arXiv subjects

Chien-Thang Tran

Publications and source records attributed to Chien-Thang Tran.

At least 19 recordsLinked to original sources

arXiv2026

Radiative and Dalitz decays of $\Upsilon(1S)$ in the light of the ATOMKI X17 anomaly

We study the radiative decay $\Upsilon(1S)\to \eta_b\gamma$ and the Dalitz decay $\Upsilon(1S)\to \eta_b e^+ e^-$ within the framework of the Covariant Confined Quark Model. We provide theoretical predictions for the hadronic form factor, the radiative decay constant, and the decay branching fractions. Within the Standard Model we predict $\Gamma(\Upsilon(1S)\to\eta_b \gamma) = 9.3(9)\, \textrm{eV}$ and $\Gamma(\Upsilon(1S)\to\eta_b e^+ e^-) = 4.8(5)\times 10^{-2}\, \textrm{eV}$. The contribution of the hypothetical ATOMKI $X17$ vector boson to the Dalitz channel is also investigated.

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arXiv2026

Detailed analysis of possible new-physics effects in the semileptonic decay $B_s \to D_s^{(*)}\tau\bar{\nu}$

We study the semileptonic decays $B_s \to D_s^{(*)}\tau\bar{\nu}$ as a promising probe for new physics (NP) beyond the standard model (SM). The extension of the SM is done through the introduction of four-fermion operators beyond the $V-A$ structure with the corresponding Wilson coefficients characterizing their contribution. The constraints on these coefficients are obtained from recent experimental data. Form factors describing hadron transitions are calculated in our covariant quark model with infrared confinement. Theoretical predictions for the full set of observables in these channels are provided. We analyze possible NP effects to be tested in future experiments.

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arXiv2025

Revisiting radiative transitions of charmonium states in covariant confined quark model

We calculate the amplitudes and branching fractions of $χ_{cJ}$, $h_c$, $J/ψ$, and $ψ(2S)$ radiative decays in the framework of the covariant confined quark model. First, we explicitly show that all calculated amplitudes are gauge invariant. Next, we use experimental data for three decay channels $J/ψ\to η_cγ$, $χ_{c0}\to J/ψγ$, and $ψ(2S)\to χ_{c0}γ$ to fit the model parameters including the charm quark mass and the meson size parameter. Finally, we use the obtained parameters to predict the branching fractions of the decays $h_c\toη_cγ$, $χ_{c1(c2)}\to J/ψγ$, and $ψ(2S)\toχ_{c1(c2)}γ$. Our predictions agree well with experimental data.

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arXiv2025

Probing the ATOMKI X17 vector boson using Dalitz decays $V\to Pe^+e^-$

Recent anomalies observed in $e^+e^-$ nuclear transitions of $^8 \textrm{Be}$, $^4 \textrm{He}$, and $^{12} \textrm{C}$ by the ATOMKI collaboration may hint at the existence of a vector boson with a mass around 17 MeV, referred to as X17. If it exists, this boson would also affect similar processes in particle physics, including the Dalitz decays of vector mesons. Recently, the BESIII collaboration measured the Dalitz decay $D^{*0}\to D^0e^+e^-$ for the first time and reported a $3.5σ$ excess over the theoretical prediction based on the vector meson dominance (VMD) model. This excess may be another signal of the X17. In this study, we investigate the possible effects of the X17 on the Dalitz decays $D^*_{(s)}\to D_{(s)}e^+e^-$, $B^*_{(s)}\to B_{(s)}e^+e^-$, and $J/ψ\to η_ce^+e^-$. The required hadronic form factors are calculated within the framework of our covariant confined quark model, without relying on heavy quark effective theory or the VMD model. We present predictions for the Dalitz decay widths and the ratios $R_{ee}(V)\equiv Γ(V\to Pe^+e^-)/Γ(V\to Pγ)$ within the Standard Model and in several new physics scenarios involving modifications due to the X17. Our results are compared with other theoretical calculations.

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arXiv2025

Polarization observables in semileptonic $V\to P$ decays of quarkonia

We investigate the weak semileptonic decays of the charmonium $J/ψ$ and bottomonium $Υ(1S)$ into a pseudoscalar meson, namely, the $J/ψ\to D_{(s)}\ell\barν_\ell$ and $Υ(1S)\to B_{(c)}\ell\barν_\ell$ decays. We focus on the polarization observables including the forward-backward asymmetry, final lepton polarization, and lepton-side convexity parameter. In order to define these quantities, we apply the helicity amplitude method and obtain a twofold angular decay distribution for a general $V\to P\ell\barν_\ell$ case. The hadronic form factors are taken from our previous work, where they were calculated in the Covariant Confined Quark Model. We provide theoretical predictions for the polarization observables and compare them with the literature.

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arXiv2024

Study of the semileptonic decays $Υ(1S)\to B_{(c)}\ell\barν_\ell$

We study the exclusive semileptonic decays $Υ(1S)\to B_{(c)}\ell\barν_\ell$, where $\ell = e,μ,τ$. The relevant hadronic form factors are calculated using the Covariant Confined Quark Model developed previously by our group. We predict the branching fractions $\mathcal{B}(Υ(1S)\to B_{(c)}\ell\barν_\ell)$ to be of the order of $10^{-13}$ and $10^{-10}$ for the case of $B$ and $B_c$, respectively. Our predictions agree well with other theoretical calculations. We also consider the effects of possible New Physics in the case of $Υ(1S)\to B_cτ\barν_τ$. We show that the branching fraction of this decay can be enhanced by an order of magnitude using constraints from the $B\to D^{(*)}\ell\barν_\ell$ and $B_c\to J/ψ\ell \barν_\ell$ experimental data.

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arXiv2024

Radiative decays \$D^*_{(s)}\to D_{(s)}γ$ in covariant confined quark model

Radiative decays $D^*_{(s)}\to D_{(s)}γ$ are revisited in light of new experimental data from the \textit{BABAR} and BESIII collaborations. The radiative couplings $g_{D^*Dγ}$ encoding nonperturbative QCD effects are calculated in the framework of the covariant confined quark model developed by us. We compare our results with other theoretical studies and experimental data. The couplings (in $\textrm{GeV}^{-1}$) $|g_{D^{*+}D^+γ}| = 0.45(9)$ and $|g_{D^{*0}D^0γ}| = 1.72(34)$ calculated in our model agree with the corresponding experimental data $|g_{D^{*+}D^+γ}|=0.47(7)$ and $|g_{D^{*0}D^0γ}|=1.77(16)$. The most interesting case is the decay $D^*_s\to D_sγ$, for which a recent prediction based on light-cone sum rules at next-to-leading order $|g_{D^*_s D_sγ}|=0.60(19)$ deviates from the first (and only to date) lattice QCD result $|g_{D^*_s D_sγ}|=0.11(2)$ at nearly $3σ$. Our calculation yields $|g_{D^*_s D_sγ}|=0.29(6)$, which falls somehow between the two mentioned results, although it is larger than those predicted in other studies using quark models or QCD sum rules.

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arXiv2021

Form-factor-independent test of lepton universality in semileptonic heavy meson and baryon decays

In the semileptonic decays of heavy mesons and baryons the lepton-mass dependence factors out in the quadratic $\cos^2θ$ coefficient of the differential $\cosθ$ distribution. We call the corresponding normalized coefficient the convexity parameter. This observation opens the path to a test of lepton universality in semileptonic heavy meson and baryon decays that is independent of form-factor effects. By projecting out the quadratic rate coefficient, dividing out the lepton-mass-dependent factor and restricting the phase space integration to the $τ$ lepton phase space, one can define optimized partial rates which, in the Standard Model, are the same for all three $(e,μ,τ)$ modes in a given semileptonic decay process. We discuss how the identity is spoiled by New Physics effects. We discuss semileptonic heavy meson decays such as $\bar{B}^0 \to D^{(\ast)+} \ell^- \barν_\ell$ and $B_c^- \to J/ψ(η_c)\ell^- \barν_\ell$, and semileptonic heavy baryon decays such as $Λ_b \to Λ_c \ell^- \barν_\ell$ for each $\ell=e,μ,τ$.

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arXiv2020

$D^\ast$ Polarization as an Additional Constraint on New Physics in the $b\to cτ\barν_τ$ Transition

Measurements of the branching fractions of the semileptonic decays $B\to D^{(*)}τ\barν_τ$ and $B_c\to J/ψτ\barν_τ$ systematically exceed the Standard Model (SM) predictions, pointing to possible signals of new physics that can violate lepton flavor universality. The unknown origin of new physics realized in these channels can be probed using a general effective Hamiltonian constructed from four-fermion operators and the corresponding Wilson coefficients. Previously, constraints on these Wilson coefficients were obtained mainly from the experimental data for the branching fractions. Meanwhile, polarization observables were only theoretically studied. The situation has changed with more experimental data having become available, particularly those regarding the polarization of the tau and the $D^*$ meson. In this study, we discuss the implications of the new data on the overall picture. We then include them in an updated fit of the Wilson coefficients using all hadronic form factors from our covariant constituent quark model. The use of our form factors provides an analysis independent of those in the literature. Several new-physics scenarios are studied with the corresponding theoretical predictions provided, which are useful for future experimental studies.

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arXiv2019

Exclusive semileptonic decays of $D$ and $D_s$ mesons in the covariant confining quark model

Recently, the BESIII collaboration has reported numerous measurements of various $D_{(s)}$ meson semileptonic decays with significantly improved precision. Together with similar studies carried out at BABAR, Belle, and CLEO, new windows to a better understanding of weak and strong interactions in the charm sector have been opened. In light of new experimental data, we review the theoretical description and predictions for the semileptonic decays of $D_{(s)}$ to a pseudoscalar or a vector meson. This review is essentially an extended discussion of our recently published results obtained in the framework of the covariant confining quark model.

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arXiv2019

Semileptonic $D_{(s)}$-meson decays in the light of recent data

Inspired by recent improved measurements of charm semileptonic decays at BESIII, we study a large set of $D(D_s)$-meson semileptonic decays where the hadron in the final state is one of $D^0$, $ρ$, $ω$, $η^{(\prime)}$ in the case of $D^+$ decays, and $D^0$, $ϕ$, $K^0$, $K^\ast(892)^0$, $η^{(\prime)}$ in the case of $D^+_s$ decays. The required hadronic form factors are computed in the full kinematical range of momentum transfer by employing the covariant confined quark model developed by us. A detailed comparison of the form factors with those from other approaches is provided. We calculate the decay branching fractions and their ratios, which show good agreement with available experimental data. We also give predictions for the forward-backward asymmetry and the longitudinal and transverse polarizations of the charged lepton in the final state.

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arXiv2019

Study of $B_s\to\ell^+\ell^-γ$ decays in covariant quark model

We study the rare radiative leptonic decays $B_s\to\ell^+\ell^-γ$ ($\ell=e,μ,τ$) within the Standard Model, considering both the structure-dependent amplitude and bremsstrahlung. In the framework of the covariant confined quark model developed by us, we calculate the form factors characterizing the $B_s\toγ$ transition in the full kinematical region of the dilepton momentum squared and discuss their behavior. We provide the analytic formula for the differential decay distribution and give predictions for the branching fractions in both cases: with and without long-distance contributions. Finally, we compare our results with those obtained in other approaches.

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arXiv2018

Analyzing lepton flavor universality in the decays $Λ_b\toΛ_c^{(\ast)}(\frac12^\pm,\frac32^-) + \ell\,\barν_\ell$

Lepton flavor universality can be tested in the semileptonic decays $Λ_b\to Λ_c^{(\ast)}$ where $Λ_c^{(\ast)}$ denotes either the ground state $Λ_c(2286)$ (with $J^P=1/2^+$) or its orbital excitations $Λ_c(2595)$ (with $J^P=1/2^-$) and $Λ_c(2625)$ (with $J^P=3/2^-$). We calculate the differential decay rates as well as the branching fractions of these decays for both tauonic and muonic modes with form factors obtained from a covariant confined quark model previously developed by us. We present results for the rate ratios of the tauonic and muonic modes which provide important tests of lepton flavor universality in forthcoming experiments.

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arXiv2018

Implications of new physics in the decays $B_c \to (J/ψ,η_c)τν$

We study the semileptonic decays of the $B_c$ meson into final charmonium states within the standard model and beyond. The relevant hadronic transition form factors are calculated in the framework of the covariant confined quark model developed by us. We focus on the tau mode of these decays, which may provide some hints of new physics effects. We extend the standard model by assuming a general effective Hamiltonian describing the $b\to cτν$ transition, which consists of the full set of the four-fermion operators. We then obtain experimental constraints on the Wilson coefficients corresponding to each operator and provide predictions for the branching fractions and other polarization observables in different new physics scenarios.

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arXiv2017

Analyzing New Physics in $\bar{B}^0 \to D^{(\ast)}τ^-\barν_τ$

We discuss possible new physics (NP) effects beyond the standard model (SM) in the exclusive decays $\bar{B}^0 \to D^{(\ast)} τ^- \barν_τ$. Starting with a model-independent effective Hamiltonian including non-SM four-Fermi operators, we show how to obtain experimental constraints on different NP scenarios and investigate their effects on a large set of physical observables. The $\bar{B}^0 \to D^{(\ast)}$ transition form factors are calculated in the full kinematic $q^2$ range by employing the covariant confined quark model developed by our group.

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arXiv2017

Probing new physics in $\bar{B}^0 \to D^{(\ast)} τ^- \barν_τ$ using the longitudinal, transverse, and normal polarization components of the tau lepton

We study the longitudinal, transverse, and normal polarization components of the tau lepton in the decays $\bar{B}^0 \to D^{(\ast)} τ^- \barν_τ$ and discuss their role in searching for new physics (NP) beyond the standard model (SM). Starting with a model-independent effective Hamiltonian including non-SM four-Fermi operators, we obtain experimental constraints on different NP scenarios and investigate their effects on the polarization observables. In the SM the longitudinal and transverse polarizations of the tau lepton differ substantially from the corresponding zero lepton mass values of $P_L=-1$ and $P_T=0$. In addition, $P_L$ and $P_T$ are very sensitive to NP effects. For the transverse polarization this holds true, in particular, for the effective tensor operator in the case of $\bar{B}^0 \to D^\ast$ and for the scalar operator in the case of $\bar{B}^0 \to D$. The $T$-odd normal polarization $P_N$, which is predicted to be negligibly small in the SM, can be very sizable assuming NP complex Wilson coefficients. We also discuss in some detail how the three polarization components of the tau lepton can be measured with the help of its subsequent leptonic and semihadronic decays.

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arXiv2017

Exclusive decays $J/ψ\to D_{(s)}^{(*)-} {\ell}^+ ν_{\ell}$ in a covariant constituent quark model with infrared confinement

We investigate the exclusive semileptonic decays $J/ψ\to D_{(s)}^{(*)-} {\ell}^+ ν_{\ell}$, where $\ell=e,μ$, within the Standard Model. The relevant transition form factors are calculated in the framework of a relativistic constituent quark model with built-in infrared confinement. Our calculations predict the branching fractions $\mathcal{B}(J/ψ\to D_{(s)}^{(*)-} {\ell}^+ ν_{\ell})$ to be of the order of $10^{-10}$ for $D_s^{(*)-}$ and $10^{-11}$ for $D^{(*)-}$. Most of our numerical results are consistent with other theoretical studies. However, some branching fractions are larger than those calculated in QCD sum rules approaches but smaller than those obtained in the covariant light-front quark model by a factor of about $2-3$.

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arXiv2016

Analyzing new physics in the decays $\bar{B}^0 \to D^{(\ast)}τ^-\barν_τ$ with form factors obtained from the covariant quark model

We study possible New Physics (NP) effects in the exclusive decays $\bar{B}^0 \to D^{(\ast)} τ^- \barν_τ$. We extend the Standard Model (SM) by taking into account right-handed vector (axial), left- and right-handed (pseudo)scalar, and tensor current contributions. The $\bar{B}^0 \to D^{(\ast)}$ transition form factors are calculated in the full kinematic $q^2$ range by employing a covariant quark model developed by us. We provide constraints on NP operators based on measurements of the ratios of branching fractions $R(D^{(\ast)}) \equiv \mathcal{B}(\bar{B}^0 \to D^{(\ast)} τ^- \barν_τ)/ \mathcal{B}(\bar{B}^0 \to D^{(\ast)} μ^- \barν_μ)$ and consider the effects of these operators on physical observables in different NP scenarios. We also derive the four-fold angular distribution for the cascade decay $\bar {B}^0\to D^{\ast\,+}(\to D^0π^+)τ^-\barν_τ$ which allows one to analyze the polarization of the $D^{\ast}$ meson in the presence of NP effects. We discuss several strategies to distinguish between various NP contributions.

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