)\times U(1)^{n-3}$ by the bifundamental link fields and the doublet-triplet splitting can be achieved.

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arXiv2003

Low Energy 6-Dimensional N=2 Supersymmertric SU(6) Models on $T^2$ Orbifolds

We propose low energy 6-dimensional N=2 supersymmetric SU(6) models on $M^4\times T^2/(Z_2)^3$ and $M^4\times T^2/(Z_2)^4$, where the orbifold $SU(3)_C\times SU(3)$ model can be embedded on the boundary 4-brane. For the zero modes, the 6-dimensional N=2 supersymmetry and the SU(6) gauge symmetry are broken down to the 4-dimensional N=1 supersymmetry and the $SU(3)_C\times SU(2)_L\times U(1)_Y\times U(1)'$ gauge symmetry by orbifold projections. In order to cancel the anomalies involving at least one $U(1)'$, we add extra exotic particles. We also study the anomaly free conditions and present some anomaly free models. The gauge coupling unification can be achieved at $100\sim 200$ TeV if the compactification scale for the fifth dimension is $3\sim 4$ TeV. The proton decay problem can be avoided by putting the quarks and leptons/neutrinos on different 3-branes. And we discuss how to break the $SU(3)_C\times SU(2)_L\times U(1)_Y\times U(1)'$ gauge symmetry, solve the $μ$ problem, and generate the $Z-Z'$ mass hierarchy naturally by using the geometry. The masses of exotic particles can be at the order of 1 TeV after the gauge symmetry breaking. We also forbid the dimension-5 operators for the neutrino masses by $U(1)'$ gauge symmetry, and the realistic left-handed neutrino masses can be obtained via non-renormalizable terms.

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arXiv2003

Flavor hierarchy in SO(10) grand unified theories via 5-dimensional wave-function localization

A mechanism to generate fermion-mass hierarchy in SO(10) Grand Unified Theories is considered. We find that the lopsided family structure, which is suitable to the Large angle MSW solution to the solar neutrino oscillation, is realized without introducing extra matter fields if the hierarchy originates from the wave-function profile in an extra dimension. Unlike the Froggatt-Nielsen mechanism, the SO(10) breaking effect may directly contribute to the source of the hierarchy, i.e., the bulk mass terms. It naturally explains the difference of the hierarchical patterns between the quark and the lepton sectors. We also find the possibility of the horizontal unification, in which three generations of the matter fields are unified to a 3 dimensional representation of an SU(2) gauge group.

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arXiv2002

Weak Mixing Angle and the SU(3)_C times SU(3) Model on M^4 times S^1/(Z_2 times Z_2')

We show that the desirable weak mixing angle sin^2 theta_W = 0.2312 at m_Z scale can be generated naturally in the SU(3)_C times SU(3) model on M^4 times S^1 / (Z_2 times Z_2') where the gauge symmetry SU(3) is broken down to SU(2)_L times U(1)_Y by orbifold projection. For a supersymmetric model with a TeV scale extra dimension, the SU(3) unification scale is about hundreds of TeVs at which the gauge couplings for SU(3)_C and SU(3) can also be equal in the mean time. For the non-supersymmetric model, SU(2)_L times U(1)_Y are unified at order of 10 TeV. These models may serve as good candidates for physics beyond the SM or MSSM.

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arXiv2002

Low Energy Gauge Unification Theory

Because of the problems arising from the fermion unification in the traditional Grand Unified Theory and the mass hierarchy between the 4-dimensional Planck scale and weak scale, we suggest the low energy gauge unification theory with low high-dimensional Planck scale. We discuss the non-supersymmetric SU(5) model on $M^4\times S^1/Z_2 \times S^1/Z_2$ and the supersymmetric SU(5) model on $M^4\times S^1/(Z_2\times Z_2') \times S^1/(Z_2\times Z_2')$. The SU(5) gauge symmetry is broken by the orbifold projection for the zero modes, and the gauge unification is accelerated due to the SU(5) asymmetric light KK states. In our models, we forbid the proton decay, still keep the charge quantization, and automatically solve the fermion mass problem. We also comment on the anomaly cancellation and other possible scenarios for low energy gauge unification.

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arXiv2002

The Z-Z' Mass Hierarchy in a Supersymmetric Model with a Secluded U(1)'-Breaking Sector

We consider the Z'/Z mass hierarchy in a supersymmetric model in which the U(1)' is broken in a secluded sector coupled to the ordinary sector only by gauge and possibly soft terms. A large mass hierarchy can be achieved while maintaining the normal sparticle spectra if there is a direction in which the tree level potential becomes flat when a particular Yukawa coupling vanishes. We describe the conditions needed for the desired breaking pattern, to avoid unwanted global symmetries, and for an acceptable effective mu parameter. The electroweak breaking is dominated by A terms rather than scalar masses, leading to tan beta ~ 1. The spectrum of the symmetry breaking sector is displayed. There is significant mixing between the MSSM particles and new standard model singlets, for both the Higgs scalars and the neutralinos. A larger Yukawa coupling for the effective mu parameter is allowed than in the NMSSM because of the U(1)' contribution to the running from a high scale. The upper bound on the tree-level mass of the lightest CP even Higgs doublet mass is about c x 174 GeV, where c is of order unity, but the actual mass eigenvalues are generally smaller because of singlet mixing.

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arXiv2002

N=2 6-dimensional Supersymmetric $E_6$ Breaking

We study the $N=2$ supersymmetric $E_6$ models on the 6-dimensional space-time where the supersymmetry and gauge symmetry can be broken by the discrete symmetry. On the space-time $M^4\times S^1/(Z_2\times Z_2') \times S^1/(Z_2\times Z_2')$, for the zero modes, we obtain the 4-dimensional $N=1$ supersymmetric models with gauge groups $SU(3)\times SU(2) \times SU(2) \times U(1)^2$, $SU(4)\times SU(2) \times SU(2) \times U(1)$, and $SU(3)\times SU(2) \times U(1)^3$ with one extra pair of Higgs doublets from the vector multiplet. In addition, considering that the extra space manifold is the annulus $A^2$ and disc $D^2$, we list all the constraints on constructing the 4-dimensional $N=1$ supersymmetric $SU(3)\times SU(2) \times U(1)^3$ models for the zero modes, and give the simplest model with $Z_9$ symmetry. We also comment on the extra gauge symmetry breaking and its generalization.

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arXiv2002

Local Discrete Symmetry in the Brane Neighborhood

With the ansatz that there exist local or global discrete symmetries in the special branes' neighborhoods, we can construct the extra dimension models with only zero modes, or the models which have large extra dimensions and arbitrarily heavy KK modes because there is no simple relation between the mass scales of extra dimensions and the masses of KK states. In addition, the bulk gauge symmetry and supersymmetry can be broken on the special branes for all the modes, and in the bulk for the zero modes by local and global discrete symmetries. To be explicit, we discuss the supersymmetric SU(5) model on $M^4\times S^1/Z_2$ in which there is a local $Z_2'$ symmetry in the special 3-brane neighborhood along the fifth dimension.

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arXiv2001

Gauge Symmmetry and Supersymmetry Breaking by Discrete Symmetry

We study the principles of the gauge symmetry and supersymmetry breaking due to the local or global discrete symmetries on the extra space manifold. We show that the gauge symmetry breaking by Wilson line is the special case of the discrete symmetry approach where all the discrete symmetries are global and act freely on the extra space manifold. As applications, we discuss the N=2 supersymmetric SO(10) and $E_8$ breaking on the space-time $M^4\times A^2$ and $M^4\times D^2$, and point out that similarly one can study any N=2 supersymmetric $SO(M)$ breaking. We also comment on the one-loop effective potential, the possible questions and generalization.

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arXiv2001

N=2 Supersymmetric GUT Breaking on $T^2$ Orbifolds

We study the 6-dimensional N=2 supersymmetric grand unified theories with gauge group SU(N) and $ SO(M)$ on the extra space orbifolds $T^2/(Z_2)^3$ and $T^2/(Z_2)^4$, which can be broken down to the 4-dimensional N=1 supersymmetric $SU(3)\times SU(2)\times U(1)^{n-3}$ model for the zero modes. We also study the models which have two SU(2) Higgs doublets (zero modes) from the 6-dimensional vector multiplet. We give the particle spectra, present the fields, the number of 4-dimensional supersymmetry and gauge group on the observable 3-brane or 4-brane and discuss some phenomenology for those models. Furthermore, we generalize our procedure for $(4+m)$-dimensional $N$ supersymmetric GUT breaking on the space-time $M^4\times T^m/(Z_2)^L$.

hep-ph↗

arXiv2001

Discrete Symmetry and GUT Breaking

We study the supersymmetric GUT models where the supersymmetry and GUT gauge symmetry can be broken by the discrete symmetry. First, with the ansatz that there exist discrete symmetries in the branes' neighborhoods, we discuss the general reflection $Z_2$ symmetries and GUT breaking on $M^4\times M^1$ and $M^4\times M^1\times M^1$. In those models, the extra dimensions can be large and the KK states can be set arbitrarily heavy. Second, considering the extra space manifold is the annulus $A^2$ or disc $D^2$, we can define any $Z_n$ symmetry and break any 6-dimensional N=2 supersymmetric SU(M) models down to the 4-dimensional N=1 supersymmetric $SU(3)\times SU(2)\times U(1)^{M-4}$ models for the zero modes. In particular, there might exist the interesting scenario on $M^4\times A^2$ where just a few KK states are light, while the others are relatively heavy. Third, we discuss the complete global discrete symmetries on $M^4\times T^2$ and study the GUT breaking.

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arXiv2001

GUT Breaking on $M^4 \times T^2/(Z_2 \times Z_2^{\prime})$

We construct two SU(5) models on the space-time $M^4 \times T^2/(Z_2 \times Z_2^{\prime})$ where the gauge and Higgs fields are in the bulk and the Standard Model fermions are on the brane at the fixed point or line. For the zero modes, the SU(5) gauge symmetry is broken down to $SU(3) \times SU(2) \times U(1) $ due to non-trivil orbifold projection. In particular, if we put the Standard Model fermions on the 3-brane at the fixed point in Model II, we only have the zero modes and KK modes of the Standard Model gauge fields and two Higgs doublets on the observable 3-brane. So, we can have the low energy unification, and solve the triplet-doublet splitting problem, the gauge hierarchy problem, and the proton decay problem.

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arXiv2001

Self-Tuning and de Sitter Brane Intersections in the 6-Dimensional Brane Models

We study the self-tuning of general brane junctions and brane networks on the 6-dimensional space-time. For the general brane junctions, there may exist one fine-tuning among the brane tensions. For the brane networks, similar to the 5-dimensional self-tuning brane models, the brane tensions can be set arbitrarily and there exists the singularity for the metric and bulk scalar. And if we want to regularize the singularity, we will introduce the fine-tuning among the brane tensions. In addition, because the 4-dimensional cosmological constant we observe may be positive and very small, we discuss the brane network with de Sitter brane intersections by introducing a bulk scalar.

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arXiv2001

Spontaneous Physical Compactification of the Extra Dimensions

We conjecture that the extra dimensions are physical non-compact at high energy scale or high temperature; after the symmetry breaking or cosmological phase transition, the bulk cosmological constant may become negative, and then, the extra dimensions may become physical compact at low energy scale. We show this in a five-dimensional toy brane model with three parallel 3-branes and a real bulk scalar whose potential is temperature dependent. We also point out that after the global or gauge symmetry breaking, or the supersymmetry breaking in supergravity theory, the spontaneous physical compactification of the extra dimensions might be realized.

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arXiv2001

Time-Like Extra Dimension and Cosmological Constant in Brane Models

We discuss the general models with one time-like extra dimension and parallel 3-branes on the space-time $M^4 \times M^1$. We also construct the general brane models or networks with $n$ space-like and $m$ time-like extra dimensions and with constant bulk cosmological constant on the space-time $M^4\times (M^1)^{n+m}$, and point out that there exist two kinds of models with zero bulk cosmological constant: for static solutions, we have to introduce time-like and space-like extra dimensions, and for non-static solutions, we can obtain the models with only space-like extra dimension(s). In addition, we give two simplest models explicitly, and comment on the 4-dimensional effective cosmological constant.

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