arXiv subjects

Lu-Hsing Tsai

Publications and source records attributed to Lu-Hsing Tsai.

18 recordsLinked to original sources

arXiv2015

CP Violation in Predictive Neutrino Mass Structures

We study the CP violation effects from two types of neutrino mass matrices with (i) $(M_ν)_{ee}=0$, and (ii) $(M_ν)_{ee}=(M_ν)_{eμ}=0$, which can be realized by the high dimensional lepton number violating operators $\bar \ell_R^cγ^μL_L (D_μΦ)Φ^2$ and $\bar \ell_R^c l_R (D_μΦ)^2Φ^2$, respectively. In (i), the neutrino mass spectrum is in the normal ordering with the lightest neutrino mass within the range $0.002\,{\rm eV}\lesssim m_0\lesssim 0.007\,{\rm eV}$. Furthermore, for a given value of $m_0$, there are two solutions for the two Majorana phases $α_{21}$ and $α_{31}$, whereas the Dirac phase $δ$ is arbitrary. For (ii), the parameters of $m_0$, $δ$, $α_{21}$, and $α_{31}$ can be completely determined. We calculate the CP violating asymmetries in neutrino-antineutrino oscillations for both mass textures of (i) and (ii), which are closely related to the CP violating Majorana phases.

hep-ph↗

arXiv2015

Connecting Neutrino Masses and Dark Matter by High-dimensional Lepton Number Violation Operator

We propose a new model with the Majorana neutrino masses generated at two-loop level, in which the lepton number violation (LNV) processes, such as neutrinoless double beta decays, are mainly induced by the dimension-7 LNV effective operator O_7=\bar l_R^c γ^μL_L(D_mu Φ) ΦΦ. Note that it is necessary to impose an Z_2 symmetry in order that O_7 dominates over the conventional dimension-5 Weinberg operator, which naturally results in a stable Z_2-odd neutral particle to be the cold dark matter candidate. More interestingly, due to the non-trivial dependence of the charged lepton masses, the model predicts the neutrino mass matrix to be in the form of the normal hierarchy. We also focus on a specific parameter region of great phenomenological interests, such as electroweak precision tests, dark matter direct searches along with its relic abundance, and lepton flavor violation processes.

hep-ph↗

arXiv2015

Comment on "A three-loop radiative neutrino mass model with dark matter" [Phys. Lett. B 741 (2015) 163]

We revisit the calculation of the three-loop diagrams for the radiative neutrino mass generation and consider some relevant constraints on the model recently proposed by L. Jin {\it et al} [Phys. Lett. B 741 (2015) 163]. We find that the previous approximation is inappropriate due to the neglect of some important contributions, and the benchmark point proposed can neither give rise to enough neutrino masses nor accommodate these additional constraints, such as the validity of the perturbation theory, the electroweak precision measurements, and the neutrinoless double beta decays.

hep-ph↗

arXiv2015

Constraints on the septet-doublet mixing models from oblique parameters

The limitations of the doublet-septet mixing models by the deviations of electroweak oblique parameters $ΔS$ and $ΔT$ are studied. In the minimal model, the mixture of the septet $η$ and the scalar doublet in the standard model (SM) is driven by a non-Hermitian dimension-7 operator. For a smaller bare mass of the septet, $ΔS$ gives a stringent constraint on the mixing angle $\sinβ$ between the CP-odd neutral parts of the SM Higgs doublet and $η$. In general, increasing the mass of the scalar septet $M_η$ will enhance the deviation of $T$ from the SM, whereas it decreases the magnitude of $ΔS$ for a larger bare mass within the range $M_η\lesssim 400\,{\rm GeV}$. We also examine two extended models from the ordinary doublet-septet mixture pattern. One of them is based on a inert doublet-septet mixing pattern, in which there is no vacuum expectation value for the neutral component of $η$, and a stable dark matter could naturally exist. For a benchmark point with this inner doublet mass of $M_χ=250{\rm}$ and $M_η=400\,{\rm GeV}$ in this model, the mixing coefficient is found to be less than $1.8$. The other extension is constructed by imposing a doubly charged scalar mixed with the doubly charged component of the septet. Apart from the contribution by the septet-doublet admixture, $ΔS$ is suppressed by a factor of $s_W^2$ and $ΔT$ has a significant constraint due to the vanishing vacuum polarization of $Z$ at the momentum transfer $p^2=0$.

hep-ph↗

arXiv2015

Study of Two-Loop Neutrino Mass Generation Models

We study the models with the Majorana neutrino masses generated radiatively by two-loop diagrams due to the Yukawa $ρ\bar \ell_R^c \ell_R$ and effective $ρ^{\pm\pm} W^\mp W^\mp$ couplings along with a scalar triplet $Δ$, where $ρ$ is a doubly charged singlet scalar, $\ell_R$ the charged lepton and $W$ the charged gauge boson. A generic feature in these types of models is that the neutrino mass spectrum has to be a normal hierarchy. Furthermore, by using the neutrino oscillation data and comparing with the global fitting result in the literature, we find a unique neutrino mass matrix and predict the Dirac and two Majorana CP phases to be $1.40π$, $1.11π$ and $1.47π$, respectively. We also discuss the model parameters constrained by the lepton flavor violating processes and electroweak oblique parameters. In addition, we show that the rate of the neutrinoless double beta decay $(0νββ)$ can be as large as the current experimental bound as it is dominated by the short-range contribution at tree level, whereas the traditional long-range one is negligible.

hep-ph↗

arXiv2014

Loop-induced Neutrino Masses: A Case Study

We study the cocktail model in which the Majorana neutrino masses are generated by the so-called "cocktail" three-loop diagrams with the dark matter particle running in the loops. In particular, we give the correct analytic expressions of the neutrino masses in the model by the detailed calculation of the cocktail diagrams. Based on the reliable numerical calculation of the loop integrals, we explore the parameter space which can give the correct orders of neutrino masses while satisfying other experimental constraints, such as those from the neutrinoless double beta decay, low-energy lepton flavor violation processes, electroweak precision tests, and collider searches. As a result, the large couplings and the large mass difference between the two singly-charged (neutral) scalars are required.

hep-ph↗

arXiv2014

X-ray Line from the Dark Transition Electric Dipole

We study a two-component dark matter (DM) model in which the two Majorana fermionic DM components with nearly degenerate masses are stabilized by an $Z_2$ symmetry and interact with the right-handed muon and tau only via real Yukawa couplings, together with an additional $Z_2$-odd singly-charged scalar. In this setup, the decay from the heavy DM to the lighter one via the transition electric dipole yields the 3.55 keV X-ray signal observed recently. The Yukawa couplings in the dark sector are assumed to be hierarchical, so that the observed DM relic abundance can be achieved with the leading s-wave amplitudes without a fine-tuning. We also consider the constraints from flavor physics, DM direct detections and collider searches, respectively.

hep-ph↗

arXiv2014

Cosmic Ray Excesses from Multi-component Dark Matter Decays

We use multi-component decaying dark matter (DM) scenario to explain the possible cosmic ray excesses in the positron fraction recently confirmed by AMS-02 and the total $e^+ +e^-$ flux observed by Fermi-LAT. In the two-component DM models, we find an interesting variation of the flavor structure along with the cutoff of the heavy DM. For the three-component DM case, we focus on a particular parameter range in which the best fits prefer to open only 2 DM decay channels with a third DM contributing nothing to the electron and positron spectra. We show that all models give the reasonable fits to both the AMS-02 positron fraction and the Fermi-LAT total $e^++e^-$ flux, which are also consistent with the measured diffuse $γ$-ray flux by Fermi-LAT.

hep-ph↗

arXiv2014

Dark radiation from a unified dark fluid model

We present a unified dark fluid model to describe the possible evolutionary behavior of $ΔN_\mathrm{eff}$ in dark radiation. This model can be viewed as an interacting model for the dark sectors, in which dark matter interacts with dark radiation. We show that the evolution of $ΔN_\mathrm{eff}$ can be nicely explained without some drawbacks, such as the blowup of $ΔN_\mathrm{eff}$ and the non-vanishing interaction at the late time.

astro-ph.CO↗

arXiv2014

Imprint of Multi-component Dark Matter on AMS-02

The multi-component decaying dark matter (DM) scenario is investigated to explain the possible excesses in the positron fraction by PAMELA and recently confirmed by AMS-02, and in the total $e^+ +e^-$ flux observed by Fermi-LAT. By performing the $χ^2$ fits, we find that two DM components are already enough to give a reasonable fit of both AMS-02 and Fermi-LAT data. The best-fitted results show that the heavier DM component with its mass 1.5 TeV dominantly decays through the $μ$-channel, while the lighter one of 100 GeV mainly through the $τ$-channel. As a byproduct, the fine structure around 100 GeV observed by AMS-02 and Fermi-LAT can be naturally explained by the dropping due to the lighter DM component. With the obtained model parameters by the fitting, we calculate the diffuse $γ$-ray emission spectrum in this two-component DM scenario, and find that it is consistent with the data measured by Fermi-LAT. We also construct a microscopic particle DM model to naturally realize the two-component DM scenario, and point out an interesting neutrino signal which is possible to be measured in the near future by IceCube.

hep-ph↗

arXiv2013

Peccei-Quinn symmetry as the origin of Dirac Neutrino Masses

We propose a model of Dirac neutrino masses generated at one-loop level. The origin of this mass is induced from Peccei-Quinn symmetry breaking which was proposed to solve the so-called strong CP problem in QCD, therefore, the neutrino mass is connected with the QCD scale, $Λ_{\rm QCD}$. We also study the parameter space of this model confronting with neutrino oscillation data and leptonic rare decays. The phenomenological implications to leptonic flavor physics such as the electromagnetic moment of charged leptons and neutrinos are studied. Axion as the dark matter candidate is one of the byproduct in our scenario. Di-photon and Z-photon decay channels in the LHC Higgs search are investigated, we show that the effects of singly charged singlet scalar can be distinguished from the general two Higgs doublet model.

hep-ph↗

arXiv2013

Correlation of $h\rightarrowγγ$ and $Zγ$ in Type-II seesaw neutrino model

We study the charged scalar contributions to the Higgs decay channels of $h\rightarrow γγ$ and $h\rightarrow Zγ$ in the Type-II seesaw neutrino model. In most of the allowed parameter space in the model, the new contribution to $h\rightarrow Zγ$ is positively correlated with that to $h\rightarrowγγ$. If the current excess of the $h\rightarrow γγ$ rate measured by the ATLAS Collaboration persists, the $h\rightarrow Zγ$ rate should be also larger than the corresponding standard model prediction. We demonstrate that the anti-correlation between $h\rightarrowγγ$ and $h\rightarrow Zγ$ only exists in some special region.

hep-ph↗

arXiv2013

New Scalar Contributions to $h\to Zγ$

We calculate the Higgs decay rate of $h\to Zγ$ by including the contributions from new scalars with arbitrary quantum numbers of the weak isospin ($T$) and hypercharge ($Y$) in the standard model. We find that our general formula for the decay rate of $h\to Zγ$ matches with that for $h\to γγ$ in the limit of $m_Z=0$, but it is different from those in the literature. To illustrate our result, by taking the current $2σ$ excess of the $h\to γγ$ rate measured by the LHC, we examine the corresponding shift for the $Zγ$ decay channel due to the new scalar. We show that the enhancement or reduction of the $h\to Zγ$ rate only depends on the relative size of $T$ and the absolute value of $Y$. Explicitly, we predict $0.76<R_{Zγ}\equiv Γ(h\to Zγ)/Γ_{SM}(h\to Zγ)<2.05$ by imposing the observed range of $1.5<R_{γγ}\equiv Γ(h\to Zγγ)/Γ_{SM}(h\toγγ)<2$, which is independent of the number of multiplets and the couplings to the Higgs particle as long as the scalars are heavier than 200 GeV. This result provides a clear signature for the future LHC measurements to test physics beyond the standard model.

hep-ph↗

arXiv2013

Multi high charged scalars in the LHC searches and Majorana neutrino mass generations

One natural way to understand the excess of the measured $H\toγγ$ rate over the standard model (SM) expectation at the Large Hadron Collider (LHC) is to have charged scalar bosons, existing in most of the SM extensions. Motivated by this LHC result, we explore if it also sheds light on solving the small neutrino mass generation problem. We concentrate on a class of models with high dimensional representations of scalars to realize Majorana neutrino masses at two-loop level without imposing any new symmetry. In these models, multi scalars with the electric charges higher than two are naturally expected, which not only enhance the $H\toγγ$ rate, but provide more searching grounds at the LHC. In particular, the rate of $H\to Zγ$ also changes similar to that of the diphoton channel.

hep-ph↗

arXiv2011

R$ν$MDM and Lepton Flavor Violation

A model relating radiative seesaw and minimal dark matter mass scales without beyond the standard model (SM) gauge symmetry (R$ν$MDM) is constructed. In addition to the SM particles, the R$ν$MDM contains, a Majorana fermion multiplet $N_R$ and scalar multiplet $χ$ that transform respectively as $(1,5,0)$ and $(1,6,-1/2)$ under the SM gauge group $SU(3)_C\times SU(2)_L\times U(1)_Y$. The neutral component $N_R^0$ plays the role of dark matter with a mass in the range of 9 to 10 TeV. This scale also sets the lower limit for the scale for the heavy degrees of freedom in $N_R$ and $χ$ which generate light neutrino masses through the radiative seesaw mechanism. The model predicts an $N_R^0$-nucleus scattering cross section that would be accessible with future dark matter direct detection searches as well as observable effects in present and searches for charged lepton flavor violating processes, such as $l_i\to l_j γ$ and $μ- e$ conversion.

hep-ph↗

arXiv2011

Effect on Higgs Boson Decays from Large Light-Heavy Neutrino Mixing in Seesaw Models

In seesaw models with more than one generation of light and heavy neutrinos, nu and N, respectively, it is possible to have sizable mixing between them for heavy-neutrino masses of order 100 GeV or less. We explore this possibility further, taking into account current experimental constraints, and study its effect on Higgs-boson decays in the contexts of seesaw models of types I and III. We find that in the type-I case the Higgs decay into a pair of light and heavy neutrinos, h -> nu N, could increase the total Higgs width in the standard model by up to almost 30% for a relatively light Higgs-boson, which would significantly affect Higgs searches at the LHC. The subsequent prompt decay of N into three light fermions makes this Higgs decay effectively a four-body decay. We further find that, in the presence of the large light-heavy mixing, these four-body Higgs decays can have rates a few times larger than their standard-model counterparts and therefore could provide a potentially important window to reveal the underlying seesaw mechanism.

hep-ph↗

arXiv2010

Spontaneous CP Violating Phase as the Phase in PMNS Matrix

We study the possibility of identifying the CP violating phases in the PMNS mixing matrix in the lepton sector and also that in the CKM mixing matrix in the quark sector with the phase responsible for the spontaneous CP violation in the Higgs potential, and some implications. Since the phase in the CKM mixing matrix is determined by experimental data, the phase in the lepton sector is therefore also fixed. The mass matrix for neutrinos is constrained leading to constraints on the Jarlskog CP violating parameter $J$, and the effective mass $ $ for neutrinoless double beta decay. The Yukawa couplings are also constrained. Different ways of identifying the phases have different predictions for $μ\to e e\bar e$ and $τ\to l_1 l_2 \bar l_3$. Future experimental data can be used to distinguish different models.

hep-ph↗

arXiv2007

Spontaneous CP Violating Phase as The CKM Matrix Phase

We propose that the CP violating phase in the CKM mixing matrix is identical to the CP phases responsible for the spontaneous CP violation in the Higgs potential. A specific multi-Higgs model with Peccei-Quinn (PQ) symmetry is constructed to realize this idea. The CP violating phase does not vanish when all Higgs masses become large. There are flavor changing neutral current (FCNC) interactions mediated by neutral Higgs bosons at the tree level. However, unlike general multi-Higgs models, the FCNC Yukawa couplings are fixed in terms of the quark masses and CKM mixing angles. Implications for meson-anti-meson mixing, including recent data on $D-\bar D$ mixing, and neutron electric dipole moment (EDM) are studied. We find that the neutral Higgs boson masses can be at the order of one hundred GeV. The neutron EDM can be close to the present experimental upper bound.

hep-ph↗