arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 289 records · Page 16Linked to original sources

arXiv2010

Light Dark Matter from the U(1)_X Sector in the NMSSM with Gauge Mediation

Cosmic ray anomalies observed by PAMELA and Fermi-LAT experiments may be interpreted by heavy (TeV-scale) dark matter annihilation enhanced by Sommerfeld effects mediated by a very light (sub-GeV) U(1)_X gauge boson, while the recent direct searches from CoGeNT and DAMA/LIBRA experiments may indicate a rather light (\sim 7 GeV) dark matter with weak interaction. Motivated by these apparently different scales, we consider a gauge mediated next-to-the minimal supersymmetric standard model (NMSSM) entended with a light U(1)_X sector plus a heavy sector (\bar H_h,H_h), which can provide both a light (\sim 7 GeV) and a heavy (TeV-scale) dark matter without introducing any ad hoc new scale. Through the Yukawa coupling between H_h and the messager fields, the U(1)_X gauge symmetry is broken around the GeV scale radiatively and a large negative m_S^2 is generated for the NMSSM singlet S. Furthermore, the small kinetic mixing parameter between U(1)_X and U(1)_Y is predicted to be θ\sim 10^{-5}-10^{-6} after integrating out the messengers. Such a light dark matter, which can have a normal relic density from the late decay of the right-handed sneutrino (assumed to be the ordinary next-to-the lightest supersymmetric particle and thermally produced in the early universe), can serve a good candidate to explain the recent CoGeNT and DAMA/LIBRA results.

hep-ph↗

arXiv2010

The Supersymmetric Standard Models with Decay and Stable Dark Matters

We propose two supersymmetric Standard Models (SMs) with decaying and stable dark matter (DM) particles. To explain the SM fermion masses and mixings and have a heavy decay DM particle S, we consider the Froggatt-Nielsen mechanism by introducing an anomalous U(1)_X gauge symmetry. Around the string scale, the U(1)_X gauge symmetry is broken down to a Z_2 symmetry under which S is odd while all the SM particles are even. S obtains a vacuum expectation value around the TeV scale, and then it can three-body decay dominantly to the second/third family of the SM leptons in Model I and to the first family of the SM leptons in Model II. Choosing a benchmark point in the constrained minimal supersymmetric SM with exact R parity, we show that the lightest neutralino DM is consistent with the CDMS II experiment. Considering S three-body decay and choosing suitable parameters, we show that the PAMELA and Fermi-LAT experiments and the PAMELA and ATIC experiments can be explained in Model I and Model II, respectively.

hep-ph↗

arXiv2010

Decaying Dark Matter in the Supersymmetric Standard Model with Freeze-in and Seesaw mechanims

Inspired by the decaying dark matter (DM) which can explain cosmic ray anomalies naturally, we consider the supersymmetric Standard Model with three right-handed neutrinos (RHNs) and R-parity, and introduce a TeV-scale DM sector with two fields ϕ_{1,2} and a $Z_3$ discrete symmetry. The DM sector only interacts with the RHNs via a very heavy field exchange and then we can explain the cosmic ray anomalies. With the second right-handed neutrino N_2 dominant seesaw mechanism at the low scale around 10^4 GeV, we show that ϕ_{1,2} can obtain the vacuum expectation values around the TeV scale, and then the lightest state from ϕ_{1,2} is the decay DM with lifetime around \sim 10^{26}s. In particular, the DM very long lifetime is related to the tiny neutrino masses, and the dominant DM decay channels to μand τare related to the approximate μ-τsymmetry. Furthermore, the correct DM relic density can be obtained via the freeze-in mechanism, the small-scale problem for power spectrum can be solved due to the decays of the R-parity odd meta-stable states in the DM sector, and the baryon asymmetry can be generated via the soft leptogensis.

hep-ph↗

arXiv2010

Quaternion Octonion Reformulation of Quantum Chromodynamics

We have made an attempt to develop the quaternionic formulation of Yang - Mill's field equations and octonion reformulation of quantum chromo dynamics (QCD). Starting with the Lagrangian density, we have discussed the field equations of SU(2) and SU(3) gauge fields for both cases of global and local gauge symmetries. It has been shown that the three quaternion units explain the structure of Yang- Mill's field while the seven octonion units provide the consistent structure of SU(3)_{C} gauge symmetry of quantum chromo dynamics.

physics.gen-ph↗

arXiv2010

SU(5) and SO(10) Models from F-Theory with Natural Yukawa Couplings

We construct the SU(5) and SO(10) models from F-theory. Turning on the U(1) fluxes, we can break the SU(5) gauge symmetry down to the Standard Model (SM) gauge symmetry, and break the SO(10) gauge symmetry down to the SU(3)_C X SU(2)_L X SU(2)_R X U(1)_{B-L} gauge symmetry. In particular, all the SM fermion Yukawa couplings preserve the enhanced U(1)_a X U(1)_b gauge or global symmetries at the triple intersections of the SM fermion and Higgs curves. And the SM fermion masses and mixings can be generated in the presence of background fluxes. In our models, the doublet-triplet splitting problem can be solved naturally. The additional vector-like particles can obtain heavy masses via the instanton effects or Higgs mechanism and then decouple at the high scale. The SM gauge couplings at the string scale, which are splitted due to the U(1) flux effects, can be explained by considering heavy threshold corrections from the extra vector-like particles. Moreover, in the SU(5) model, we have the Yukawa coupling unification for the bottom quark and tau lepton. In the SO(10) models, we have the Yukawa coupling unification for the top and bottom quarks, and the Yukawa coupling unification for the tau lepton and tau neutrino.

hep-th↗

arXiv2010

Flipped SU(5) X U(1)_X Models from F-Theory

We systematically construct flipped SU(5) X U(1)_X models without and with bulk vector-like particles from F-theory. To realize the decoupling scenario, we introduce sets of vector-like particles in complete SU(5) X U(1) multiplets at the TeV scale, or at the intermediate scale, or at the TeV scale and high scale. To avoid the Landau pole problem for the gauge couplings, we can only introduce five sets of vector-like particles around the TeV scale. These vector-like particles can couple to the Standard Model singlet fields, and obtain suitable masses by Higgs mechanism. We study gauge coupling unification in detail. We show that the U(1)_X flux contributions to the gauge couplings preserve the SU(5) X U(1)_X gauge coupling unification. We calculate the SU(3)_C X SU(2)_L unification scales, and the SU(5) X U(1)_X unification scales and unified couplings. In most of our models, the high-scale or bulk vector-like particles can be considered as string-scale threshold corrections since their masses are close to the string scale. Futhermore, we discuss the phenomenological consequences of our models. In particular, in the models with TeV-scale vector-like particles, the vector-like particles can be observed at the Large Hadron collider, the proton decay is within the reach of the future Hyper-Kamiokande experiment, the lightest CP-even Higgs boson mass can be increased, the hybrid inflation can be naturally realized, and the correct cosmic primodial density fluctuations can be generated.

hep-th↗

arXiv2009

A Modified Holographic Dark Energy Model with Infrared Infinite Extra Dimension(s)

We propose a modified holographic dark energy (MHDE) model with the Hubble scale as the infrared (IR) cutoff. Introducing the infinite extra dimension(s) at very large distance scale, we consider the black hole mass in higher dimensions as the ultraviolet cutoff. Thus, we can probe the effects of the IR infinite extra dimension(s). As a concrete example, we consider the Dvali-Gabadadze-Porrati (DGP) model and its generalization. We find that the DGP model is dual to the MHDE model in five dimensions, and the $Λ$CDM model is dual to the MHDE model in six dimensions. Fitting the MHDE model to the observational data, we obtain that $Ω_{m0}=0.269^{+0.030}_{-0.027}$, $Ω_{k0}=0.003^{+0.011}_{-0.012}$, and the number of the spatial dimensions is $N=4.78^{+0.68}_{-0.44}$. The best fit value of $N$ implies that there might exist two IR infinite extra dimensions.

hep-th↗

arXiv2009

Electroweak Baryogenesis, CDM and Anomaly-free Supersymmetric U(1)-prime Models

We construct two anomaly-free supersymmetric $U(1)'$ models with a secluded $U(1)'$-breaking sector. For the one with $E_6$ embedding we show that there exists a strong enough first order electroweak phase transition for electroweak baryogenesis (EWBG) because of the large soft trilinear terms in the Higgs potential. Unlike the Minimal Supersymmetric Standard Model (MSSM), the stop masses can be very heavy. We then discuss possible CP violation in the Higgs sector, which can be both spontaneous and explicit, even at tree level. The spontaneous violation provides a direct source for baryogenesis, while its magnitude is mediated by an explicit phase from the secluded sector. These new CP sources do not introduce significant new contributions to electric dipole moments. EWBG in the thin wall ($τ$ leptons) and thick wall regimes (top squarks, charginos and top quarks) are systematically discussed. We find that the CP-violating stop and chargino currents are very different from those obtained in the MSSM. Due to the space-dependence of the relevant CP phases, they do not require a variation of $\tan β$ in the bubble wall to have a non-trivial structure at the lowest order of Higgs insertion. In addition to $τ$ leptons, top squarks and charginos, we find that top quarks can also play a significant role. Numerical results show that the baryon asymmetry is large enough to explain the cosmological observation today. We illustrate that EWBG and neutralino cold dark matter can be accommodated in the same framework, i.e., there exists parameter space where a strong enough first order EWPT, large CP phase variations across the bubble wall, a reasonable baryon asymmetry as well as an acceptable neutralino dark matter relic density can be achieved simultaneously.

hep-ph↗

arXiv2009

Discrete Symmetries and Generalized Fields of Dyons

We have studied the different symmetric properties of the generalized Maxwell's - Dirac equation along with their quantum properties. Applying the parity (\mathcal{P}), time reversal (\mathcal{T}), charge conjugation (\mathcal{C}) and their combined effect like parity time reversal (\mathcal{PT}), charge conjugation and parity (\mathcal{CP}) and \mathcal{CP}T transformations to varius equations of generalized fields of dyons, it is shown that the corresponding dynamical quantities and equations of dyons are invariant under these discrete symmetries. Abstract Key words- parity, time reversal, charge-conjugation, dyons Abstract PACS No.- 14.80 Hv.

hep-th↗

arXiv2009

Realistic Flipped SU(5) from Orbifold SO(10)

We propose a realistic flipped SU(5) model derived from a five-dimensional orbifold SO(10) model. The Standard Model (SM) fermion masses and mixings are explained by combining the traditional Froggatt-Nielsen mechanism with the five-dimensional wave function profiles of the SM fermions. Employing tree-level spontaneous R-symmetry breaking in the hidden sector and extra(ordinary) gauge mediation, we obtain realistic supersymmetry breaking soft mass terms with non-vanishing gaugino masses. Including the messenger fields at the intermediate scale and Kaluza-Klein states at the compactification scale, we study gauge coupling unification. We show that the SO(10) unified gauge coupling is very strong and the unification scale can be much higher than the compactification scale. We briefly discuss proton decay as well.

hep-ph↗

arXiv2009

Low-Scale SU(4)_W Unification

We embed the minimal left-right model SU(2)_L*SU(2)_R* U(1)_{B-L} into an SU(4)_W gauge group, and break the unified group via five-dimensional S^1/(Z_2*Z_2) orbifolding. Leptons are fitted into SU(4)_W multiplets and located on a symmetry preserving O brane, while quarks are placed onto an O' brane where the symmetry is broken. This approach predicts \sin^2θ_W=0.25 for the weak mixing angle at tree level and leads to a rather low weakly (strongly) coupled unification scale of order 3*10^2 TeV (several TeV) with supersymmetry, or as low as several TeV in the non-supersymmetric case. Another symmetry breaking chain with the low-energy gauge group SU(2)_L*U(1)_{3R}*U(1)_{B-L} can also give rise to a weak mixing angle \sin^2θ_W=0.25 at tree level after gauge symmetry breaking by orbifolding. Such theories with low-scale unification have interesting phenomenological consequences.

hep-ph↗

arXiv2009

The SU(3)_C * SU(4)_W * U(1)_{B-L} Models with Left-Right Unification

The left-right models provide an explanation for the parity asymmetry in the Standard Model (SM). To further understand the origin of the left-right symmetry, we study a partial unification model based on SU(4)_W*U(1)_{B-L} which can be broken down to the minimal left-right model either through the Higgs mechanism in four dimensions or through the five-dimensional orbifold gauge symmetry breaking with inner or outer automorphism. We scrutinize all these breaking mechanisms and find that for the orbifold breaking in five dimensions, the rank-reducing outer automorphism is better than the inner automorphism and can make the low energy theory free of the U(1)_Z anomaly. For the four-dimensional model with the Higgs mechanism, we study in detail both its structure and its typical phenomenology. It turns out that this four-dimensional scenario may predict some new phenomenology since the new mirror fermions (which are introduced in order to fill the SM fermions into SU(4)_W without anomaly) are preserved at low energy scale and mix with the SM fermions. We also examine the gauge coupling unification in each case, and discuss the possibility for unifying this partial unification group with the Pati-Salam group SU(4)_{PS} to realize a grand unification.

hep-ph↗

arXiv2009

Time Delays of Strings in D-particle Backgrounds and Vacuum Refractive Indices

Using standard techniques in string/D-brane scattering amplitude computations, we evaluate the scattering of open strings off D-particles in brane world scenarios. The D-particles are viewed as D3 branes wrapped up around three cycles, and their embedding in brane worlds constitutes a case of intersecting branes, among which strings are stretched, representing various types of excitations of the Standard Model (SM) particles in the low-energy limit. Our analysis, reveals interesting and novel selection rules for the resulting causal time delays, proportional to the energy of the incident matter state, from the processes of splitting, capture and re-emission of the latter by the D-particles. In particular, we show that there are relatively large time delays only for excitations that belong to the Cartan subalgebra of the SM gauge group, which notably includes photons. We discuss the possible relevance of these results to the models of space time foam predicting a non-trivial vacuum refractive index for photons and the associated cosmic γ-ray phenomenology. In particular, we demonstrate how low-string-scale models can be falsified already in this context, by current astrophysical observations of cosmic photons, and how such observations serve as a way to discriminate low- from high-string scale models.

hep-th↗

arXiv2009

ATIC and PAMELA Results on Cosmic e^\pm Excesses and Neutrino Masses

Recently the ATIC and PAMELA collaborations released their results which show the abundant e^\pm excess in cosmic rays well above the background, but not for the \bar{p}. Their data if interpreted as the dark matter particles' annihilation imply that the new physics with the dark matter is closely related to the lepton sector. In this paper we study the possible connection of the new physics responsible for the cosmic e^\pm excesses to the neutrino mass generation. We consider a class of models and do the detailed numerical calculations. We find that these models can natually account for the ATIC and PAMELA e^\pm and \bar{p} data and at the same time generate the small neutrino masses.

hep-ph↗

arXiv2009

Non-Thermal Production of WIMPs, Cosmic $e^\pm$ Excesses and $γ$-rays from the Galactic Center

In this paper we propose a dark matter model and study aspects of its phenomenology. Our model is based on a new dark matter sector with a U(1)' gauge symmetry plus a discrete symmetry added to the Standard Model of particle physics. The new fields of the dark matter sector have no hadronic charges and couple only to leptons. Our model can not only give rise to the observed neutrino mass hierarchy, but can also generate the baryon number asymmetry via non-thermal leptogenesis. The breaking of the new U(1)' symmetry produces cosmic strings. The dark matter particles are produced non-thermally from cosmic string loop decay which allows one to obtain sufficiently large annihilation cross sections to explain the observed cosmic ray positron and electron fluxes recently measured by the PAMELA, ATIC, PPB-BETS, Fermi-LAT, and HESS experiments while maintaining the required overall dark matter energy density. The high velocity of the dark matter particles from cosmic string loop decay leads to a low phase space density and thus to a dark matter profile with a constant density core in contrast to what happens in a scenario with thermally produced cold dark matter where the density keeps rising towards the center. As a result, the flux of gamma rays radiated from the final leptonic states of dark matter annihilation from the Galactic center is suppressed and satisfies the constraints from the HESS gamma-ray observations.

hep-ph↗

arXiv2009

F-SU(5)

We construct three flipped SU(5) X U(1)_X models from F-theory, and consider two such models from free fermionic string model building. To achieve the decoupling scenario in F-theory models and the string-scale gauge coupling unification in free fermionic models, we introduce vector-like particles at the TeV scale that can be observed at the Large Hadron Collider. We study gauge coupling unification, and find that proton decay is within the reach of the future Hyper-Kamiokande experiment. In these models, the doublet-triplet splitting problem and monopole problem can be solved, the neutrino masses and mixings can be explained via the double seesaw or seesaw mechanism, the observed baryon asymmetry can be obtained through leptogenesis, the hybrid inflation can be realized, and the correct cosmic primodial density fluctuations can be generated.

hep-th↗

arXiv2008

Realistic Type IIB Supersymmetric Minkowski Flux Models without the Freed-Witten Anomaly

We show that there exist supersymmetric Minkowski vacua on Type IIB toroidal orientifold with general flux compactifications where the RR tadpole cancellation conditions can be relaxed and the Freed-Witten anomaly can be cancelled elegantly. We present a realistic Pati-Salam like flux model without the Freed-Witten anomaly. At the string scale, we can break the gauge symmetry down to the Standard Model (SM) gauge symmetry, achieve the gauge coupling unification naturally, and decouple all the extra chiral exotic particles. Thus, we have the supersymmetric SMs with/without SM singlet(s) below the string scale. Also, we can explain the SM fermion masses and mixings. In addition, the unified gauge coupling and the real parts of the dilaton and Kähler moduli are functions of the four-dimensional dilaton. The complex structure moduli and one linear combination of the imaginary parts of the Kähler moduli can be determined as functions of the fluxes and the dilaton.

hep-th↗

arXiv2008

Three-Dimensional N \geq 5 Superconformal Chern-Simons Gauge Theories And Their Relations

We propose three-dimensional N=6 superconformal U(N) X U(M) and SU(N) X SU(N) Chern-Simons gauge theories with two pairs of bifundamental chiral superfields in the (N, M) and (\overline{N}, \overline{M}) representations and in the (N, N) and (\overline{N}, \overline{N}) representations, respectively. We also propose the superconformal U(1) X U(1) gauge theories that have n pairs of bifundamental chiral superfields with U(1) X U(1) charges (\pm 1, \mp 1) or (\pm 1, \pm 1). Although these U(1) X U(1) gauge theories have global symmetry SU(2n), the R-symmetry is SO(6) for n=2, and might be SO(2n) or SO(2n+1) for 3 \leq n \leq 8. In addition, we show that from either the generalized ABJM theories, or our U(N) X U(M) theories, or the N=5 superconformal O(N) X USp(2M) gauge theories, we can derive all the N \geq 5 superconformal Chern-Simons gauge theories except the N=5 superconformal G_2 X USp(2) gauge theory and our U(1) X U(1) gauge theories with n \not= 2 and 4. Furthermore, we derive the three-dimensional N=8 superconformal U(1) X U(1) gauge theory from the BLG theory, and study the corresponding moduli space. With the novel Higgs mechanism in the unitary gauge, we suggest that it describes a D2-brane and a decoupled D2-brane.

hep-th↗