arXiv subjects

We-Fu Chang

Publications and source records attributed to We-Fu Chang.

At least 19 recordsLinked to original sources

arXiv2024

Exploring a Gauge Horizontal Model for Charged Fermion Masses

We investigate an extension of the Standard Model (SM) incorporating a gauge $ U(1) $ horizontal symmetry that is free of anomalies. This extension introduces four additional un-Higgsed scalar doublets that do not develop vacuum expectation values, two scalar singlets, and a pair of vector-like fermionic singlets. Within this framework, the masses of third-generation charged fermions are generated through the conventional SM Yukawa interactions, while the masses of second-generation charged fermions are suppressed via a mechanism reminiscent of Froggatt-Nielsen. In contrast, the masses of first-generation charged fermions are predominantly determined by radiative corrections. Unlike traditional implementations of the Froggatt-Nielsen mechanism, our model does not require additional colored vector or chiral fermions beyond the SM. This model provides an economical ultraviolet-complete mechanism to explain the observed patterns in charged fermion masses and Cabibbo-Kobayashi-Maskawa matrix elements. Notably, the electron electric dipole moment vanishes automatically at the two-loop level, and there is no charged lepton flavor violation to all orders. We also discuss potential experimental signatures that could distinguish this model from other $ Z' $ models, such as specific patterns in gauge boson decays and associated collider signatures.

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arXiv2022

Non-universal gauged lepton number for charged lepton masses hierarchy and $(g-2)_{e,\mu}$

We construct a novel flavor-dependent gauged lepton number $U(1)_\ell$ model for the hierarchical charged lepton masses and the observed $(g-2)_{e,\mu}$. Only tau participates in the tree-level Standard Model ( SM ) Yukawa interaction. At the same time, the masses of electron and muon are light due to radiative generation and(or) the heavy-mediator-suppressed Yukawa coupling to the SM Higgs. Not only can the measured anomalous magnetic dipole moment of the muon, $\triangle a_\mu$, be explained, but the positive (or negative) $\triangle a_e$ can also be accommodated in this model. Without additional discrete symmetries introduced, charged lepton flavor violation is highly suppressed by the $U(1)_\ell$ symmetry. Corresponding to two equally viable $U(1)_\ell$ charge assignments, this model predicts either $A_{FB}^\mu A_{FB}^\tau >A_{FB}^e$, which can be tested at the $e^+e^-$ machines before discovering the $U(1)_\ell$ gauge boson. Moreover, the effective muon and electron Yukawa couplings can depart significantly from the SM predictions, and those deviations could be probed at future $e^+e^-$ colliders and High-Luminosity LHC.

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arXiv2022

Possibly heteroclite electron Yukawa coupling and small $\triangle a_\mu$ in a hidden Abelian gauge model for neutrino masses

We attempt to simultaneously explain the neutrino oscillation data and the observed $(g-2)_{e,\mu}$ in a hidden gauge $U(1)_X$ model where all the Standard Model(SM) fields are $U(1)_X$ singlets. The minimal version of this model calls for four exotic scalars and two pairs of vector fermions, and all are charged under $U(1)_X$. We carefully consider the experimental limits on charge lepton flavor violation without assuming any flavor symmetry and explore the viable model parameter space. The model can accommodate the neutrino oscillation data for both the normal and the inverted mass ordering while explaining the central value of $\triangle a_e$ by adopting the fine structure constant determined by using either Cesium or Rubidium atoms. However, mainly constrained by the current experimental bound on ${\cal B}(\tau\rightarrow \mu \gamma)$, this model predicts $\triangle a_\mu <5.5(8.0)\times 10^{-10}$ for the normal(inverted) neutrino ordering. Moreover, while the muon Yukawa coupling is close to the SM one, we find the magnitude of the electron Yukawa coupling could be one order of magnitude larger than the SM prediction. This abnormal electron Yukawa could be probed in the future FCC-ee collider and plays an essential role in testing flavor physics.

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arXiv2021

One colorful resolution to the neutrino mass generation, three lepton flavor universality anomalies, and the Cabibbo angle anomaly

We propose a simple model to simultaneously explain four observed flavor anomalies while generating the neutrino mass at the one-loop level. Specifically, we address the measured anomalous magnetic dipole moments of the muon, $\Delta a_\mu$ , and electron, $\Delta a_e$, the observed anomaly of $b\rightarrow s l^+ l^-$ in the $B$-meson decays, and the Cabibbo-angle anomaly. The model consists of four colorful new degrees of freedom: three scalar leptoquarks with the Standard Model quantum numbers $(3,3,-1/3),(3,2,1/6)$, and $(3,1,2/3)$, and one pair of down-quark-like vector fermion in $(3,1,-1/3)$. The baryon number is assumed to be conserved for simplicity. Phenomenologically viable solutions with the minimal number of real parameters can be found to accommodate all the above-mentioned anomalies and produce the approximate, close to $1\sigma$, neutrino oscillation pattern at the same time. From general consideration, the model robustly predicts: (1) neutrino mass is of the normal hierarchy type, and (2) ${\cal M}^\nu_{ee}\lesssim 3\times 10^{-4}\mbox{eV}$. The possible UV origin to explain the flavor pattern of the found viable parameter space is briefly discussed. The parameter space can be well reproduced within a simple split fermion toy model.

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arXiv2020

Constraints on light singlet fermion interactions from coherent elastic neutrino-nucleus scattering

The exotic singlet fermions $\chi$, with a mass $m_\chi\lesssim 50$ MeV, could be produced at the coherent elastic neutrino-nucleus scattering (CE$\nu$NS) experiments through the $\nu {\mathcal N} \rightarrow \chi {\mathcal N}$ process. Due to the coherent enhancement, it offers a unique way to study how $\chi$ interacts with the Standard Model (SM) sector. Based on the most general dimension-6 effective Lagrangian, we perform a comprehensive study on the relevant interaction between $\chi$ and the SM sector. From the current and future COHERENT and future CONUS experiments, we obtain the upper bounds on the Wilson coefficients for the dipole, scalar, vector, and tensor interactions. For $m_\chi $ below 10 MeV, future CONUS data has the best sensitivity, while for $m_\chi$ between 10 MeV$-50$ MeV, the current and future COHERENT bounds dominate. These limits are complementary to those from neutrino oscillation and collider searches. Moreover, the bounds do not depend on the charge conjugation property of $\chi$, nor whether $\chi$ is dark matter or not.

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arXiv2019

KeV scale new fermion from a hidden sector

We studied a simple model of hidden sector consists of a Dirac fermion $\chi$ and a spontaneously broken $U(1)_s$ symmetry. The dark sector is connected to the Standard Model(SM) via three righthanded SM singlet neutrinos, $N_R$'s, and the kinetic mixing between $U(1)_s$ and $U(1)_Y$. A mixing between the scalar $\phi$ that breaks $U(1)_s$ and the SM Higgs boson, $H$, is implemented via the term $\phi^\dagger \phi H^\dagger H$ and this provides a third connection to the SM. Integrating out the $N_R$ at a high scale not only gives the active neutrinos, $\nu$, masses but generates effective Dirac type of couplings between $\nu$ and $\chi$. This changes the usual Type-I seesaw results for active neutrino masses and makes $\chi$ behave like a sterile neutrino even though its origin is in the hidden sector. $\chi$ is also split into a pair of Majorana fermions. The amount of splitting depends on the parameters. If the lighter of the pair has a mass around keV, its lifetime is longer than the age of the universe and it can be a warm dark matter candidate. Signatures of $\chi$ in high precision Kurie plots of nuclei $\beta$ decays and low energy neutrino nuclei coherent scatterings are discussed. The model also induces new invisible $Z$ decay modes that can be searched for in future Z factories.

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arXiv2018

Alternative Perspective on Gauged Lepton Number and Implications for Collider Physics

A new anomaly-free gauged $U(1)_\ell$ lepton-number model is studied. Two standard model lepton generations acquire the same but oppositive sign $U(1)_\ell$ charges, while four exotic chiral leptons cancel the anomalies of the remaining lepton family. We discuss a simplified case which has the universal Yukawa couplings. It agrees with all the experimental constraints and predicts $m_e, m_\mu \ll m_\tau$, and the latter is of the electroweak scale. Due to the interference between the SM and $U(1)_\ell$ gauge interactions, this model robustly predicts that $e,\mu,\tau$ have distinctive forward-backward asymmetries at the $e^+e^-$ colliders. It can be searched for at the $e^+e^-$ machine with $\sim$ TeV center-of-mass energy and an integrated luminosity $\sim ab^{-1}$.

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arXiv2018

Neutrino masses and gauged $U(1)_\ell$ lepton number

We investigate the tree-level neutrino mass generation in the gauged $U(1)_\ell$ lepton model recently proposed by us [arXiv:1805.10382]. With the addition of one Standard Model(SM) singlet, $\phi_1(Y=0, \ell=1)$, and one SM triplet scalar, $T(Y=-1,\ell=0)$, realistic lepton masses can be accommodated. The resulting magnitude of neutrino mass is given by $\sim v_t^3/v_L^2$, where $v_t$ and $v_L$ are the vacuum expectation values of $T$ and $\phi_1$, respectively, and it is automatically of the inverse see-saw type. Since $v_L$ is the lepton number violation scale we take it to be high, i.e. ${\cal O} \gtrsim (\mbox{TeV})$. Moreover, the induced lepton flavor violating processes and the phenomenology of the peculiar triplet are studied. An interesting bound, $0.1\lesssim v_t\lesssim24.1$ GeV, is obtained when taking into account the neutrino mass generation, $Br(\mu\rightarrow e \gamma)$, and the limits from oblique parameters, $\Delta S$ and $\Delta T$. Collider phenomenology of the SM triplets is also discussed.

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arXiv2018

Study of Gauged Lepton Symmetry Signatures at Colliders

We construct a new gauged $U(1)_\ell$ lepton number model which is anomaly-free for each SM generation. The active neutrino masses are radiatively generated with a minimal scalar sector. The phenomenology and collider signals are studied. The interference effects among the new gauge boson, $Z_\ell$, photon, and $Z$-boson can be probed at the future $e^+e^-$ colliders even if the center-of-mass energy is below the mass of $Z_\ell$. Moreover, the electroweak precision sets a stringent bound on the mass splitting of the new lepton doublets.

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arXiv2018

Prospects for Detecting light bosons at the FCC-ee and CEPC

We look at the prospects for detecting light bosons, $X$, at proposed Z factories assuming a production of $10^{12}$ Z bosons. Such a large yield is within the design goals of future FCC-ee and CEPC colliders. Specifically we look at the cases where $X$ is either a singlet scalar which mixes with the standard model Higgs or a vector boson with mass $1\lesssim M_X \lesssim 80$ GeV. We find that several channels are particularly promising for discovery prospects. In particular $Z\rightarrow f \bar{f} X$ and $Z \rightarrow V_Q X$ gives a promising signal above a very clean standard model background. We also discuss several channels that have too large a background to be useful.

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arXiv2017

Signal for a light singlet scalar at the LHC

In the general Higgs portal like models, the extra neutral scalar, $S$, can mix with the Standard Model (SM) Higgs boson, $H$. We perform an exploratory study focusing on the direct search for such a light singlet $S$ at the Large Hadron Collider (LHC). After careful study of the SM background, we find the process $pp\rightarrow t\bar{t}S$ followed by $S\rightarrow b\bar{b}$ can be used to investigate $S$ with mass in the $20<M_S<100$ GeV range, which has not been well explored at the LHC. The signal significance becomes meaningful with a luminosity around a few $\mbox{ab}^{-1}$. Also, we study the prospects of finding the light scalar at the future 100 TeV $pp$ collider, the $Z$ and Higgs factories. With similar luminosity, the current LEP limits on the mixing between $S$ and $H$ can be improved by at least one or two orders of magnitude.

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arXiv2016

Charged Lepton Flavor Violating Processes and Scalar Leptoquark Decay Branching Ratios in the Colored Zee-Babu Model

We consider a neutrino mass generating model which employs a scalar leptoquark, $Δ$, and a scalar diquark, $S$. The new scalars $Δ$ and $S$ carry the standard model $SU(3)_c\times SU(2)_L\times U(1)_Y$ quantum numbers $(3,1,-1/3)$ and $(6,1,-2/3)$, respectively. The neutrino masses are generated at the two-loop level, as in the Zee-Babu model\cite{Zee-Babu}, and $Δ/S$ plays the role of the doubly/singly charged scalar in the Zee-Babu model. With a moderate working assumption that the magnitudes of the six Yukawa couplings between $S$ and the down-type quarks are of the same order, strong connections are found between the neutrino masses and the charged lepton flavor violating processes. In particular, we study $Z\rightarrow \overline{l} l'$, and $l\rightarrow l' γ$ and find that some portions of the parameter space of this model are within the reach of the planned charged lepton flavor violating experiments. Interesting lower bounds are predicted that $B(Z\rightarrow \overline{l} l')\gtrsim 10^{-16}-10^{-14}(10^{-14})\times(1\mbox{TeV}\cdot m_S/7 m_Δ^2)^2$ and $B(l\rightarrow l' γ)\gtrsim 10^{-17}-10^{-16}(10^{-18}-10^{-16})\times(1 \mbox{TeV}\cdot m_S/7 m_Δ^2)^2$ for neutrino masses being the normal(inverted) hierarchical pattern. The type of neutrino mass hierarchy could also be determined by measuring the charged lepton flavor violating double ratios. Moreover, definite leptoquark decay branching ratios are predicted when there is no Yukawa interaction between the right-handed fermions and $Δ$ ( the branching fraction of $Δ$ to a charged lepton and a quark is 50\%), which could help refine the collider search limit on the scalar leptoquark mass.

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arXiv2016

Renormalization Group Study of the Minimal Majoronic Dark Radiation and Dark Matter Model

We study the 1-loop renormalization group equation running in the simplest singlet Majoron model constructed by us earlier to accommodate the dark radiation and dark matter content in the universe. A comprehensive numerical study was performed to explore the whole model parameter space. A smaller effective number of neutrinos $\triangle N_{eff}\sim 0.05$, or a Majoron decoupling temperature higher than the charm quark mass, is preferred. We found that a heavy scalar dark matter, $ρ$, of mass $1.5-4$ TeV is required by the stability of the scalar potential and an operational type-I see-saw mechanism for neutrino masses. A neutral scalar, $S$, of mass in the $10-100$ GeV range and its mixing with the standard model Higgs as large as $0.1$ is also predicted. The dominant decay modes are $S$ into $b\bar{b}$ and/or $ωω$. A sensitive search will come from rare $Z$ decays via the chain $Z\rightarrow S+ f\bar{f}$, where $f$ is a Standard Model fermion, followed by $S$ into a pair of Majoron and/or b-quarks. The interesting consequences of dark matter bound state due to the sizable $Sρρ$-coupling are discussed as well. In particular, shower-like events with an apparent neutrino energy at $M_ρ$ could contribute to the observed effective neutrino flux in underground neutrino detectors such as IceCube.

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arXiv2014

A Minimal Model of Majoronic Dark Radiation and Dark Matter

We extend the singlet Majoron model of dark radiation by adding another singlet scalar of unit lepton charge. The spontaneous breaking of global $U(1)_L$ connects dark radiation with neutrino mass generation via the type-I seesaw mechanism. The model naturally has a stable scalar dark matter field. It also predicts the existence of a light scalar of mass less than 1 GeV that mixes with the Standard Model Higgs boson. We perform a numerical analysis of the parameters of the model by imposing constraints from giving correct relic abundance and satisfying bounds from direct dark matter detection, rare decays of B-meson, and invisible width of the Higgs boson. The viability of the model in accommodating the gamma rays from the Galactic center is discussed as well. The model gives rise to new rare Higgs boson decays such as four-muon final states with displaced vertices. Another unique signal is two muons and missing energy recoil against the muon pair. Our result also shows that such a bridge between dark radiation and the seesaw mechanism will put the seesaw scale in the range of 1-100 TeV.

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arXiv2013

Some consequences of the Majoron being the dark radiation

We discuss some phenomenological consequences in a scenario where a singlet Majoron plays the role of dark radiation. We study the interrelations between neutrino mass generation and the scalar potential arising from this identification. We find the extra scalar has to be light with a mass at or below the GeV level. The mixing of this scalar with the Standard Model Higgs impacts low energy phenomena such as the muonic hydrogen Lamb shift and muon anomalous magnetic moment. Demanding that the light scalar solves the puzzle in the muon magnetic moment requires the scalar to be lighter still with mass at or below the 10 MeV level. The cross-sections for the production of heavy neutrinos at LHC14 are also given.

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arXiv2013

An effective gauge-Higgs operators analysis of new physics associated with the Higgs

We study the new physics(NP) related to the recent discovered 125 GeV Higgs by employing an important subset of the standard model(SM) gauge invariant dimension-six operators constructed by the the SM Higgs and gauge fields. Explicitly, we perform a model-independent study on the production and decays of the Higgs, the electric dipole moments(EDM) of the neutron and the electron, and we take into account the anomalous magnetic dipole moments of muon and electron as well. We find that, even all Higgs decay channels agree with the SM predictions, the SM theoretical uncertainties provide a lot of room to host NP associated with the 125 GeV boson. A linear relation is revealed in our numerical study that $μ_{ZZ}\simeq μ_{WW}$ and $ 0.6 \lesssim μ_{ZZ,WW} \lesssim 1.4$ at 95% CL with or without the EDM's constraints. The neutron and electron EDM's severely constrain the relevant Wilson coefficients. Therefore the CP violating components in the $h\rightarrow WW, ZZ$ channels are too small, $\sim{\cal O}(10^{-5})$, to be detected at LHC. However, we point out that even the parity of the 125GeV boson has been largely determined to be even in the $h\to ZZ$ channel, one should pay special attention to the potentially large CP violation in the $h\to γγ$ and $h\to γZ$ channels. This should be seriously checked in the future spin correlation experiments.

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arXiv2013

Two Higgs Doublets Model in Gauge-Higgs Unification framework

We discuss the realization of two Higgs doublets model in the framework of 6 dimensional Gauge-Higgs Unification model with a simple Lie group G_M. Two Higgs SU(2)_L doublets can emerge at the low energy effective theory, and the quartic coupling terms in the scalar potential, essential for the electroweak symmetry breaking, are now G_M gauge invariant and permissive. A realistic two Higgs doublets model can possibly be obtained only when two of the root vectors associated with the would-be Higgs doublets and the root vector for SU(2)_L form an isosceles triangle with vertex angle either of Pi/3, Pi/2, or 2Pi/3. Moreover, depending on G_M, the scalar potential of resulting two Higgs doublets model can admit only a few limited forms. The mass spectrum of the physical Higgs and the weak mixing angle are briefly discussed.

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arXiv2012

Constraints on New Scalars from the LHC 125 GeV Higgs Signal

We study the implications the recent results from the LHC Higgs searches have on scalar new physics. We study the impact on both the Higgs production and decay from scalars with and without colour, and in cases where decoupling do and do not happen. We investigate possible constraints on scalar parameters from the production rate in the diphoton channel, and also the two vector boson channels. Measurements from both channels can help disentangle new physics due to colour from that due to charge, and thus reveal the nature of the new scalar states.

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