arXiv subjects

Zhong-Lv Huang

Publications and source records attributed to Zhong-Lv Huang.

8 recordsLinked to original sources

arXiv2026

Leptogenesis Determined By Low Energy Parameters

We study thermal leptogenesis in three predictive type-I seesaw models in which the neutrino Dirac mass matrix is equal to the mass matrix of up-type quarks, or down-type quarks, or charged leptons. In this framework, the seesaw relation permits a full reconstruction of the heavy right-handed neutrino mass matrix from low-energy neutrino parameters, which greatly reduces the parameter freedom. A systematic numerical scan based on density matrix Boltzmann equations is performed to examine whether the observed baryon asymmetry of the Universe can be obtained. Successful leptogenesis occurs for normal ordering of light neutrino masses with nonzero Majorana phases. In this case, viable solutions are found in model B, associated with down-type quarks, and model C, associated with charged leptons. Both point to a close-mass pair of heavy neutrinos satisfying $|M_i-M_j|/M_i<10^{-3}$, while remaining outside the conventional quasi-degenerate resonant regime. Four representative benchmark points are selected to show the evolution of the asymmetry and the impact of different treatments of spectator effects. Neutrinoless double beta decay is further studied for all parameter points that can generate an acceptable baryon asymmetry $η_B = (6.12 \pm 0.20)\times 10^{-10}$. The predicted effective Majorana mass for certain cases can be probed by next generation experiments with sub-10 meV sensitivity, such as LEGEND-1000, nEXO, JUNO 50 tons, and CUPID-1T. This framework therefore provides clear targets for future searches.

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arXiv2026

Probing the imaginary parts and their $q^2$ dependences for the tau $g-2$ and EDM

The $τ$ anomalous magnetic dipole moment (MDM) $a_τ= (g-2)_τ/2$ and electric dipole moment (EDM) $d_τ$, are precision probes of electroweak dynamics and possible new physics sources, yet both remain weakly constrained experimentally. Treated as generalized form factors, these quantities exhibit a generic $q^2$ dependence for an off-shell interacting photon. For timelike momentum transfer above the $τ^+τ^-$ threshold, $q^2 = s > 4m_τ^2$, the form factors can acquire absorptive imaginary parts. We investigate how such a $q^2$ dependence and the associated imaginary parts are generated from two complementary perspectives: the model-independent Standard Model Effective Field Theory (SMEFT) and a UV-complete Two-Higgs-Doublet Model (2HDM). The effective framework reveals the intimate correlation between $a_τ$ and $d_τ$. New CP-violating interactions which generate a non-zero $d_τ$, can also generically have non-zero contributions to $a_τ$, thereby deeply linking their phenomenological studies. Within the 2HDM, we demonstrate that sizable imaginary parts and significant $q^2$ running can be generated at levels accessible by $e^+e^-$ colliders. Motivated by these features, we propose experimental methods to extract both the real and imaginary components of the dipole form factors. Utilizing these techniques, we show that Belle II and the Super Tau-Charm Facility (STCF) can improve current bounds on $a_τ$ by more than one order of magnitude. Finally, we highlight that combining measurements across the distinct center-of-mass energies of Belle II and STCF provides a unique, previously unexplored avenue to explicitly obtain information about the $q^2$ evolution of these dipole form factors.

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arXiv2026

Nonstandard Solution for Anomaly Cancellation as Seesaw Neutrino Origin in the SM

For fixed Standard Model (SM) non-Abelian representations of 15 chiral fermions with arbitrary hypercharges, anomaly cancellation admits the usual assignment and a distinct nonstandard solution. In the latter, the colored exotic quark and exotic lepton weak doublets and exotic lepton singlet have zero hypercharge, whereas the two colored exotic quark singlets carry opposite hypercharges $-q$ and $q$. The exotic neutral lepton singlets naturally play the role of the heavy neutrinos. The minimal model with two exotic lepton copies gives a rank-two seesaw, the minimal seesaw model, with one massless active neutrino and predicts $m_{ββ} = 1.2 \text{-} 4.1 \, \mathrm{meV}$ for normal ordering or $15.9\text{-}48.9 \,\mathrm{meV}$ for inverted ordering. In a direct SM realization, generating exotic quark masses through the SM Higgs mechanism forces the exotic quarks to carry electric charges $\pm 1/2$. A separate $\mathrm{SU}(2)_{L'}$ realization of the nonstandard solution can allow exotic quarks from several TeV to $10\,\mathrm{TeV}$ with order-one Yukawa couplings. In this model, the charged exotic quarks and leptons carry electric charges $\pm q$. In both cases, the lightest exotic quark and lepton are stable, but suitable choices of their charges and masses can satisfy experimental constraints.

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arXiv2026

Complex $τ$ Electric Dipole Moment from GeV-Scale New Physics

Among the charged leptons, the $τ$ electric dipole moment~($d_τ$) is the least constrained. We show that the Im[$d_τ$] imposes strong constraints on new physics that have yet to be discussed. Motivated in particular by the Super Tau-Charm Facility (STCF), which will provide a uniquely clean environment for precision $τ$-physics, we study the momentum-transfer dependence of $d_τ(q^2)$ and compare the projected sensitivities of STCF and Belle II. Our analysis shows that an axion-like coupling of the $τ$ lepton can induce sizable real and imaginary components of the EDM. The predicted EDM values may approach the present experimental sensitivities, making them accessible to future measurements at Belle II and the STCF.

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arXiv2025

Constraints on $ΔL=2$ Vector Bosons with Tree Couplings to SM Particles

We investigate phenomenological implications of vector bosons $V$ transforming as (1, 2, -3/2) under the standard model (SM) product gauge group $SU(3)_C$, $SU(2)_L$ and $U(1)_Y$. These vector bosons can couple to two SM leptons at tree-level forming dimension-4 operators. These operators dictate $V$ to have two units of global lepton number, $ΔL = 2$. The operators generated conserve the global lepton number but can violate generational lepton numbers. We study constraints on the couplings $Y$ of $V$ to SM particles using tree-level processes such as $l_α^{-} \to l_β^{+} l_ρ^{-} l_σ^{-}$, muonium and antimuonium oscillation, neutrino trident scattering, inverse muon decay, $e^- e^+ \to l^- l^+$, and also one-loop level processes such as the magnetic dipole moment of a charged lepton and $l_i \to l_j γ$. Strong constraints are obtained from $l_α^{-} \to l_β^{+} l_ρ^{-} l_σ^{-}$ with $\left|Y_{e e} Y^{*}_{μe}\right| < 3.29 \times 10^{-11}\left(m_{V}/ \mathrm{GeV}\right)^{2},\left|Y_{e e} Y^{*}_{e μ}\right| < 3.29 \times 10^{-11}\left(m_{V}/ \mathrm{GeV}\right)^{2}$ and from $l_i \to l_j γ$ with $ \left|Y_{τe}Y_{μτ}^{*}\right|<3.46\times10^{-12}(m_{V}/ \mathrm{GeV})^2, \left|Y_{e τ}Y_{τμ}^{*}\right|<3.46\times10^{-12}\left(m_{V}/ \mathrm{GeV}\right)^2$, respectively. Interestingly, the imaginary part of the coupling constant in our model induces CP violation, which is constrained by experimental limits on the electric dipole moment.

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arXiv2024

The SM expected branching ratio for $h \to γγ$ and an excess for $h \to Z γ$

The recent measurements of $h \to Z γ$ from ATLAS and CMS show an excess of the signal strength $μ_Z = (σ\cdot{\cal B})_{\mathrm{obs}}/(σ\cdot{\cal B})_{\mathrm{SM}}=2.2\pm 0.7$, normalized as 1 in the standard model~(SM). If confirmed, it would be a signal of new physics (NP) beyond the SM. We study NP explanation for this excess. In general, for a given model, it also affects the process $h \to γγ$. Since the measured branching ratio for this process agrees well with the SM prediction, the model is severely constrained. We find that a minimally fermion singlets and doublet extended NP model can explain simultaneously the current data for $h \to Z γ$ and $h\to γγ$. There are two solutions. Although both solutions enhance the amplitude of $h \to Z γ$ to the observed one, in one of the solutions the amplitude of $h \to γγ$ flips sign to give the observ ed branching ratio. This seems to be a contrived solution although cannot be ruled out simply using branching ratio measurements alone. However, we find another solution that naturally enhances $h \to Z γ$ to the measured value, but keeps the amplitude of $h \to γγ$ close to its SM prediction. We also comment on the phenomenology associated with these new fermions.

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arXiv2024

Majorana Phase And Matter Effects In Neutrino Chiral Oscillation

Due to finite masses and mixing, for neutrinos propagation in space-time, there is a chiral oscillation between left- and right- chiral neutrinos, besides the usual oscillation between different generations. The probability of chiral oscillation is suppressed by a factor of $m^2/E^2$ making the effect small for relativistic neutrinos. However, for non-relativistic neutrinos, this effect can be significant. In matter, the equation of motion is modified. When neutrinos produced in weak interaction pass through the matter, the eigen-energies are split into two different ones depending on the helicity of the neutrino. This results in different oscillation behavior for neutrinos with different helicity, in particular there is a new resonant effect related to the helicity state of neutrino different than the usual MSW effect. For Majorana neutrinos, chiral oscillation also depends on Majorana phases.

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arXiv2023

Type-II Seesaw Triplet Scalar Effects on Neutrino Trident Scattering

In Type-II seesaw model, an electroweak triplet scalar field $Δ$ with a non-zero vacuum expectation value (vev) $v_Δ$ is introduced to facilitate the generation of small neutrino masses. A non-zero $v_Δ$ also affects the W mass through the electroweak $ρ$ parameter, making it to be less than 1 as predicted by standard model (SM). The component fields in $Δ$ come along introduce additional contributions to reduce the SM rare neutrino trident scattering cross section. These fields also induce new processes not existed in SM, such as $l_i \to \overline{ l_j} l_k l_l$ and $l_i \to l_j γ$. There are severe constraints on these processes which limit the effects on neutrino trident scattering and the $ρ$ parameter and therefore the W mass. The newly measured W mass by CDF makes the central value of $ρ$ parameter to be larger than 1, even larger than previously expected. Combining neutrinoless double beta decay, direct neutrino mass and oscillation data, we find a lower limit for $v_Δ$ as a function of the triplet scalar mass $m_Δ$, $v_Δ> (6.3 \sim 8.4) \mathrm{eV} (100 \mathrm{GeV}/m_Δ)$. To have significant effect on $ρ$ in this model, $v_Δ$ needs to be in the range of a GeV or so. However this implies a very small $m_Δ$ which is ruled out by data. We conclude that the effect of triplet vev $v_Δ$ on the W mass can be neglected. We also find that at 3$σ$ level, the deviation of the ratio for Type-II Seesaw to SM neutrino trident scattering cross section predictions is reduced to be below 1, but is restricted to be larger than 0.98.

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