arXiv subjects

Feng-Zhi Chen

Publications and source records attributed to Feng-Zhi Chen.

13 recordsLinked to original sources

arXiv2026

$τ^- \to ωπ^- ν_τ$ decay in R$χ$T with tensor sources

We present a study of the $τ^- \to ωπ^-ν_τ$ decay in the framework of low-energy effective field theory. By analyzing the $J^{PG}$ quantum numbers of the quark currents and the $ωπ$ final state, we find that only the Standard Model (SM) vector interaction and the non-standard tensor interaction can contribute to this decay. We construct the resonance chiral theory Lagrangian with external tensor sources and calculate both the vector and tensor form factors, with resonance couplings determined through QCD short-distance constraints, spectral function fitting, and chiral perturbation theory matching. The new physics (NP) effect is investigated in the spectral function and forward-backward asymmetry distributions. Our results show that the spectral function is dominated by the SM, while the forward-backward asymmetry, which can only arise from a non-zero tensor interaction, provides a sensitive probe of this NP effect. Future measurements at Belle II, Tera-Z, and STCF facilities are therefore strongly motivated.

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arXiv2026

Study of $η^\prime \to ηππ$ Decays in Large-$N_C$ Chiral Perturbation Theory

We investigate the $η^\prime \to ηππ$ decays within the framework of large-$N_{C}$ chiral perturbation theory, by calculating the decay amplitudes up to next-to-next-to-leading order in a simultaneous expansion in powers of external momenta, quark masses, and $1/N_C$. Projecting the amplitudes onto partial waves allows us to implement a unitarization procedure to account for the $S$- and $D$-wave $ππ$ final-state interactions. The relevant low-energy constants are determined by fitting our theoretical results to the precise experimental data from the A2 collaboration. A comparison of fits with and without $ππ$ final-state interactions demonstrates that including these effects significantly improves the agreement of our theoretical predictions with the experimental measurements. Consequently, the Dalitz-plot parameters are extracted as $a=-0.085(18)_{\mathrm{stat}}(4)_{\mathrm{syst}}$, $b=-0.081(10)_{\mathrm{stat}}(6)_{\mathrm{syst}}$, and $d=-0.045(6)_{\mathrm{stat}}(8)_{\mathrm{syst}}$. Our results provide therefore a refined theoretical description of the $η^\prime \to ηππ$ decay dynamics.

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arXiv2026

Higgs Decays to $Zγ$ and $γγ$ in the Flavor-Gauged Two Higgs Doublet Model

This work examines the $h\to Zγ$ and $h\toγγ$ decays in the flavor-gauged two Higgs doublet model (FG2HDM), which augments the Standard Model (SM) with an additional scalar doublet, a singlet, and a $U(1)'$ flavor gauge symmetry. Beyond the SM spectrum, FG2HDM predicts five additional physical scalars and a new neutral gauge boson, $Z'$. We demonstrate that while both decay channels are sensitive to charged Higgs loops, $h \to Zγ$ is uniquely modified by fermion-antifermion-$Z$ ($f\bar{f}Z$) vertex corrections. These vertex corrections further impact top-quark observables and the flavor-changing neutral current (FCNC) process $b\to s\ell^+\ell^-$. Our analysis identifies a viable parameter space ($m_{H^\pm}>200$~GeV and $λ_{hH^+H^-}<0$) consistent with current $1σ$ experimental limits, where the signal strength $μ_{γγ}$ remains the primary constraint on scalar sector parameters. Regarding the $f\bar{f}Z$ couplings, we delineate the allowed regions in the $\mathcal{Q}_{tL}$-$\mathcal{Q}_{tR}$ plane by evaluating the leading top-quark contributions, revealing that $b\to s\ell^+\ell^-$ imposes the most stringent bounds. Finally, we highlight that the $14\%$ projected precision for $μ_{Zγ}$ at the High-Luminosity LHC (HL-LHC) will significantly enhance sensitivity to the FG2HDM.

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arXiv2026

Probing neutrino-night-quark effective scalar interactions from neutrino masses

In this work, we use neutrino masses as a probe of the neutrino-light-quark effective scalar interactions. It is found that neutrinos can acquire masses not only from the usual light quark loop corrections but also from the light quark condensates. The latter contribution has been overlooked in the literature. We show that both contributions are comparable for operators involving $u$ and $d$ quarks, while quark loop corrections dominate for operators involving the $s$ quark. Using the low-energy effective field theory extended with light right-handed neutrinos and matching to chiral perturbation theory, we systematically analyze these contributions, deriving constraints on the corresponding Wilson coefficients from neutrino mass bounds, coherent elastic neutrino-nucleus scattering, and light pseudoscalar meson invisible decays. Our analysis shows that electron neutrino mass measurements provide the most stringent constraints on these scalar couplings, significantly improving upon limits from other observables. The results highlight the importance of including both perturbative and nonperturbative contributions in complete phenomenological analyses of neutrino mass generation mechanisms.

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arXiv2024

Correlating $B\to K^{(\ast)} ν\barν$ and flavor anomalies in SMEFT

The recent measurement of $\mathcal{B}(B^+\to K^+ν\barν)$ by Belle-II reveals a $2.8~σ$ deviation from the Standard Model (SM) prediction. Combining this with a prior Belle measurement of $\mathcal{B}(B^{0}\to K^{\ast0}ν\barν)$, the upper bound of the ratio $\mathcal{B}(B^{0}\to K^{\ast0}ν\barν)/\mathcal{B}(B^+\to K^+ν\barν)$ is notably smaller than the SM prediction. In this work, tensions are solved within the framework of Standard Model Effective Field Theory (SMEFT). Flavor observables, described by Low-Energy Effective Field Theory (LEFT) operators, are interconnected by SMEFT at the electroweak scale. Utilizing a set of only four SMEFT operators, the FCNC process $b\to sν\barν$ is correlated with $b\to s\ell^+\ell^-$, $b\to u_i\ell\barν$, $u_j\to s\ell\barν$, $u_j\to u_iν\barν$, and $u_j\to u_i\ell^+\ell^-$. Subsequently, we obtain the latest ranges of Wilson coefficients for these four operators through a global fit that accommodates flavor anomalies such as $R_{K^{(\ast)}}$, $R_{D^{(\ast)}}$, and $\mathcal{B}(B\to K^{(\ast)}ν\barν)$. Our findings reveal that predictions for $\mathcal{B}(B^+\to τ^+ν_τ)$ and $\mathcal{B}(D_s^+\to τ^+ν_τ)$ align well with measured values from Belle and BESIII, based on the fitted coefficients. The predicted branching fraction for $B^0\to K^{\ast0}ν\barν$ is $(1.42\pm 0.74)\times 10^{-5}$, closely approaching the current experimental upper limit. Anticipation surrounds the rare decay $B_s\to τ^+ τ^-$, expected in the near future with a branching fraction on the order of $10^{-4}$.

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arXiv2024

Study of $τ^- \to ωπ^- ν_τ$ decay in resonance chiral theory with tensor sources

In this work, we make a study of the $τ^- \to ωπ^-ν_τ$ decay in the framework of low-energy effective field theory. The $J^{\mathcal{P}G}$ decompositions of the quark currents and the $ωπ$ final state show that, besides the Standard Model vector interaction, only the non-standard tensor interaction can have a non-zero contribution to the decay. To discuss its effect, a reliable calculation of the $ωπ$ tensor form factors is necessary. After constructing the Lagrangian of resonance chiral theory with external tensor sources, we calculate both the vector and tensor form factors with the relevant resonance couplings determined by combining the QCD short-distance constraints, the fit to the spectral function of $τ^- \to ωπ^-ν_τ$ decay, as well as the matching between the $\mathcal{O}(p^4)$ odd-intrinsic-parity operators after integrating out the vector resonances and the $\mathcal{O}(p^6)$ operators of chiral perturbation theory. The new physics effect is then investigated in the distributions of the spectral function and the forward-backward asymmetry of $τ^- \to ωπ^-ν_τ$ decay. We find that the spectral function is dominated by the Standard Model, and the non-standard tensor contribution is negligible. However, since the forward-backward asymmetry can be only generated with a non-zero tensor interaction, the observable is quite sensitive to this kind of new physics. A future measurement of the observable at the Belle II experiment as well as at the proposed Tera-Z and STCF facilities is, therefore, strongly called for to check the existence of such a non-standard tensor interaction.

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arXiv2024

$CP$ asymmetries in $τ\to K_Sπν_τ$ decays

We present here the CP asymmetries in the decay rate and angular distributions of $τ\to K_Sπν_τ$ decays in the Standard Model (SM) and beyond (BSM). The CP asymmetries in the SM are induced by the CP violation in $K^0-\bar{K}^0$ mixing. To investigate the BSM CP-violating (CPV) effects, a model-independent analysis is performed by using the low-energy effective field theory (LEFT) framework at $μ=2$~GeV. If one further assumes the BSM physics to stem from above the electroweak scale, the LEFT shall then be matched onto the SM effective field theory (SMEFT), the operators of which contributing to $τ\to K_Sπν_τ$ decays will also contribute to the neutron electric dipole moment (EDM) and $D^0-\bar{D}^0$ mixing. The stringent bounds from the latter suggest that no remarkable CPV effects can be observed in either the decay rate or the angular distributions. The prospects for future measurements of these observables are also mentioned.

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arXiv2023

The $W\ellν$-vertex corrections to W-boson mass in the R-parity violating MSSM

Inspired by the astonishing $7σ$ discrepancy between the recent CDF-II measurement and the standard model prediction on the mass of $W$-boson, we investigate the $λ'$-corrections to the vertex of $μ\toν_μe\bar{ν_e}$ decay in the context of the $R$-parity violating minimal supersymmetric standard model. These corrections can raise the $W$-boson mass independently. Combined with recent $Z$-pole and kaon decay measurements, $m_W \lesssim 80.37$ GeV can be reached. We find that these vertex corrections cannot explain the CDF result entirely at the $2σ$ and even $3σ$ levels. However, these corrections together with the oblique contributions can be accordant with the CDF-II result and relevant bounds at the $3σ$ level.

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arXiv2022

Nonperturbative effects in neutrino magnetic moments

In this paper, we calculate the QCD nonperturbative contributions of the neutrino-quark tensor operators to the neutrino magnetic moments by matching onto the chiral perturbation theory at low energies. These nonperturbative contributions can be compared to the perturbative ones, which are induced from one-loop mixing when performing the renormalization group evolutions from $μ=m_W$ down to $μ=2~\mathrm{GeV}$. We then constrain the dipole and tensor Wilson coefficients of the low-energy neutrino effective field theory (LNEFT) separately from the neutrino-electron scattering with Borexino data and coherent elastic neutrino-nucleus scattering (CE$ν$NS) with COHERENT data to show the competition between these two contributions, at the renormalization scales $μ=2~\mathrm{GeV}$ and $μ=m_W$ in the $\bar{\mathrm{MS}}$ scheme. In the neutrino-electron scattering, it is found that the nonperturbative contributions dominate for the coefficients involving up and down quarks, while they are expected to be of the same order of magnitude as the perturbative contributions for the coefficients involving strange quark. As for constraints in the CE$ν$NS, the tensor operators can contribute to the process through either direct or indirect way. As a result, the indirect contributions including nonperturbative and perturbative parts for all couplings become negligible in comparison with the direct ones. As the nonperturbative contributions crucially depend on the value of $c_T$, its inputs will affect the extraction of limits on the tensor LNEFT Wilson coefficients. We compute the upper bounds on these coefficients with $c_T$ quoting from the model and lattice estimates.

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arXiv2022

Explaining anomalies of $B$-physics, muon $g-2$ and $W$ mass in $R$-parity violating MSSM with seesaw mechanism

The recent experimental results including $R_{K^{(\ast)}}$, $R_{D^{(\ast)}}$, $(g-2)_μ$ and $W$ mass show deviations from the standard model (SM) predictions, implying the clues of new physics (NP). In this work, we investigate the explanations of these anomalies in the $R$-parity violating minimal supersymmetric standard model (RPV-MSSM) extended with the inverse seesaw mechanism. The non-unitarity extent $η_{ee}$ and the loop corrections from the interaction $λ'\hat L \hat Q \hat D$ are utilized to raise the prediction of $W$ mass through muon decays. We also find that the interaction $λ'\hat L \hat Q \hat D$ involved with right-handed (RH)/singlet (s)neutrinos can explain the $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$ anomalies simultaneously when considering nonzero $λ'_{1jk}$. For nonzero $λ'_{2jk}$, this model fulfills the whole $b\to s\ell^+\ell^-$ fit but cannot be accordant with $R_{D^{(\ast)}}$ measurements. The explanations in both cases are also favored by $(g-2)_μ$ data, neutrino oscillation data and the relevant constraints we scrutinized. Furthermore, this model framework can be tested in future experiments covering, e.g., the predicted lepton flavor violations (LFV) at Belle II and the Future Circular Collider with $e^+e^-$ beams (FCC-ee), as well as the heavy neutrinos at future colliders.

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arXiv2022

$CP$ asymmetry in the angular distributions of $τ\to K_Sπν_τ$ decays -- II: general effective field theory analysis

We proceed to study the $CP$ asymmetry in the angular distributions of $τ\to K_Sπν_τ$ decays within a general effective field theory framework including four-fermion operators up to dimension-six. It is found that, besides the commonly considered scalar-vector interference, the tensor-scalar interference can also produce a non-zero $CP$ asymmetry in the angular distributions. Bounds on the effective couplings of the non-standard scalar and tensor interactions are obtained under the combined constraints from the measured $CP$ asymmetries and the branching ratio of $τ^-\to K_Sπ^-ν_τ$ decay, with $\mathrm{Im}[\hatε_S]=-0.008\pm0.027$ and $\mathrm{Im}[\hatε_T]=0.03\pm0.12$, at the scale $μ_τ=2~\mathrm{GeV}$ in the $\mathrm{\overline{MS}}$ scheme. Using the best-fit values, we also find that the distributions of the $CP$ asymmetries can deviate significantly from the SM expectation in almost the whole $Kπ$ invariant-mass region. Nevertheless, the current bounds are still plagued by large experimental uncertainties, but will be improved with more precise measurements from Belle II as well as the proposed Tera-Z and STCF facilities. Assuming further that the non-standard scalar and tensor interactions originate from a weakly-coupled heavy new physics well above the electroweak scale, the $SU(2)_L$ invariance of the resulting SMEFT Lagrangian would indicate that very strong limits on $\mathrm{Im}[\hatε_S]$ and $\mathrm{Im}[\hatε_T]$ could also be obtained from the neutron electric dipole moment and the $D^0-\bar{D}^0$ mixing. With the bounds from these processes taken into account, it is then found that, unless there exist extraordinary cancellations between the new physics contributions, neither the scalar nor the tensor interaction can produce any significant effects on the $CP$ asymmetries in the processes considered.

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arXiv2020

$CP$ asymmetry in the angular distribution of $τ\to K_Sπν_τ$ decays

In this work, we study the $CP$ asymmetry in the angular distribution of $τ\to K_Sπν_τ$ decays, taking into account the known $CP$ violation in $K^0-\bar{K}^0$ mixing. It is pointed out for the first time that, once the well-measured $CP$ violation in the neutral kaon system is invoked, a non-zero $CP$ asymmetry would appear in the angular observable of the decays considered, even within the Standard Model. By employing the reciprocal basis, which is most convenient when a $K_{S(L)}$ is involved in the final state, the $CP$-violating angular observable is derived to be two times the product of the time-dependent $CP$ asymmetry in $K\to π^+π^-$ and the mean value of the angular distribution in $τ^\pm\to K^0(\bar{K}^0)π^\pm\barν_τ(ν_τ)$ decays. Compared with the Belle results measured in four different bins of the $Kπ$ invariant mass, our predictions lie within the margins of these measurements, except for a $1.7~σ$ deviation for the lowest mass bin. While being below the current Belle detection sensitivity that is of $\mathcal{O}(10^{-3})$, our predictions are expected to be detectable at the Belle II experiment, where $\sqrt{70}$ times more sensitive results will be obtained with a $50~\text{ab}^{-1}$ data sample.

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arXiv2019

CP asymmetry in $τ\to K_Sπν_τ$ decays within the Standard Model and beyond

Motivated by the $2.8σ$ discrepancy observed between the BaBar measurement and the Standard Model prediction of the CP asymmetry in $τ\to K_Sπν_τ$ decays, as well as the prospects of future measurements at Belle II, we revisit this observable in this paper. Firstly, we reproduce the known CP asymmetry due to $K^0 -\bar{K}^0$ mixing by means of the reciprocal basis, which is convenient when a $K_{S(L)}$ is involved in the final state. As the $Kπ$ tensor form factor plays a crucial role in generating a non-zero direct CP asymmetry that can arise only from the interference of vector and tensor operators, we then present a dispersive representation of this form factor, with its phase obtained in the context of chiral theory with resonances, which fulfills the requirements of unitarity and analyticity. Finally, the $τ\to K_Sπν_τ$ decays are analyzed both within a model-independent low-energy effective theory framework and in a scalar leptoquark scenario. It is observed that the CP anomaly can be accommodated in the model-independent framework, even at the $1σ$ level, together with the constraint from the branching ratio of $τ^-\to K_Sπ^-ν_τ$ decay; it can be, however, marginally reconciled only at the $2σ$ level, due to the specific relation between the scalar and tensor operators in the scalar leptoquark scenario. Once the combined constraints from the branching ratio and the decay spectrum of this decay are taken into account, these possibilities are however both excluded, even without exploiting further the stronger bounds from the (semi-)leptonic kaon decays under the assumption of lepton-flavour universality, as well as from the neutron electric dipole moment and $D-\bar{D}$ mixing under the assumption of $SU(2)$ invariance of the weak interactions.

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