arXiv subjects

Hiroyuki Umeeda

Publications and source records attributed to Hiroyuki Umeeda.

At least 19 recordsLinked to original sources

arXiv2026

Polarized observables in $\tau \to VV \nu_{\tau}$ decays

We study polarization observables in $\tau^- \to V_1^- V_2^0 \nu_\tau$ decays for $(V_1^-, V_2^0)=(\rho^-, \rho^0), (\rho^-, \omega), (K^{*-}, \rho^0), (K^{*-}, \omega), (\rho^-, \bar{K}^{*0})$ using the helicity formalism within the generalized hidden local symmetry framework. The predicted branching fractions are comparable with those from the effective chiral model, but smaller than those from the angular momentum algebra model. Our results for the longitudinal polarization fractions $f_L$ indicate that nonnegligible corrections to the endpoint limit ($f_L = 1/3$), a consequence of Lorentz symmetry, are present at the scale of tau mass. Future measurements of $f_L$ at Belle II for $\tau^- \to V_1^- V_2^0 \nu_\tau$ decays will provide crucial information on polarization patterns observed in nonleptonic charm decays.

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arXiv2026

$b\to c \bar u q$ decay and CP violating observables in the presence of new physics contributions

In this work, a comprehensive analysis for processes related to $b\to c\bar{u}q~(q=d, s)$ transitions are carried out, including new physics contributions. In light of a recent tension between branching fractions for $B_{(s)}\to D_{(s)}^{(*)}M$ ($M$ represents a meson) decays in the QCD factorization approach and relevant experimental results, phenomenological constraints on complex-valued Wilson coeffients are discussed. Analyzed observables contain direct CP asymmetry ($A_{\text{CP}}$) in $B^-\to D^0\pi^-$ decays and $\gamma/\phi_3$, one of the angles in the unitarity triangle, combined with others from $\tau_{B^+}/\tau_{B_d}$, $\Delta\Gamma_{q}/\Gamma_q$, and $A_{\rm SL}^{q}~(q=d, s)$. We constrain the complex Wilson coefficients at $1\sigma$ and $2\sigma$ levels under color-singlet and color-rearranged scenarios. These constraints yield correlated predictions for $\Delta\Gamma_d/\Gamma_d$, $A_{\rm SL}^d$ and $A_{\text{CP}}$.

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arXiv2025

$D^0-\bar{D}^0$ mixing in the Dyson-Schwinger approach

In view of difficulty to reproduce observables in the $D^0-\bar{D}^0$ mixing via the operator product expansion, we discuss the Dyson-Schwinger approach to this process. Formulated by the parametrization of quark propagators, SU(3) breaking relevant to charm mixing is evaluated in such a way that properly takes account of dynamical chiral symmetry breaking. The $\bar{D}^0\to D^0$ transition is discussed in the vacuum-insertion approximation with locality of the light valence-quark field, represented by the decay constant of $D^0$ meson as well as relevant momentum integrals. It is found that dimensionless mass-difference observable in this approach leads to $|x|=(1.3-2.9)\times 10^{-3}$, the order of magnitude comparable to the HFLAV data, and thereby offering a certain improvement as a theoretical framework.

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arXiv2024

$B\to X_c\ell\bar{\nu}$ Decay as a Probe of Complex Conjugate Poles

In this work, nontrivial analytic structure of the quark propagator is discussed for $B$-meson inclusive decays. Attributed to invalidity of the standard K\"{a}ll\'{e}n-Lehman spectral representation, complex conjugate poles alter the evaluation of decay rates, which lead to violation of quark-hadron duality. As phenomenological observables, widths in $B\to X_c\ell\bar{\nu}$ decay as well as the lifetime of the $B_d^0$ meson are discussed. In the presence of the mentioned nonanalytical contributions, the possibility for resolving the $|V_{cb}|$ puzzle is addressed. It is demonstrated that there exists a parameter region, which explains $|V_{cb}|$ and $B_d^0$-meson lifetime simultaneously within 1$\sigma$ via the complex conjugate poles from a charm quark.

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arXiv2024

$B_{(s)}\to D_{(s)}^{(*)}M$ decays in the presence of final-state interaction

In light of the recent data for $\bar{B}_{(s)}\to D^{(*)}_{(s)}P$ and $\bar{B}_{(s)}\to D_{(s)}V$ decays, we perform a model-independent phenomenological analysis in the presence of quasi-elastic rescattering. With the Wilson coefficients including contributions beyond the standard model, lifetimes of $B$ meson as well as the $B^0_d-\bar{B}^0_d$ mixing are investigated for clarifying correlations among the observables. We show that parameter regions for quasi-elastic rescattering, the size of color-suppressed tree amplitudes and new physics are constrained due to the lifetime data. As a consequence, it is revealed that this scenario can be testable by the future LHCb measurement of width difference in $B^0_d-\bar{B}^0_d$ mixing and semi-leptonic CP asymmetry.

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arXiv2022

Is right-handed current contribution to $\bar{B}\to X_ulν$ decays corrected by non-trivial vacuum in QCD?

We study violation of quark-hadron duality for $\bar{B}\to X_ulν$ and $D\to X_dlν$ decays in the presence of the right-handed current operator. For this case, we show that duality violation has some aspects different from the standard model due to the existence of an instanton. In particular, the fermionic zero mode of an intermediate light quark can give a nonvanishing contribution owing to its chirality structure. A duality-violating component that arises from a single instanton is then analytically extracted as a finite-distance singularity, leading to an oscillatory correction to the observable in Minkowski space. Finally, we show that the size of duality violation for the lepton energy distribution in $\bar{B}$ decays can be comparable to the chirally suppressed perturbative contribution although the absolute size depends on the detail of the QCD vacuum.

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arXiv2022

Theoretical aspects of $D^0-\bar{D}^0$ mixing

Observables in the $D^0-\bar{D}^0$ mixing can be theoretically analyzed by the operator product expansion (OPE), in which $1/m_c$ is regarded as an expansion parameter. Since the contributions of four-quark operators are strongly suppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism, the order of magnitude of the width difference is still not reproduced in the OPE analysis. In view of this issue, quark-hadron duality, an assumption that is tacitly made in the OPE, is studied for the $D^0-\bar{D}^0$ mixing. In particular, the exclusive width difference and the inclusive counterpart can be compared within the 't Hooft model, two-dimensional QCD in the large-$N_c$ limit. It is shown that the order of magnitude of the exclusive width difference is enhanced relative to the 4D-like inclusive contributions of the four-quark operators, that is qualitatively consistent with the realistic observation of the $D^0-\bar{D}^0$ mixing.

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arXiv2021

Revisiting quark-hadron duality for heavy meson non-leptonic decays in two-dimensional QCD

We study lifetimes of heavy mesons in the 't Hooft model, a large-$N_c$ theory of strong interaction in two-dimensional spacetime. Since this model is solvable, one can evaluate the total decay widths through hadronic amplitudes that are determined unambiguously within the formalism. We investigate quark-hadron duality by numerically comparing the exclusive result and one from the heavy quark expansion, with the contribution of Pauli interference being consistently incorporated. Within certain numerical accuracy, we find that the maximal difference between inclusive and exclusive rates for $τ[D^+_s]/τ[D^0]$ $(τ[B^+]/τ[B^0])$ is as large as the current experimental (theoretical) error in four-dimensions, except for $τ[D^+]/τ[D^0]$, where an excellent agreement between the rates is seen.

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arXiv2021

Quark-hadron duality for heavy meson mixings in the 't Hooft model

We study local quark-hadron duality and its violation for the $D^0-\bar{D}^0$, $B^0_d-\bar{B}^0_d$ and $B^0_s-\bar{B}^0_s$ mixings in the 't Hooft model, offering a laboratory to test QCD in two-dimensional spacetime together with the large-$N_c$ limit. With the 't Hooft equation being numerically solved, the width difference is calculated as an exclusive sum over two-body decays. The obtained rate is compared to inclusive one that arises from four-quark operators to check the validity of the heavy quark expansion (HQE). In view of the observation in four-dimensions that the HQE prediction for the width difference in the $D^0-\bar{D}^0$ mixing is four orders of magnitude smaller than the experimental data, in this work we investigate duality violation in the presence of the GIM mechanism. We show that the order of magnitude of the observable in the $D^0-\bar{D}^0$ mixing is enhanced in the exclusive analysis relative to the inclusive counterpart, when the 4D-like phase space function is used for the inclusive analysis. By contrast, it is shown that for the $B^0_d-\bar{B}^0_d$ and $B^0_s-\bar{B}^0_s$ mixings, small yet non-negligible corrections to the inclusive result emerge, which are still consistent with what is currently indicated in four-dimensions.

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arXiv2020

QCD sum rules with spectral densities solved in inverse problems

We construct QCD sum rules for nonperturbative studies without assuming the quark-hadron duality for the spectral density at low energy on the hadron side. Instead, both resonance and continuum contributions to the spectral density are solved with the operator-product-expansion input on the quark side by treating sum rules as an inverse problem. This new formalism does not involve the continuum threshold, does not require the Borel transformation and stability analysis, and can be extended to extract properties of excited states. Taking the two-current correlator as an example, we demonstrate that the series of $ρ$ resonances can emerge in our formalism, and the decay constants $f_{ρ(770)} (f_{ρ(1450)},f_{ρ(1700)},f_{ρ(1900)})\approx$ 0.22 (0.19, 0.14, 0.14) GeV for the masses $m_{ρ(770)} (m_{ρ(1450)},m_{ρ(1700)},m_{ρ(1900)})\approx$ 0.78 (1.46, 1.70, 1.90) GeV are determined. We also show that the decay width $Γ_{ρ(770)}\approx 0.17$ GeV can be obtained by substituting a Breit-Wigner parametrization for the $ρ(770)$ pole on the hadron side. It is observed that quark condensates of dimension-six on the quark side are crucial for establishing those $ρ$ resonances. Handling the conventional sum rules with the duality assumption as an inverse problem, we find that the multiple pole sum rules widely adopted in the literature do not describe the $ρ$ excitations reasonably. The precision of our theoretical outcomes can be improved systematically by including higher-order and higher-power corrections on the quark side. Broad applications of this formalism to abundant low energy QCD observables are expected.

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arXiv2020

Vacuum polarization contribution to muon $g-2$ as an inverse problem

We analyze the electromagnetic current correlator at an arbitrary photon invariant mass $q^2$ by exploiting its associated dispersion relation. The dispersion relation is turned into an inverse problem, via which the involved vacuum polarization function $Π(q^2)$ at low $q^2$ is solved with the perturbative input of $Π(q^2)$ at large $q^2$. It is found that the result for $Π(q^2)$, including its first derivative $Π^\prime(q^2=0)$, agrees with those from lattice QCD, and its imaginary part accommodates the $e^+e^-$ annihilation data. The corresponding hadronic vacuum polarization contribution $a^{\rm HVP}_μ= (641^{+65}_{-63})\times 10^{-10}$ to the muon anomalous magnetic moment $g-2$, where the uncertainty arises from the variation of the perturbative input, also agrees with those obtained in other phenomenological and theoretical approaches. We point out that our formalism is equivalent to imposing the analyticity constraint to the phenomenological approach solely relying on experimental data, and can improve the precision of the $a^{\rm HVP}_μ$ determination in the Standard Model.

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arXiv2020

$D$ meson mixing as an inverse problem

We calculate the parameters $x$ and $y$ for the $D$ meson mixing in the Standard Model by considering a dispersion relation between them. The dispersion relation for a fictitious charm quark of arbitrary mass squared $s$ is turned into an inverse problem, via which the mixing parameters at low $s$ are solved with the perturbative inputs $x(s)$ and $y(s)$ from large $s$. It is shown that nontrivial solutions for $x$ and $y$ exist, whose values around the physical charm scale agree with the data in both CP-conserving and CP-violating cases. We then predict the observables $|q/p|-1\approx 2\times 10^{-4}$ and $Arg(q/p)\approx 6\times 10^{-3}$ degrees associated with the coefficient ratio for the $D$ meson mixing, which can be confronted with more precise future measurements. Our work represents the first successful quantitative attempt to explain the $D$ meson mixing parameters in the Standard Model.

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arXiv2019

Hidden relations in three generation seesaw model with Dirac mass matrix of four-zero texture

We present predictions for CP violating phases of the Type-I seesaw model with four-zero textures on the Dirac mass matrix. For the four-zero textures, the effective low energy Majorana mass matrix is parametrized with seven parameters. They are three mass-dimensional parameters, two angles and two CP violating sources. The number of these parameters is less than that of the general description of the Majorana mass matrix with three neutrino masses, three mixing angles and three CP violating phases. In particular, only two independent CP violating sources give rise to three CP violating phases. The efficient and comprehensive method is proposed in this paper to investigate four-zero textures in Type-I seesaw. We numerically show the possible range of CP violating phases in the plane of a Dirac CP violating phase and one of Majorana phases. Some cases show the strong correlations among two phases. These correlations can be explained by hidden relations among the elements of Majorana matrix with the four-zero textures. The hidden relations are classified according to the position of one vanishing off-diagonal element of Majorana mass matrix. The Majorana mass matrix all of whose elements are non-vanishing also produces other hidden relations particularly in the case of four-zero textures. By applying the hidden relations, we describe the concrete correlations among CP violating phases.

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arXiv2018

Analysis of Dalitz decays with intrinsic parity violating interactions in resonance chiral perturbation theory

Observables of light hadron decays are analyzed in a model of chiral Lagrangian which includes resonance fields of vector mesons. In particular, transition form factors are investigated for Dalitz decays of $V\to Pl^+l^-$ and $P\to γl^+l^-$ $(V=1^-, P=0^-)$. Moreover, the differential decay width of $P\to π^+π^-γ$ and the partial widths of $P\to2γ, V\to Pγ, η^\prime\to Vγ, ϕ(1020)\toω(782)π^0$ and $V\to 3P$ are also calculated. In this study, we consider a model which contains octet and singlet fields as representation of $SU(3)$. As an extension of chiral perturbation theory, we include 1-loop ordered interaction terms. For both pseudoscalar and vector meson, we evaluate mixing matrices in which isospin/$SU(3)$ breaking is taken into account. Furthermore, intrinsic parity violating interactions are considered with singlet fields. For parameter estimation, we carry out $χ^2$ fittings in which a spectral function of $τ$ decays, vector meson masses, decay widths of $V\to Pγ$ and transition form factor of $V\to Pl^+l^-$ are utilized as input data. Using the estimated parameter region in the model, we give predictions for decay widths and transition form factors of intrinsic parity violating decays. As further model predictions, we calculate the transition form factors of $ϕ(1020)\to π^0l^+l^-$ and $η^\prime\toγl^+l^-$ in the vicinity of resonance regions, taking account of the contribution for intermediate $ρ(770)$ and $ω(782)$.

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arXiv2018

Direct CP Violation in Cabibbo-Favored Charmed Meson Decays and $ε'/ε$ in $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ Model

Since the standard model contribution is virtually absent, any observation of direct CP violation in the Cabibbo-favored charmed meson decays would be evidence of new physics. In this paper, we conduct a quantitative study on direct CP violation in $D^0\to K^-π^+$, $D_s^+\to ηπ^+$ and $D_s^+\to η'π^+$ decays in the $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge extension of the standard model. In the model, direct CP violation arises mainly from the interference between the decay amplitude coming from the SM left-left current operators and that from the right-right current operators induced by $W_R^+$ gauge boson exchange. Interestingly, the strong phase between the two amplitudes is evaluable, since it stems from difference in QCD corrections to the left-left and right-right current operators, which is a short-distance QCD effect given by $\sim(α_s(M_{W_L}^2)/4π)\log(M_{W_R}^2/M_{W_L}^2)$. We assess the maximal direct CP violation in the above decays in the $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model. Additionally, we present a correlation between direct CP violation in these modes and one in $K\toππ$ decay parametrized by $ε'$, since $W_R^+$ gauge boson has a sizable impact on the latter.

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arXiv2018

$ε'/ε$ Anomaly and Neutron EDM in $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model with Charge Symmetry

The Standard Model prediction for $ε'/ε$ based on recent lattice QCD results exhibits a tension with the experimental data. We solve this tension through $W_R^+$ gauge boson exchange in the $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model with `charge symmetry', whose theoretical motivation is to attribute the chiral structure of the Standard Model to the spontaneous breaking of $SU(2)_R\times U(1)_{B-L}$ gauge group and charge symmetry. We show that $M_{W_R}<58$ TeV is required to account for the $ε'/ε$ anomaly in this model. Next, we make a prediction for the neutron EDM in the same model and study a correlation between $ε'/ε$ and the neutron EDM. We confirm that the model can solve the $ε'/ε$ anomaly without conflicting the current bound on the neutron EDM, and further reveal that almost all parameter regions in which the $ε'/ε$ anomaly is explained will be covered by future neutron EDM searches, which leads us to anticipate the discovery of the neutron EDM.

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arXiv2018

Effective theory analysis for vector-like quark model

We study a model with a down-type SU(2) singlet vector-like quark (VLQ) as a minimal extension of the standard model (SM). In this model, flavor changing neutral currents (FCNCs) arise at tree level and the unitarity of the $3\times 3$ Cabibbo-Kobayashi-Maskawa (CKM) matrix does not hold. In this paper, we constrain the FCNC coupling from $b\rightarrow s$ transitions, especially $B_s\rightarrow μ^+μ^-$ and $\bar{B}\rightarrow X_sγ$ processes. In order to analyze these processes, we derive an effective Lagrangian which is valid below the electroweak symmetry breaking scale. For this purpose, we first integrate out the VLQ field and derive an effective theory by matching Wilson coefficients up to one-loop level. Using the effective theory, we construct the effective Lagrangian for $b\rightarrow sγ^{(*)}$. It includes the effects of the SM quarks and the violation of the CKM unitarity. We show the constraints on the magnitude of the FCNC coupling and its phase by taking account of the current experimental data on $ΔM_{B_s}$, $\mathrm{Br}[B_s\rightarrowμ^+μ^-]$, $\mathrm{Br}[\bar{B}\rightarrow X_sγ]$ and CKM matrix elements as well as theoretical uncertainties. We find that the constraint from the $\mathrm{Br}[B_s\rightarrowμ^+μ^-]$ is more stringent than that from the $\mathrm{Br}[\bar{B}\rightarrow X_sγ$]. We also obtain the bound for the mass of the VLQ and the strength of the Yukawa couplings related to the FCNC coupling of $b\rightarrow s$ transition. Using the CKM elements which satisfy above constraints, we show how the unitarity is violated on the complex plane.

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arXiv2018

Phenomenological Aspects of Possible Vacua of a Neutrino Flavor Model

We discuss a supersymmetric model with discrete flavor symmetry $A_4\times Z_3$. The additional scalar fields which contribute masses of leptons in the Yukawa terms are introduced in this model. We analyze their scalar potential and find that they have various vacuum structures. We show the relations among 24 different vacua and classify them into two types. We derive expressions of the lepton mixing angles, Dirac CP violating phase and Majorana phases for the two types. The model parameters which are allowed by the experimental data of the lepton mixing angles are different for each type. We also study the constraints on the model parameters which are related to Majorana phases. The different allowed regions of the model parameters for the two types are shown numerically for a given region of two combinations of the CP violating phases.

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