arXiv subjects

Ya-Dong Yang

Publications and source records attributed to Ya-Dong Yang.

At least 19 recordsLinked to original sources

arXiv2026

Matching $ν$2HDM onto $ν$SMEFT up to one-loop accuracy

The neutrinophilic two-Higgs-doublet model ($ν$2HDM) represents one of the most economical extensions of the Standard Model (SM) while still giving rise to a rich phenomenology. In this work, we carry out a full tree-level and one-loop matching of this ultraviolet completion onto the SM Effective Field Theory extended with right-handed neutrinos ($ν$SMEFT), including all operators up to mass dimension six. This is achieved by integrating out the heavy scalar doublet at a scale well above the electroweak scale. For the matching procedure, we employ the functional method in combination with the naive dimensional regularization and modified minimal subtraction schemes. After converting from the Green's basis to the minimal, non-redundant Warsaw-extended basis, the effects of high-scale new physics are encoded in the Wilson coefficients of both physical and evanescent operators in the $ν$SMEFT. These coefficients are given in terms of the $ν$2HDM parameters prior to spontaneous symmetry breaking. Our final expressions at the high-energy matching scale are provided as ancillary files on \href{https://github.com/dufewe/v2HDM-to-vSMEFT}{GitHub}. This setup forms the foundation for forthcoming systematic and global analyses of the $ν$2HDM parameter space using effective field theory techniques.

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arXiv2026

Radiative generation of chiral vector operators in $b\to s ν\barν$ transition

The recent Belle II evidence for $B^+ \to K^+ ν\barν$, combined with a suppressed branching fraction ratio $R \equiv Br(B^0 \to K^{*0} ν\barν) / Br(B^+ \to K^+ ν\barν)$, necessitates new physics contributing to both left- and right-handed vector operators. We perform a systematic topological classification of one-loop completions that radiatively generate both operators without tree-level mediation, and construct two minimal benchmark scenarios: a scalar-rich model and a fermion-rich model. Evaluating these frameworks under specific benchmark mass schemes and four distinct flavor structures, we find a generic anti-correlation where enhancing one decay channel typically suppresses the other. A notable exception is a decoupled flavor configuration within the scalar-rich model, which yields simultaneous constructive interference, reducing $R$ by $4.22\%$ while satisfying all complementary flavor bounds. Under a universal coupling assumption, the scalar-rich model yields exact cancellation in the charged mode, highlighting the non-trivial interplay between flavor structure and loop-generated Wilson coefficients.

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arXiv2026

Light top-flavoured dark matter in dark SMEFT: one-loop matching and phenomenology

We study light top-flavoured dark matter (DM) below the electroweak (EW) scale within the dark Standard Model effective field theory (DSMEFT). We perform the one-loop matching of the DSMEFT onto the dark low-energy effective field theory (DLEFT) at the EW scale for all relevant operators that couple scalar, fermionic or vector DM, as well as axion-like particles, exclusively to third-generation quarks. After QCD evolution to the relevant low-energy scales, we match the DLEFT onto the non-relativistic EFT for DM-nucleus scattering. This EFT chain provides a unified framework for phenomenological studies of rare $B$- and $K$-meson decays with missing energy, direct and indirect detection, and the thermal relic abundance. Interpreting the Belle II excess in $B^+\to K^+ν\barν$ as a decay into DM, we find that only the DSMEFT operators contributing at tree level can accommodate it with $O(1)$ couplings for a new-physics scale of $1\,\mathrm{TeV}$. The decay $B^0\to K^{*0}+\mathrm{inv}$ provides the strongest constraint over large portions of the Belle II-favoured parameter space, while $K^+\toπ^++\mathrm{inv}$ can dominate at low and $B_s\to\mathrm{inv}$, $B^+\toπ^++\mathrm{inv}$ and $Υ(1S)\to\mathrm{inv}$ become important at high DM masses, respectively. Direct-detection searches by CRESST-III and DarkSide-50 are competitive for operators involving scalar quark currents and, depending on the operator, exclude the Belle~II-favoured band for $m_{\mathrm{DM}}\gtrsim1$-$2\,\mathrm{GeV}$. We also perform detailed analyses of the thermal relic abundance and the indirect-detection constraints from the CMB and Fermi-LAT.

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arXiv2026

Phenomenological study of diquark effects in $\bar{B}_{(s)}^0 \to D_{(s)}^{(*)+} L^-$ decays

Motivated by the deviations between experimental data and SM predictions for the branching fractions of $\bar{B}_{(s)}^{0} \to D_{(s)}^{(*)+}L^-$ decays with $L=π,K^{(*)},ρ$, we investigate whether the type-IV scalar diquark can offer a plausible explanation for them. By matching the renormalized amplitudes in the full theory onto those in the low-energy effective theory formulated in the diquark operator basis, we extract the corresponding Wilson coefficients up to NLO in $α_s$ at the NP scale $μ_ϕ=M_ϕ$. We then rewrite the obtained operators in the SM basis and apply the renormalization-group running to evolve the Wilson coefficients down to the hadronic scale $μ_b=m_b$. For the hadronic matrix elements of the relevant NP operators, we employ the QCD factorization approach to evaluate the non-factorizable one-loop vertex corrections, and confront our theoretical predictions to the latest data to constrain the diquark parameter space. In the single-coupling case, we find that the purely left-handed coupling $Δ_{d(s)}^{LL}$ can reconcile the discrepancies at $1σ$, while the mixed right-left coupling $Δ_{d(s)}^{RL}$ remains compatible with data only at about $2σ$. Extending to the two-coupling case, we find viable parameter regions as long as $Δ_{d(s)}^{LL}\neq 0$. Incorporating the LHCb measurement of the ratio $R_{π/μν_μ}$ further reduces the allowed range of $Δ_{d}^{LL}$, but does not yield meaningful extra restriction on $Δ_{d}^{RL}$. Hence, while either $Δ_{d(s)}^{LL}$ or $Δ_{d(s)}^{RL}$ can separately provide an acceptable NP interpretation, our phenomenological results favor a scenario dominated by purely left-handed diquark-quark couplings. Finally, we present updated predictions for the branching fractions and the ratios $R_{(s)L}^{(*)}$, which can be probed by forthcoming LHCb and Belle II measurements.

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arXiv2026

Heavy-quark expansion at leading power for local QCD operators in gradient flow

Gradient flow provides finite-flow-time QCD operators, whose matrix elements can be computed on the lattice and related to renormalized QCD operators through perturbative matching. We develop a matching framework that relates local flowed-QCD operators to the corresponding operators in heavy-quark effective theory (HQET) at leading power in the heavy-quark expansion. By combining the flowed-QCD-to-HQET matching relation with conventional QCD-to-HQET matching, we obtain a mass-dependent conversion relation between flowed-QCD operators and renormalized QCD operators. As a first application, we calculate the one-loop matching coefficients for heavy-light currents and derive the corresponding conversion coefficients. We find that heavy-quark mass effects in the matching and conversion coefficients can be numerically important when the flow scale is not much larger than the heavy-quark mass scale, highlighting the need for mass-dependent matching in precision lattice applications of gradient flow to heavy-quark observables.

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arXiv2026

Probing strange-sector flavor-changing neutral currents at DUNE-like facilities

We investigate the sensitivity of DUNE-like facilities to flavor-changing neutral-current interactions between down and strange quarks through inclusive deep-inelastic neutrino scattering. In the framework of a general low-energy effective Lagrangian for Dirac and Majorana neutrinos, we evaluate projected sensitivities for CP- and $τ$-optimized beam configurations. The strongest bounds on the dimensionless Wilson coefficients, defined relative to the Fermi constant $G_F$, reach $\mathcal{O}(10^{-3})\text{--}\mathcal{O}(10^{-2})$ at the near detector, whereas far-detector limits are roughly one order of magnitude weaker. When systematic uncertainties are included, the near-detector sensitivity is dominated by the background-rate uncertainty, while the far-detector sensitivity is limited primarily by the signal-rate uncertainty. At the far detector, the dependence of the projected bounds on the neutrino mass ordering and the Dirac CP-violating phase $δ_{\mathrm{CP}}$ is largely confined to coefficients involving the electron flavor.

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arXiv2026

Probing TeV-Scale Inverse-Seesaw Leptogenesis and Majorana Dark Matter in $U(1)_{B-L}$ Models at Multi-TeV Muon Colliders

We investigate a predictive and testable framework in which dark matter (DM), heavy-neutrino dynamics, and the BAU originate from correlated interactions within a local $U(1)_{B-L}$ extension of the SM. Unlike conventional $B-L$ constructions based on the type-I seesaw, we employ an inverse-seesaw mechanism realized through a sterile fermion $S_{1}$ and a complex scalar field $ϕ$, whose vacuum expectation value simultaneously generates the masses of the heavy neutrinos $N_{1,2}$ and the Majorana DM fermion $χ$ via Yukawa couplings. The small lepton-number-violating parameter induced by a higher-dimensional operator leads to naturally light active neutrinos together with TeV-scale heavy neutrinos and sizable active-sterile mixing, yielding distinctive collider signatures unavailable in minimal $B-L$ models. The relic abundance of $χ$ is governed by annihilation channels mediated by the same scalar and gauge interactions, producing a direct and model-specific correlation between successful leptogenesis and the observed DM relic density. A combined parameter-space analysis incorporating neutrino oscillation data, lepton-flavor-violating processes, direct-detection limits, and collider bounds on $N_{1,2}$ and $Z^\prime$ reveals a narrow yet robust region consistent with all these constraints. Representative benchmark points in this region are examined at a future multi-TeV muon collider. Heavy-neutrino production through electroweak processes yields striking signatures in the dilepton plus missing energy ($2\ell + E\!\!\!/_T$) and single-lepton plus di-jet plus missing energy ($1\ell + 2j + E\!\!\!/_T$) final states. These channels demonstrate that next-generation muon colliders offer a powerful and complementary probe of the inverse-seesaw origin of neutrino masses, the DM relic density, and the TeV-scale leptogenesis within such an extended $U(1)_{B-L}$ framework.

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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

$\bar{B}_{s,d}^{0} \to J/ψμ^{+}μ^{-}$ Decays in QCD Factorization

Motivated by the first LHCb searches for the rare $\bar{B}_{s,d}^{0} \to J/ψμ^{+}μ^{-}$ decays, we perform a detailed study of these processes within the QCD factorization formalism. Since the transverse size of the $J/ψ$ meson is small in the heavy quark mass limit, this formalism is generally expected to hold for these decays. We include both the leading- and next-to-leading-order QCD corrections to the hard-scattering kernels, which are convoluted with the light-cone distribution amplitudes (LCDAs) of the initial- and final-state hadrons. It is numerically found that, depending on the model parameters for the leading-twist $B$-meson LCDA, the maximum branching ratios of $\bar{B}_{s}^{0}\to J/ψμ^{+}μ^{-}$ and $\bar{B}_{d}^{0} \to J/ψμ^{+}μ^{-}$, integrated over the dimuon invariant mass squared $q^2$ from $1\,\mathrm{GeV}^2$ to $(m_{B_{s,d}}-m_{J/ψ})^2$, can reach up to $2.21\times10^{-9}$ and $7.69\times10^{-11}$ at the leading order in $α_s$, respectively. After incorporating the non-factorizable one-loop vertex corrections, these branching ratios are further reduced by about one order of magnitude, with $\mathcal{B}(\bar{B}_{s}^{0} \to J/ψμ^{+}μ^{-})|_{q^2 \geq 1\,\mathrm{GeV}^2}=2.88\times10^{-10}$ and $\mathcal{B}(\bar{B}_{d}^{0} \to J/ψμ^{+}μ^{-})|_{q^2 \geq 1\,\mathrm{GeV}^2}=1.07\times10^{-11}$. In addition, we have presented the dimuon invariant mass distributions of the individual and total helicity amplitudes squared, as well as the differential and integrated longitudinal polarization fractions of the $J/ψ$ meson, which could be probed by the future LHCb and Belle II experiments with more accumulated data.

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arXiv2025

On sum rules for semi-leptonic $b \to c$ and $b \to u$ decays

The semi-leptonic $b \to c l ν$ processes are receiving a lot of attention, as the lepton flavor universality violation has been hinted by the measured ratios $R_{D^{(*)}} = Γ(B \to D^{(*)} τν)/Γ(B \to D^{(*)} \ellν)$ for $\ell = e,μ$. Recently, it has also been pointed out that the baryonic counterpart, $R_{Λ_c} = Γ(Λ_b \to Λ_c τν)/Γ(Λ_b \to Λ_c \ellν)$, has a strong correlation with $R_{D^{(*)}}$, referred to as the R ratio sum rule in this paper. The correlation is almost independent of the new physics (NP) contributions and hence can predict $R_{Λ_c}$ from the measured $R_{D^{(*)}}$. On the other hand, we have fewer measurements and/or theoretical studies of the semi-leptonic $b \to u l ν$ processes, although the same arguments can be applied to the ratios $R_π$, $R_ρ$, and $R_p$ as above. Since these processes are measurable at the ongoing LHCb run-3 and/or Belle~II experiments, precise studies on them are important as well. In this paper, we obtain the semi-analytic formulae for all the aforementioned $R_X$ ratios in the presence of model-independent NP contributions by using the available lattice QCD and/or light-cone sum rule fits to the form factors. Two novel points are highlighted: (i) We evaluate uncertainties of $R_X$ including both the Standard Model (SM) and NP terms, inherited from the form factor fits, and discuss how the uncertainties affect the $R$ ratio sum rules. (ii) We obtain the R ratio sum rule among the semi-leptonic $b \to u l ν$ processes for the first time, which provides a complementary motivation for observing these processes. In addition, based on our model-independent results, we investigate how the different NP scenarios work in the $b\to c$ and $b\to u$ sectors and perform a combined study in the framework of SM effective field theory with specific flavor symmetries.

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arXiv2025

Revisiting $B_{c}^-\to J/ψ(η_c) L^-$ decays within the SM and beyond in QCD factorization

Motivated by the deviations observed between the data and the SM predictions of $\mathcal{B}(\bar{B}_s^0\to D_s^+ π^-)$ and $\mathcal{B}(\bar{B}_d^0\to D^+ K^-)$, we revisit the $B_{c}^{-}\to J/ψ(η_{c}) L^{-}$ decays, with $L=π, K^{(*)}, ρ$, both within the SM and beyond. Since these processes are also mediated by $b\to c \bar{u} d(s)$ transitions and hence dominated by the colour-allowed tree topology, the QCD factorization (QCDF) is expected to hold in the heavy-quark limit. Firstly, we update the SM predictions of these decays by including the nonfactorizable vertex corrections up to the NNLO in $α_s$. It is found that, relative to the LO results, the branching ratios of these decays up to the NLO and NNLO corrections are always enhanced, with a relative amount given by $δ_{\text{NLO}} = (\mathcal{B}^\text{NLO}-\mathcal{B}^\text{LO})/\mathcal{B}^\text{LO} \approx +6\%$ and $δ_{\text{NNLO}} = (\mathcal{B}^\text{NNLO}-\mathcal{B}^\text{LO})/\mathcal{B}^\text{LO} \approx +9\%$, respectively. To minimize the uncertainties brought by $V_{cb}$ and the transition form factors, we construct the ratios $R_{J/ψ(η_{c}) L}$, $R_{(s)L}^{(\ast)}$, and $R_{π/μν_μ}$, which are then used to constrain the model-independent new physics (NP) Wilson coefficients. After considering the latest Belle data and the updated $B_{(s)}\to D_{(s)}^{(*)}$ form factors, we find that the deviations can still be explained by the NP four-quark operators with $(1+γ_{5}) \otimes (1-γ_{5})$ and $(1+γ_{5}) \otimes (1+γ_{5})$ structures, while the solution with $γ^μ(1+γ_{5}) \otimes γ_μ(1-γ_{5})$ structure does not work anymore, under the combined constraints from $R_{(s)L}^{(\ast)}$ at the $2σ$ level. Furthermore, the ratio $R_{π/μν_μ}$, once measured precisely, could provide complementary constraint.

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arXiv2025

Polarization Probes of New Physics in Lepton-Flavor-Violating Hyperon Production from $e^- N \to τ^- Y$ Scattering

While charged lepton-flavor-violating tau decays to strange mesons provide powerful probes of the underlying $τ\to\ell d \bar{s}$ transition, the corresponding decay modes involving hyperons are kinematically forbidden. To address this gap, we propose the quasi-elastic scattering processes $e^- N \to τ^- Y$. Within a general low-energy effective Lagrangian, we perform a comprehensive analysis of the final-state polarizations of both the $τ$ lepton and the hyperon $Y$, systematically addressing the theoretical uncertainty driven by the model dependence of form factors. Our analysis demonstrates that while this uncertainty leads to large variations in predicted rates, the polarization observables can potentially distinguish between the form factor models, and help to determine the chiral nature of the vectorial new physics interaction. Finally, we forecast the event rates for future facilities, revealing a strong dependence on the form factor models, with predicted yields ranging from potentially observable levels to rates far below current detection thresholds.

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arXiv2025

Rare $W \to B_c + γ$ decay up to the NNLO and NLL accuracy in QCD

We perform a detailed theoretical study of the rare radiative decay of the $W$ boson into a $B_c$ meson and an on-shell photon. The decay amplitude is described by two independent form factors, which are calculated up to the next-to-next-to-leading order (NNLO) in QCD within the nonrelativistic QCD (NRQCD) factorization formalism. Since the two typical energy scales, the $W$-boson mass $m_W$ and the $B_c$-meson mass $m_{B_c}$, involved in the process are widely separated, large logarithms of $m_W^2/m_{B_c}^2$ present in the NRQCD short-distance coefficients are also resummed to all orders in $α_s$ up to the next-to-leading logarithmic (NLL) accuracy, by employing the light-cone factorization approach. Taking into account all these corrections, we then perform a phenomenological exploration of this rare decay. It is found that, relative to the leading-order result, the decay width of the process is reduced by the next-to-leading-order and NNLO corrections, with a net effect of $\sim19\%$ and of $\sim31\%$, respectively. Furthermore, the NLL resummation can considerably alter the fixed-order NRQCD predictions, especially for the $\mathcal{O}(α_s)$ correction. We also find that the radiative corrections increase the renormalization scale dependence of the branching fraction, which is however significantly reduced by the NLL resummation. The dependence of the branching fraction on the heavy-quark masses $m_{b,c}$ is also investigated, which shows a monotonic decrease (increase) with $m_c$ ($m_b$).

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arXiv2025

CP asymmetries of $t \to c γ$ and $t \to cg$ decays in the aligned two-Higgs-doublet model

We study the CP asymmetries of the rare top-quark decays $t \to c γ$ and $t \to cg$ in the aligned two-Higgs-doublet model (A2HDM), which is generically characterized by new sources of CP violation beyond the Standard Model (SM). Specifically, the branching ratios and CP asymmetries of these rare top-quark decays are explicitly formulated, with an emphasis on the origins of weak and strong phases in the A2HDM. Taking into account the most relevant constraints on this model, we evaluate the variations of these observables with respect to the model parameters. It is found that the branching ratios of $t \to c γ$ and $t \to cg$ decays can maximally reach up to $1.47\times10^{-10}$ and $4.86\times10^{-9}$ respectively, which are about four and three orders of magnitude higher than the corresponding SM predictions. While the branching ratios are almost independent of the relative phase $φ$ between the two alignment parameters $ς_u$ and $ς_d$ within the allowed parameter space, the CP asymmetries are found to be very sensitive to $φ$. When the two alignment parameters are complex with a non-zero $φ$ varied within the range $[50^\circ,150^\circ]$, the magnitudes of the CP asymmetries can be significantly enhanced relative to both the SM and the real case. In particular, the maximum absolute values of the CP asymmetries can even reach up to $\mathcal{O}(1)$ for these two decay modes, in the range $φ\in [70^\circ,100^\circ]$. These interesting observations could be utilized to discriminate the SM and the different scenarios of the A2HDM.

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arXiv2025

Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet

Scalar singlets under the Standard Model gauge group appear naturally in many well-motivated New Physics scenarios, such as the composite Higgs models. Unlike the Higgs boson in the Standard Model, they can induce large flavour-changing neutral currents (FCNCs) in the top sector. We investigate systematically the effects of a light scalar singlet $S$ with top-quark FCNC couplings, by including the low-energy constraints from the $B_s \to μ^+ μ^-$ decay, the muon anomalous magnetic moment $(g-2)_μ$ and the neutron Electric Dipole Moment (EDM). We also perform a detailed Monte-Carlo simulation of the channel $pp \to t S +j$ with $S \to μ^+ μ^-$ and $S \to b \bar b$, and investigate the LHC sensitivity to the $tcS$ couplings. It is found that the scalar singlet $S$ can induce scalar-type contributions to the $B_s \to μ^+ μ^-$ decay, which do not suffer from the helicity suppression and contain a large CKM factor $V_{cs}^*V_{tb}$. As a result, constraints on the $tcS$ couplings from the measured branching ratio $\mathcal{B}(B_s \to μ^+ μ^-)$ are quite stringent, being even stronger than the expected LHC sensitivity in some parameter spaces. Besides the CP-conserving $tcS$ couplings, we have also considered the case of CP-violating $tcS$ couplings, with $y_{R,L}^{ct}=|y_{R,L}^{ct}|e^{iθ_{R,L}}$. It is found that the CP observables $\mathcal{A}_{ΔΓ_s}^{μμ}$ and $\mathcal{S}_{μμ}$ of the $B_s \to μ^+ μ^-$ decay are sensitive to the phase $θ_R$, while the neutron EDM can provide bounds on the phase difference $θ_L-θ_R$. Therefore, they are complementary to each other in probing the CP phases of the $tcS$ couplings.

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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

Probing new physics with polarization components of the tau lepton in quasielastic $e^- p \to Λ_c τ^-$ scattering process

Kinematics restrict the ability of rare charm decays to explore the charged Lepton Flavor Violation processes mediated by the quark-level $c\to u \ell τ$ transition. To fill the gap, we propose exploring new physics (NP) through the quasielastic scattering process $e^-p\to τ^-Λ_c$ and the polarization of the $τ$ lepton. As analyzing modes for the $τ$ polarization, we consider the decays $τ^-\to π^-ν_τ$, $τ^-\to ρ^-ν_τ$, and $τ^- \to \ell^-\barν_{\ell}ν_τ$, and show that the $τ$ polarization components can be extracted from analyzing the kinematics of the $τ$ visible decay products. In the framework of a general low-energy effective Lagrangian, we then perform a detailed analysis of the polarization components in various aspects and scrutinize possible NP signals. With one upcoming experimental setup, we finally demonstrate promising event rate can be expected for the cascade process and, even in the worst-case scenario -- no signals is observed at all -- it can still provide a competitive potential for constraining the NP, compared with those from the high-$p_T$ dilepton invariant mass tails at high-energy colliders.

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arXiv2024

Deciphering the Belle II data on $B\to K ν\barν$ decay in the (dark) SMEFT with minimal flavour violation

Recently, the Belle II collaboration announced the first measurement of $\mathcal B(B^+\to K^+ν\barν)$, which is found to be about $2.7σ$ higher than the SM prediction. We decipher the data with two new physics scenarios: the underlying $b\to s ν\barν$ transition is, besides the SM contribution, further affected by heavy new mediators that are much heavier than the EW scale, or amended by an additional decay channel with undetected light final states like dark matter or axion-like particles. These two scenarios can be most conveniently analyzed in the SMEFT and the dark SMEFT (DSMEFT) framework, respectively. We consider the flavour structures of the resulting effective operators to be either generic or satisfy the minimal flavour violation (MFV) hypothesis, both for the quark and lepton sectors. In the first scenario, once the MFV is assumed, only one SM-like low-energy effective operator induced by the SMEFT dim-6 operators can account for the Belle II excess, the parameter space of which is, however, excluded by the Belle upper bound on $\mathcal B(B^0\to K^{*0}ν\barν)$. In the second scenario, it is found that the Belle II excess can be accommodated by 22 of the DSMEFT operators involving one or two scalar, fermionic, or vector dark matters as well as ALPs. These operators also receive dominant constraints from the $B^0\to K^{*0}+$inv and $B_s\to$inv decays. In the MFV hypothesis, the number of viable operators is reduced to 14, and the $B^+\toπ^+ +$inv and $K^+\toπ^++$inv decays start to put further constraints. Within the parameter space allowed by all the current experimental data, the $q^2$ distributions (and $F_L$) of the $B\to K^{(*)}+$inv decays are studied for each viable operator. In addition, we, for the first time, calculate systematically the longitudinal polarization fraction $F_L$ of $K^*$ in the $B\to K^*+$inv decays within the DLEFT.

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