arXiv subjects

Quan-Yi Hu

Publications and source records attributed to Quan-Yi Hu.

14 recordsLinked to original sources

arXiv2026

Pseudoscalar meson $P\to τ(\to πν_τ, ρν_τ, \ell \barν_\ell ν_τ) \barν_τ$ decays in the Standard Model and beyond

In this work, we have conducted a comprehensive and systematic theoretical investigation of the full cascade decays of charged pseudoscalar mesons, specifically $D_s$, $D$, $B$, and $B_c$, into $τν_τ$, followed by the subsequent decay of the $τ$ via its dominant experimentally reconstructible channels: $τ\to πν_τ$, $τ\to ρν_τ$, $τ\to e \barν_e ν_τ$, and $τ\to μ\barν_μν_τ$. Our study is framed within the model-independent low-energy effective field theory approach, which incorporates the most general set of four-fermion operators, including those coupling to right-handed neutrinos. We provide precise Standard Model predictions for differential decay rate as a function of the final-state charged particle energy, develop an innovative and robust methodology for extracting the magnitudes of the new physics couplings using energy moments, and identify and characterize fixed points in the normalized energy distributions. The fixed points are invariant under new physics contributions described by the considered effective Hamiltonian.

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arXiv2025

The influence of invisible light particles on $Λ_b \to ΛE_{\mathrm{miss}}$

In this work, we study the contribution of invisible light particles to $Λ_b \to ΛE_{\mathrm{miss}}$, particularly the three-body decays $Λ_b \to Λϕ\barϕ$ and $Λ_b \to Λψ\barψ$. The differential branching ratio of $Λ_b \to ΛE_{\mathrm{miss}}$ and the $q^2$-dependent longitudinal polarization asymmetry of $Λ$ in scenarios explaining the Belle II excess are presented. In the chiral basis, we investigate the correlations between $\mathcal{B}(B \to K E_{\mathrm{miss}})$ and $\mathcal{B}(Λ_b \to ΛE_{\mathrm{miss}})$, as well as between $\mathcal{B}(B \to K E_{\mathrm{miss}})$ and $P^Λ_{L}$, in eight distinct new physics scenarios. We find that the $P^Λ_{L}$ can be used to distinguish the chirality of the hadronic current part in the effective operators, which is similar to the cases in the two-body decays $Λ_b \to Λϕ$ and $Λ_b \to ΛV$.

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arXiv2025

Are the new particles heavy or light in $b \to s E_{\mathrm{miss}}$?

In this work, we study the $B^+\to K^+ E_{\mathrm{miss}}$, $B^0\to K^{*0} E_{\mathrm{miss}}$, and $Λ_b^0\to Λ^0 E_{\mathrm{miss}}$ decays under three different new physics hypotheses: the heavy new particles, the light neutral vector particles, and the axion-like particles. We find that all three hypotheses can resolve the Belle-II excess, and they can be clearly distinguished by the longitudinal polarization fraction of $K^*$. Furthermore, we discover that the longitudinal polarization fraction of $Λ$ can be used to distinguish the chirality of the effective operators.

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arXiv2024

Disentangling left-handed and right-handed neutrino effects in $B_c \to τ\barν_τ$ decay

Inspired by the anomalies in $B\to D^{(*)} τ\barν_τ$ decay, in this work, we carefully study the influence of the most general dimension-six effective operators involving right-handed neutrinos on the four cascade decays $B_c \to τ(\to ν_τh ) \barν_τ$ ($h=π,ρ$) and $B_c \to τ(\to ν_τ\ell \barν_\ell) \barν_τ$ ($\ell =μ, e$). We calculate for the first time the analytical results of the distributions of the differential decay rates of these cascade decays with respect to the energy of the charged particles in the final state. We select a total of 13 new physics benchmark points, among which 9 are derived from the effect of purely left-handed neutrinos, and the other 4 are from the effect of purely right-handed neutrinos. We find that the benchmark points BP4, BP6, BP11, and BP13 can significantly increase the values of these distributions while the BP3 causes a sharp decrease in these distributions. The end-point behavior of the differential distribution $dΓ/dE_π$ can be used to disentangle the effects of left-handed and right-handed neutrinos. Finally, in order to eliminate the uncertainties brought by the CKM matrix element $V_{cb}$ and the decay constant $f_{B_c}$, we introduce the normalized distributions $dΓ/(ΓdE_a)$ ($a=π,ρ,μ,e$), which are only sensitive to the right-handed neutrinos. We find that in each normalized distribution there exists a fixed point that is not related to any dimension-six effective operators considered in this work.

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arXiv2021

New physics in the angular distribution of $B_c^- \to J/ψ(\to μ^+ μ^-)τ^- (\to π^- ν_τ)\barν_τ$ decay

In $B_c^- \to J/ψ(\to μ^+ μ^-)τ^-\barν_τ$ decay, the three-momentum $\boldsymbol{p}_{τ^-}$ cannot be determined accurately due to the decay products of $τ^-$ inevitably include an undetected $ν_τ$. As a consequence, the angular distribution of this decay cannot be measured. In this work, we construct a {\it measurable} angular distribution by considering the subsequent decay $τ^- \to π^- ν_τ$. The full cascade decay is $B_c^- \to J/ψ(\to μ^+ μ^-)τ^- (\to π^- ν_τ)\barν_τ$, in which the three-momenta $\boldsymbol{p}_{μ^+}$, $\boldsymbol{p}_{μ^-}$, and $\boldsymbol{p}_{π^-}$ can be measured. The five-fold differential angular distribution containing all Lorentz structures of the new physics (NP) effective operators can be written in terms of twelve angular observables $\mathcal{I}_i (q^2, E_π)$. Integrating over the energy of pion $E_π$, we construct twelve normalized angular observables $\widehat{\mathcal{I}}_i(q^2)$ and two lepton-flavor-universality ratios $R(P_{L,T}^{J/ψ})(q^2)$. Based on the $B_c \to J/ψ$ form factors calculated by the latest lattice QCD and sum rule, we predict the $q^2$ distribution of all $\widehat{\mathcal{I}}_i$ and $R(P_{L,T}^{J/ψ})$ both within the Standard Model and in eight NP benchmark points. We find that the benchmark BP2 (corresponding to the hypothesis of tensor operator) has the greatest effect on all $\widehat{\mathcal{I}}_{i}$ and $R(P_{L,T}^{J/ψ})$, except $\widehat{\mathcal{I}}_{5}$. The ratios $R(P_{L,T}^{J/ψ})$ are more sensitive to the NP with pseudo-scalar operators than the $\widehat{\mathcal{I}}_{i}$. Finally, we discuss the symmetries in the angular observables and present a model-independent method to determine the existence of tensor operators.

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arXiv2021

The measurable angular distribution of $Λ_b^0 \to Λ_c^+ (\to Λ^0 π^+)τ^- (\to π^- ν_τ)\barν_τ$ decay

In $Λ_b^0 \to Λ_c^+ (\to Λ^0 π^+) τ^- \barν_τ$ decay, the solid angle of the final-state particle $τ^-$ cannot be determined precisely since the decay products of the $τ^-$ include an undetected $ν_τ$. Therefore, the angular distribution of this decay cannot be measured. In this work, we construct a {\it measurable} angular distribution by considering the subsequent decay $τ^- \to π^- ν_τ$. The full cascade decay is $Λ_b^0 \to Λ_c^+ (\to Λ^0 π^+)τ^- (\to π^- ν_τ)\barν_τ$. The three-momenta of the final-state particles $Λ^0$, $π^+$, and $π^-$ can be measured. Considering all Lorentz structures of the new physics (NP) effective operators and an unpolarized initial $Λ_b$ state, the five-fold differential angular distribution can be expressed in terms of ten angular observables ${\cal K}_i (q^2, E_π)$. By integrating over some of the five kinematic parameters, we define a number of observables, such as the $Λ_c$ spin polarization $P_{Λ_c}(q^2)$ and the forward-backward asymmetry of $π^-$ meson $A_{FB}(q^2)$, both of which can be represented by the angular observables $\widehat{\cal K}_i (q^2)$. We provide numerical results for the entire set of the angular observables $\widehat{\cal K}_i (q^2)$ and $\widehat{\cal K}_i$ both within the Standard Model and in some NP scenarios, which are a variety of best-fit solutions in seven different NP hypotheses. We find that the NP which can resolve the anomalies in $\bar{B} \to D^{(*)} τ^- \barν_τ$ decays has obvious effects on the angular observables $\widehat{\cal K}_i (q^2)$, except $\widehat{\cal K}_{1ss} (q^2)$ and $\widehat{\cal K}_{1cc} (q^2)$.

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arXiv2020

Probing new physics in semileptonic $Σ_b$ and $Ω_b$ decays

Recently, several hints of lepton non-universality have been observed in the semileptonic B meson decays in terms of both in the neutral current ($b\to s l \bar{l}$) and charged current ($b\to c l \bar{ν_{l}}$) transitions. Motivated by these inspiring results, we perform the analysis of the baryon decays ${Σ_b}\to Σ_c l \bar{ν_{l}}$ and $Ω_b\to Ω_c l \bar{ν_{l}} (l=e,μ,τ)$ which are mediated by $ b \to c l \bar{ν_{l}}$ transitions at the quark level, to scrutinize the nature of new physics (NP) in the model independent method. We first use the experimental measurements of $\mathcal{B}( B\to D^{(*)} l\bar{ν_{l}})$, ${\rm R}_{D^{(*)}}$ and ${\rm R}_{J/ψ}$ to constrain the NP coupling parameters in a variety of scenarios. Using the constrained NP coupling parameters, we report numerical results on various observables related to the processes ${Σ_b}\to Σ_c l \bar{ν_{l}}$ and $Ω_b\to Ω_c l \bar{ν_{l}}$, such as the branching ratios, the ratio of branching fractions, the lepton side forward-backward asymmetries, the hadron and lepton longitudinal polarization asymmetries and the convexity parameter. We also provide the $q^2$ dependency of these observables and we hope that the corresponding numerical results in this work will be testified by future experiments.

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arXiv2020

Revisiting the $B$-physics anomalies in $R$-parity violating MSSM

In recent years, several deviations from the Standard Model predictions in semileptonic decays of $B$-meson might suggest the existence of new physics which would break the lepton-flavour universality. In this work, we have explored the possibility of using muon sneutrinos and right-handed sbottoms to solve these $B$-physics anomalies simultaneously in $R$-parity violating minimal supersymmetric standard model. We find that the photonic penguin induced by exchanging sneutrino can provide sizable lepton flavour universal contribution due to the existence of logarithmic enhancement for the first time. This prompts us to use the two-parameter scenario $(C^{\rm V}_9, \, C^{\rm U}_9)$ to explain $b \to s \ell^+ \ell^-$ anomaly. Finally, the numerical analyses show that the muon sneutrinos and right-handed sbottoms can explain $b \to s \ell^+ \ell^-$ and $R(D^{(\ast)})$ anomalies simultaneously, and satisfy the constraints of other related processes, such as $B \to K^{(\ast)} ν\barν$ decays, $B_s-\bar B_s$ mixing, $Z$ decays, as well as $D^0 \to μ^+ μ^-$, $τ\to μρ^0$, $B \to τν$, $D_s \to τν$, $τ\to K ν$, $τ\to μγ$, and $τ\to μμμ$ decays.

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arXiv2020

Explaining $b\to s \ell^+ \ell^-$ data by sneutrinos in the $R$-parity violating MSSM

The recent measurements on $b\to s \ell^+ \ell^-$ processes suggest the existence of lepton-flavour-universality breaking new physics. In this work, we have explored the possibility of explaining these data by sneutrinos in the $R$-parity violating minimal supersymmetric standard model. We study the light sneutrinos, of order 1 TeV, and suppose that the rest of the sfermions are much heavier than them. This setup can solve the $b \to s μ^+ μ^-$ anomaly well, and it is almost unconstrained by other related processes, such as $B_s-\bar{B}_s$ mixing, as well as $B_s^0 \to τ^+ τ^-$, $B^+ \to K^+ τ^+ τ^-$, $B_s^0 \to τ^\pm μ^\mp$, $B^+ \to K^+ τ^\pm μ^\mp$, and $B \to K^{(\ast)} ν\barν$ decays.

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arXiv2019

$B_{s(d)}-\bar{B}_{s(d)}$ Mixing and $B_s\toμ^+μ^-$ Decay in the NMSSM with the Flavour Expansion Theorem

In this paper, motivated by the observation that the Standard Model predictions are now above the experimental data for the mass difference $ΔM_{s(d)}$, we perform a detailed study of $B_{s(d)}-\bar{B}_{s(d)}$ mixing and $B_s\toμ^+μ^-$ decay in the $\mathbb{Z}_3$-invariant NMSSM with non-minimal flavour violation, using the recently developed procedure based on the Flavour Expansion Theorem, with which one can perform a purely algebraic mass-insertion expansion of an amplitude written in the mass eigenstate basis without performing any diagrammatic calculations in the interaction/flavour basis. Specifically, we consider the finite orders of mass insertions for neutralinos but the general orders for squarks and charginos, under two sets of assumptions for the squark flavour structures (\textit{i.e.}, while the flavour-conserving off-diagonal element $δ_{33}^\text{LR}$ is kept in both of these two sectors, only the flavour-violating off-diagonal elements $δ_{23}^\text{LL}$ and $δ_{i3}^\text{RR}$ ($i=1,2$) are kept in the \text{LL} and \text{RR} sectors, respectively). Our analytic results are then expressed directly in terms of the initial Lagrangian parameters in the interaction/flavour basis, making it easy to impose the experimental bounds on them. It is found numerically that the NMSSM effects with the above two assumptions for the squark flavour structures can accommodate the observed deviation for $ΔM_{s(d)}$, while complying with the experimental constraints from the branching ratios of $B_s\to μ^+ μ^-$ and $B\to X_sγ$ decays.

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arXiv2019

$b\to cτν$ Transitions in the Standard Model Effective Field Theory

The $R(D^{(\ast)})$ anomalies observed in $B\to D^{(\ast)}τν$ decays have attracted much attention in recent years. In this paper, we study the $B\to D^{(\ast)}τν$, $Λ_b\toΛ_cτν$, $B_c\to (J/ψ,\,η_c)τν$, $B\to X_cτν$, and $B_c\toτν$ decays, all being mediated by the same quark-level $b\to cτν$ transition, in the Standard Model Effective Field Theory. The most relevant dimension-six operators for these processes are $Q_{lq}^{(3)}$, $Q_{ledq}$, $Q^{(1)}_{lequ}$, and $Q^{(3)}_{lequ}$ in the Warsaw basis. Evolution of the corresponding Wilson coefficients from the new physics scale $Λ=1$~TeV down to the characteristic scale $μ_b\simeq m_b$ is performed at three-loop in QCD and one-loop in EW/QED. It is found that, after taking into account the constraint ${\cal B}(B_c\toτν)\lesssim 10\%$, a single $\left[C_{lq}^{(3)}\right]_{3323}(Λ)$ or $\left[C^{(3)}_{lequ}\right]_{3332}(Λ)$ can still be used to resolve the $R(D^{(\ast)})$ anomalies at $1σ$, while a single $\left[C^{(1)}_{lequ}\right]_{3332}(Λ)$ is already ruled out by the measured $R(D^{(\ast)})$ at more than $3σ$. By minimizing the $χ^2(C_i)$ function constructed based on the current data on $R(D)$, $R(D^\ast)$, $P_τ(D^\ast)$, $R(J/ψ)$, and $R(X_c)$, we obtain eleven most trustworthy scenarios, each of which can provide a good explanation of the $R(D^{(\ast)})$ anomalies at $1σ$. To further discriminate these different scenarios, we predict thirty-one observables associated with the processes considered under each NP scenario. It is found that most of the scenarios can be differentiated from each other by using these observables and their correlations.

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arXiv2019

R-parity violating solutions to the $R_{D^{(\ast)}}$ anomaly and their GUT-scale unifications

Recently, several $B$-physics experiments report interesting anomalies in the semi-leptonic decays of $B$-mesons, such as the excess in the $R_{D^{(\ast)}}$ measurements. These anomalies seem to suggest intriguing hints of lepton flavor non-universality, and the R-parity violating (RPV) interactions are candidates for explaining this non-universality. In this paper, we discuss the RPV interactions for resolving the $R_{D^{(\ast)}}$ anomaly with the Grand Unified Theory (GUT) assumption. To solve the $R_{D^{(\ast)}}$ anomaly, it is known that large RPV couplings and around $1~{\rm TeV}$ sfermion masses are required. At the same time, large RPV couplings are conducive to realize the bottom-tau Yukawa unification which appears in the GUT models. On the other hand, there are problems for realizing favorable sfermion masses in the constrained minimal supersymetric standard model. To resolve these problems, we show that two non-universalities, the non-universal sfermion masses and the non-universal gaugino masses, are favorable.

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arXiv2017

The $Λ_b\to Λ(\to pπ^-)μ^+μ^-$ decay in the aligned two-Higgs-doublet model

The rare baryonic decay $Λ_b\to Λ(\to pπ^-)μ^+μ^-$ provides valuable complementary information compared to the corresponding mesonic $b\to sμ^+μ^-$ transition. In this paper, using the latest high-precision lattice QCD calculation of the $Λ_b\toΛ$ transition form factors, we study this interesting decay within the aligned two-Higgs-doublet model, paying particularly attention to effects of the chirality-flipped operators generated by the charged scalars. In order to extract the full set of angular coefficients in this decay, we consider the following ten angular observables that can be derived from the analysis of the subsequent parity-violating $Λ\to pπ^-$ decay: the differential branching fraction ${\rm d}{\cal B}/{\rm d}q^2$, the longitudinal polarization fraction $F_L$, the lepton-, hadron- and combined lepton-hadron-side forward-backward asymmetries $A_{\rm FB}^\ell$, $A_{\rm FB}^Λ$ and $A_{\rm FB}^{\ellΛ}$, as well as the other five asymmetry observables $Y_i$~($i={\rm 2,\,3s,\,3sc,\,4s,\,4sc}$). Detailed numerical comparisons are made between the SM and NP values for these angular observables. It is found that, under the constraints from the inclusive $B\to X_sγ$ branching fraction and the latest global fit results of $b\to s\ell\ell$ data, the contributions of right-handed semileptonic operators $O^{\prime}_{9,10}$, besides reconciling the $P_5^{\prime}$ anomaly observed in $B^0\to K^{\ast 0}μ^+μ^-$ decay, could also enhance the values of ${\rm d}{\cal B}/{\rm d}q^2$ and $A_{\rm FB}^\ell$ in the bin $[15,20]~{\rm GeV}^2$, leading to results consistent with the current LHCb measurements.

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arXiv2017

$B^0\to K^{\ast 0}μ^+μ^-$ Decay in the Aligned Two-Higgs-Doublet Model

In the aligned two-Higgs-doublet model, we perform a complete one-loop computation of the short-distance Wilson coefficients $C_{7,9,10}^{(\prime)}$, which are the most relevant ones for $b\to s\ell^+\ell^-$ transitions. It is found that, when the model parameter $\left|ς_u\right|$ is much smaller than $\left|ς_d\right|$, the charged-scalar contributes mainly to chirality-flipped $C_{9,10}^\prime$, with the corresponding effects being proportional to $\left|ς_d\right|^2$. Numerically, the charged-scalar effects fit into two categories: (A) $C_{7,9,10}^\mathrm{H^\pm}$ are sizable, but $C_{9,10}^{\prime\mathrm{H^\pm}}\simeq0$, corresponding to the (large $\left|ς_u\right|$, small $\left|ς_d\right|$) region; (B) $C_7^\mathrm{H^\pm}$ and $C_{9,10}^{\prime\mathrm{H^\pm}}$ are sizable, but $C_{9,10}^\mathrm{H^\pm}\simeq0$, corresponding to the (small $\left|ς_u\right|$, large $\left|ς_d\right|$) region. Taking into account phenomenological constraints from the inclusive radiative decay $B\to X_sγ$, as well as the latest model-independent global analysis of $b\to s\ell^+\ell^-$ data, we obtain the much restricted parameter space of the model. We then study the impact of the allowed model parameters on the angular observables $P_2$ and $P_5'$ of $B^0\to K^{\ast 0}μ^+μ^-$ decay, and find that $P_5'$ could be increased significantly to be consistent with the experimental data in case B.

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