arXiv subjects

We-Fu Chang

Publications and source records attributed to We-Fu Chang.

At least 37 records · Page 2Linked to original sources

arXiv2012

A model for Neutrino Masses and Dark Matter with the Discrete Gauge Symmetry

A simple renormalizable U(1) gauge model is constructed to explain the smallness of the active neutrino masses and provide the stable cold dark matter candidate simultaneously. The local U(1) symmetry is assumed to be spontaneously broken by a scalar field around the TeV scale. The active neutrino masses are then generated at one-loop level. This model contains several cold dark matter candidates whose stability is guaranteed by a residue discrete gauge $Z_2$ symmetry a la the Krauss-Wilczek mechanism. Unlike the other dark matter models, no further global discrete or continuous symmetry is introduced. Moreover, all the new degrees of freedom beyond the Standard Model acquire their masses only after the spontaneous breaking of U(1) thus they could be probed at or below the TeV scale. The possible cosmological and phenomenological consequences are briefly discussed.

hep-ph↗

arXiv2010

Neutrino Masses via the Zee Mechanism in 5D split fermions model

We study the Zee model in the framework of the split fermion model in $M_4\times S_1/Z_2$ spacetime. Neutrino masses are generated through 1-loop diagrams without the right-handed neutrinos introduced. By assuming an order one anarchical complex 5D Yukawa couplings, all the effective 4D Yukawa couplings are determined by the wave function overlap between the split fermions and the bulk scalars in the fifth dimension. The predictability of the Yukawa couplings is in sharp contrast to the original Zee model in 4D where the Yukawa couplings are unknown free parameters. This setup exhibits a geometrical alternative to the lepton flavor symmetry. By giving four explicit sets of the split fermion locations, we demonstrate that it is possible to simultaneously fit the lepton masses and neutrino oscillation data by just a handful free parameters without much fine tuning. Moreover, we are able to make definite predictions for the mixing angle $θ_{13}$, the absolute neutrino masses, and the lepton flavor violation processes for each configuration.

hep-ph↗

arXiv2010

An Unusual Two Higgs Doublet Model from Warped Space

We study a simple two Higgs doublet model (2HDM) in the Randall-Sundrum scenario, with an IR brane localized Higgs field and a second doublet arising from a $t\bar{t}$ condensate due to strong Kaluza-Klein gluon effects. The effective 2HDM predicts that the ratio of the brane to condensate vacuum expectation values $\tan β\sim 3$. It also predicts a standard model like Higgs boson of mass ${\mathcal{O}}(100)$ GeV and a heavier scalar at the scale of the lowest KK gluon mass, which we take to be $M_{KK} \gtrsim 1.5$ TeV. The pseudoscalar and the charged scalars are degenerate in mass at tree-level and are ${\mathcal{O}}(M_{KK})$. There are no tree-level flavor changing neutral currents (FCNC) for the down-type quarks and the standard model results hold there. In contrast, FCNC decays of the t-quark larger than in the SM are expected.

hep-ph↗

arXiv2009

Discrete Dirac Neutrino in Warped Extra Dimensions

We implement Dirac neutrinos in the minimal custodial Randall-Sundrum setting via the Krauss-Wilczek mechanism. We demonstrate by giving explicit lepton mass matrices that with neutrinos in the normal hierarchy, lepton mass and mixing patterns can be naturally reproduced at the scale set by the constraints from electroweak precision measurements, and at the same time without violating bounds set by lepton flavour violations. Our scenario generically predicts a nonzero neutrino mixing angle $θ_{13}$, as well as the existence of sub-TeV right-handed Kaluza-Klein neutrinos, which partner the right-handed Standard Model charged leptons. These relatively light KK neutrinos may be searched for at the LHC.

hep-ph↗

arXiv2009

Exploring Non-Supersymmetric New Physics in Polarized Møller Scattering

We study in an effective operator approach how the effects of new physics from various scenarios that contain an extra $Z'$ neutral gauge boson or doubly charged scalars, can affect and thus be tested by the precision polarized Møller scattering experiments. We give Wilson coefficients for various classes of generic models, and we deduce constraints on the parameter space of the relevant coupling constants or mixing angles from the results of the SLAC E158 experiment where applicable. We give also constraints projected from the upcoming 1 ppb JLAB experiment. In the scenario where the extra $Z'$ is light ($M_{Z'} \ll M_W$), we obtain further constraints on the parameter space using the BNL $g-2$ result where it is useful. We find that the BNL deviation from the Standard Model cannot be attributed to a light extra $Z'$ neutral gauge boson.

hep-ph↗

arXiv2008

Flavour Changing Neutral Current Constraints from Kaluza-Klein Gluons and Quark Mass Matrices in RS1

We continue our previous study on what are the allowed forms of quark mass matrices in the Randall-Sundrum (RS) framework that can reproduce the experimentally observed quark mass spectrum and the CKM mixing pattern. We study the constraints the $ΔF = 2$ processes in the neutral meson sector placed on the admissible forms found there, and we found only the asymmetrical type of quark mass matrices arising from anarchical Yukawa structures stay viable at the few TeV scale reachable at the LHC. We study also the decay of the first Kaluza-Klein (KK) excitation of the gluon. We give the decay branching ratios into quark pairs, and we point out that measurements of the decay width and just one of the quark spins in the dominant $\bar{t}t$ decays can be used to extract the effective coupling of the first KK gluon to top quarks for both chiralities. This provides further probe to the flavour structure of the RS framework.

hep-ph↗

arXiv2008

Testing Realistic Quark Mass Matrices in the Custodial Randall-Sundrum Model with Flavor Changing Top Decays

We study quark mass matrices in the Randall-Sundrum (RS) model with bulk symmetry $SU(2)_L \times SU(2)_R \times U(1)_{B-L}$. The Yukawa couplings are assumed to be within an order of magnitude of each other, and perturbative. We find that quark mass matrices of the symmetrical form proposed by Koide \textit{et. al.} [Y. Koide, H. Nishiura, K. Matsuda, T. Kikuchi and T. Fukuyama, Phys. Rev. D {\bf 66}, 093006 (2002)] can be accommodated in the RS framework with the assumption of hierarchyless Yukawa couplings, but not the hermitian Fritzsch-type mass matrices. General asymmetrical mass matrices are also found which fit well simultaneously with the quark masses and the Cabibbo-Kobayashi-Maskawa matrix. Both left-handed (LH) and right-handed (RH) quark rotation matrices are obtained that allow analysis of flavour changing decay of both LH and RH top quarks. At a warped down scale of 1.65 TeV, the total branching ratio of $t \ra Z$ + jets can be as high as $\sim 5 \times 10^{-6}$ for symmetrical mass matrices and $\sim 2 \times 10^{-5}$ for asymmetrical ones. This level of signal is within reach of the LHC.

hep-ph↗

arXiv2007

Phenomenology from a U(1) gauged hidden sector

We consider the phenomenological consequences of a hidden Higgs sector extending the Standard Model (SM), in which the matter content are uncharged under the SM gauge groups. We consider a simple case where the hidden sector is gauged under a U(1) with one Higgs singlet. The only couplings between SM and the hidden sector are through mixings between the neutral gauge bosons of the two respective sectors, and between the Higgs bosons. We find signals testable at the LHC that can reveal the existence and shed light on the nature of such a hidden sector.

hep-ph↗

arXiv2007

Shadow Higgs from a scale-invariant hidden $U(1)_s$ model

We study a scale invariant $SU(2)\times U(1)_Y \times U(1)_s$ model which has only dimensionless couplings. The shadow $U(1)_s$ is hidden, and it interacts with the Standard Model (SM) solely through mixing in the scalar sector and kinetic mixing of the U(1) gauge bosons. The gauge symmetries are broken radiatively by the Coleman-Weinberg mechanism. Lifting of the flat direction results in a light shadow Higgs or "scalon", and a heavier scalar which we identify as the SM Higgs boson. The phenomenology of this model is discussed. It is possible that shadow Higgs boson can be discovered in precision $t$-quark studies at the LHC. The conditions that it be a dark matter candidate is also discussed.

hep-ph↗

arXiv2006

A Very Narrow Shadow Extra Z-boson at Colliders

We consider the phenomenological consequences of a hidden Higgs sector extending the Standard Model (SM), in which the ``shadow Higgs'' are uncharged under the SM gauge groups. We consider a simple U(1) model with one Higgs singlet. One mechanism which sheds light on the shadow sector is the mixing between the neutral gauge boson of the SM and the additional U(1) gauge group. The mixing happens through the usual mass-mixing and also kinetic-mixing, and is the only way the ``shadow $Z$'' couples to the SM. We study in detail modifications to the electroweak precision tests (EWPTs) that the presence of such a shadow sector would bring, which in turn provide constraints on the kinetic-mixing parameter, $s_ε$, left free in our model. The shadow $Z$ production rate at the LHC and ILC depends on $s_ε$. We find that observable event rate at both facilities is possible for a reasonable range of $s_ε$ allowed by EWPTs.

hep-ph↗

arXiv2005

An Effective Operators Analysis of CP Violation : The Semileptonic Case

Aiming at a model-independent analysis of possible new physics effects in semileptonic processes at various energy scales, we list and study a complete set of $SU(3)_c\times SU(2)_L\times U(1)_Y$ invariant 4-Fermi operators which consist of a pair of quarks and a pair of leptons above the electroweak symmetry breaking. We give a full 1-loop renormalization group treatment of the evolution of the Wilson coefficients associated with these 4-Fermi operators between low energy ($\sim$ meson masses) and the cutoff scale $Λ$, $\sim (1-10)$ TeV, where we assume new degree of freedom beyond standard model will begin to appear and an ultra-violet completion of our effective theory will take place. Motivated by the existing phenomenological bounds, we argue that the new CP violation can only stem from the scalar and tensor types of 4-Fermi interaction. Some interesting constraints are obtained by studying the universality of kaon and pion leptonic decays, CP violating polarization of $K^+_{μ3}$, charged lepton anomalous magnetic moments, and $(μ\ra e γ)$ like rare decays. In particular, we can use the limit of electron dipole moment to constrain the size of the CP violating triplet correlation in the $e^+ e^- \ra t \bar{t}$ process.

hep-ph↗

arXiv2005

An Effective Operators Analysis of Leptonic CP Violation : Bridging High and Low Energy Processes

We study the leptonic CP violation by employing the complete set of dimension-six pure leptonic effective operators. Connection among the observable at different energy scales can be made by the running of the renormalization group equations. Explicitly, we study the charged lepton electric dipole moment, muon Michel decay, and the triple spin-momentum correlations at the Linear Collider. We found the electron electric dipole moment, which starts at 2-loop level, severely constrains the possibilities to detect the CP violating signatures in muon decay and at the linear colliders.

hep-ph↗

arXiv2005

Electric Dipole Moment in the Split Supersymmetry Models

We study an important contribution to the electric dipole moment (EDM) of the electron (or quarks) at the two-loop level due to the $W$-EDM in the recently proposed scenario of split supersymmetry. This contribution is independent of the Higgs mass, and it can enhance the previous estimation of the electron (neutron) EDM by $20-50%$ ($40-90%$). Our formula is new in its analytical form.

hep-ph↗

arXiv2005

Charged Lepton Electric Dipole Moments from TeV Scale Right-handed Neutrinos

We study the connection between charged lepton electric dipole moments, $d_l$ $(l=e,μ,τ)$, and seesaw neutrino mass generation in a simple two Higgs doublet extension of the Standard Model plus three right-handed neutrinos (RHN) $N_a$, $a=1,2,3$. For RHN with hierarchical masses and at least one with mass in the 10 TeV range we obtain the upper bounds of $|d_e|< 9\times 10^{-30}$ e-cm and $|d_μ|<2 \times 10^{-26}$ e-cm. Our scenario favors the normal mass hierarchy for the light neutrinos. We also calculated the cross section for $e^-e^- \ra W^- W^-$ in a high luminosity collider with constraints from neutrinoless double beta decay of nuclei included. Among the rare muon decay experiments we find that $μ\ra eγ$ is most sensitive and the upper limit is $<8\times 10^{-13}$.

hep-ph↗

arXiv2005

Lepton Flavor Violation in Extra Dimension Models

Models involving large extra spatial dimension(s) have interesting predictions on lepton flavor violating processes. We consider some 5D models which are related to neutrino mass generation or address the fermion masses hierarchy problem. We study the signatures in low energy experiments that can discriminate the different models. The focus is on muon-electron conversion in nuclei, $μ\ra e γ$ and $μ\ra 3e$ processes and their $τ$ counterparts. Their links with the active neutrino mass matrix are investigated. We show that in the models we discussed the branching ratio of $μ\ra e γ$ like rare process is much smaller than the ones of $μ\ra 3e$ like processes. This is in sharp contrast to most of the traditional wisdom based on four dimensional gauge models. Moreover, some rare tau decays are more promising than the rare muon decays.

hep-ph↗

arXiv2003

Phenomenology of a 5D Orbifold $SU(3)_W$ Unification model

We study the phenomenology of a 5D $SU(3)_W$ model on a $S_1/(Z_2\times Z'_2)$ orbifold in which the minimal scalar sector plays an essential role of radiatively generating neutrino Majorana masses without the benefits of right-handed singlets. We carefully examine how do the exotic scalars affect the renormalization group (RG) equations for the gauge couplings and the 5D $SU(3)_W$ unification. We found that the compactification scale of extra dimension is in the range of $1/R\sim 1.5-5$ TeV. The possibility of the existence of relatively low mass Kaluza-Klein excitations makes the phenomenology of near term interest. Some possible bilepton signatures can be searched for in future colliders and in neutrino scattering experiments with intense neutrino beams. The low energy constraints from muon physics and lepton number violating decay process induced by bilepton are also discussed. These constraints can provide new information on the structure of Yukawa couplings which might be useful for future model building.

hep-ph↗

arXiv2003

Neutrino Masses in 5D Orbifold SU(5) Unification Models without Right-handed Singlets

We explore a mechanism for radiatively generating neutrino Majorana masses in a 5 dimensional orbifold SU(5) unification model without introducing right-handed singlets. The model is non-supersymmetric and the extra dimension is compactified via a $s_1/(Z_2\times Z'_2)$ orbifold geometry. The necessary lepton number violating interaction arises from the Yukawa interactions either between a 10-plet or a 15-plet bulk scalar field and the fermion quintuplets which are residents on the SU(5) symmetrical brane located at one of the orbifold fixed points. The model is engineered to give realistic charged fermion masses and mixing and in the same time avoiding the rapid proton and neutron decays by geometric construction. The gauge unification can be maintained by adding extra fermion or scalar fields. The unification scale is found to be larger then $10^{15}$ GeV by adding a bulk vector decuplet pair whose zero mode has masses around $10\sim 100$ TeV range. We found that neutrino mass matrix of the normal hierarchy type is favored by using 15-plet scalar. We give a solution of this type which has detectable $μ\ra 3e$ transition. On the other hand, by introducing 10-plet scalar, the leading neutrino mass matrix can only be inverted hierarchical and gives at most bi-maximal mixing.

hep-ph↗

arXiv2003

Neutrino Masses in a 5D $SU(3)_W$ TeV Unification Model

We study the generation of neutrino masses in the $SU(3)_W$ electroweak unified theory in $M_4\times S_1/(Z_2\times Z'_2)$ spacetime. By appropriate orbifolding, the bulk symmetry $SU(3)_W$ is broken into $SU(2)_L \times U(1)_Y$ at one of the fixed points, where the quarks reside. The leptons form $SU(3)_W$ triplets, localized at the other symmetric fixed point. The fermion masses arise from the bulk Higgs sector containing a triplet and an anti-sextet. We construct neutrino Majorana masses via 1-loop quantum corrections by adding a parity odd bulk triplet scalar. No right-handed neutrino is needed. The neutrino mass matrix is of the inverted hierarchy type. We show that the model can easily accommodate the bi-large mixing angle solution favored by the recent neutrino experiments without much fine tuning of parameters. The constraints from $μ\ra 3e$ transition and neutrinoless double beta decays are discussed.

hep-ph↗