arXiv subjects

Qin Chang

Publications and source records attributed to Qin Chang.

At least 55 records · Page 3Linked to original sources

arXiv2019

Revisiting the form factors of $P\to V$ transition within the light-front quark models

We investigate the self-consistency and Lorentz covariance of the covariant light-front quark model~(CLF QM) via the matrix elements and form factors~(${\cal F}=g$, $a_{\pm}$ and $f$) of $P\to V$ transition. Two types of correspondence schemes between the manifest covariant Bethe-Salpeter approach and the light-front quark model are studied. We find that, for $a_{-}(q^2)$ and $f(q^2)$, the CLF results obtained via $\lbd=0$ and $\pm$ polarization states of vector meson within the traditional type-I correspondence scheme are inconsistent with each other; and moreover, the strict covariance of the matrix element is violated due to the nonvanishing spurious contributions associated with noncovariance. We further show that such two problems have the same origin and can be resolved simultaneously by employing the type-II correspondence scheme, which advocates an additional replacement $M\to M_0$ relative to the traditional type-I scheme; meanwhile, the results of ${\cal F}(q^2)$ in the standard light-front quark model~(SLF QM) are exactly the same as the valence contributions and equal to numerally the full results in the CLF QM, {\it i.e.}, $[{\cal{F}}]_{\rm SLF}=[{\cal{F}}]_{\rm val.}\doteq[{\cal{F}}]_{\rm full}$. The numerical results for some $P\to V$ transitions are updated within the type-II scheme. Above findings confirm the conclusion obtained via the decay constants of vector and axial-vector mesons in the previous works.

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arXiv2018

Self-consistency and covariance of light-front quark models: testing via $P$, $V$ and $A$ meson decay constants, and $P\to P$ weak transition form factors

In this paper, we test the self-consistencies of the standard and the covariant light-front quark model and study the zero-mode issue via the decay constants of pseudoscalar ($P$), vector ($V$) and axial-vector ($A$) mesons, as well as the $P\to P$ weak transition form factors. With the traditional type-I correspondence between the manifestly covariant and the light-front approach, the resulting $f_{V}$ as well as $f_{^1\!A}$ and $f_{^3\!A}$ obtained with the $\lbd=0$ and $\lbd=\pm$ polarization states are different from each other, which presents a challenge to the self-consistency of the covariant light-front quark model. However, such a self-consistency problem can be "resolved" within the type-II scheme, which requires an additional replacement $M\to M_0$ relative to the type-I case. Moreover, the replacement $M\to M_0$ is also essential for the self-consistency of the standard light-front quark model. In the type-II scheme, the valence contributions to the physical quantities~(${\cal Q}$) considered in this paper are alway the same as that obtained in the standard light-front quark model, $[{\cal Q}]_{\rm val.}=[{\cal Q}]_{\rm SLF}$, and the zero-mode contributions to $f_{V,^1\!A,^3\!A}$ and $f_-(q^2)$ exist only formally but vanish numerically, which implies further that $[{\cal Q}]_{\rm val.}\dot{=} [{\cal Q}]_{\rm full}$. In addition, the manifest covariance of the covariant light-front quark model is violated in the traditional type-I scheme, but can be recovered by taking the type-II scheme.

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arXiv2018

$B_{u}$ ${\to}$ $ψM$ decays and $S$-$D$ wave mixing effects

The $B_{u}$ ${\to}$ $ψM$ decays are studied with the perturbative QCD approach, where the psion $ψ$ $=$ $ψ(2S)$, $ψ(3770)$, $ψ(4040)$ and $ψ(4160)$, and the light meson $M$ $=$ $π$, $K$, $ρ$ and $K^{\ast}$. The factorizable and nonfactorizable contributions, and the $S$-$D$ wave mixing effects on the psions are considered in the calculation. With appropriate inputs, the branching ratios for the $B_{u}$ ${\to}$ $ψK$ decays are generally coincident with the experimental data within errors. However, due to the large theoretical and experimental errors, it is impossible for the moment to give a severe constraint on the $S$-$D$ wave mixing angles.

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arXiv2018

Probing the effects of new physics in $\bar{B}^* \to P \ell \barν_\ell$ decays

The significant divergence between the SM predictions and experimental measurements for the ratios, $R_{D^{(*)}}\equiv \mathcal{B}(\bar{B}\to D^{(*)}τ^- \barν_τ)/ \mathcal{B}(\bar{B}\to D^{(*)} \ell^{\prime-} \barν_{\ell^{\prime}})$ with $(\ell^{\prime}=e\,,μ)$, implies possible hint of new physics in the flavour sector. In this paper, motivated by the "$R_{D^{(*)}}$ puzzle" and abundant $B^*$ data samples at high-luminosity heavy-flavor experiments in the future, we try to probe possible effects of new physics in the semileptonic $\bar{B}^*_{u,d,s} \to P \ell^- \barν_\ell$ $(P=D\,,D_s\,,π\,,K)$ decays induced by $b \to (u,\,c)\ell^- \barν_{\ell}$ transitions in the model-independent vector and scalar scenarios. Using the spaces of NP parameters obtained by fitting to the data of $R_{D}$ and $R_{D^{*}}$, the NP effects on the observables including branching fraction, ratio $R_P^{\ast}$, lepton spin asymmetry and lepton forward-backward asymmetry are studied in detail. We find that the vector type couplings have large effects on the branching fraction and ratio $R_P^{\ast}$. Meanwhile, the scalar type couplings provide significant contributions to all of the observables. The future measurements of these observables in the $\bar{B}^* \to P \ell^- \barν_\ell$ decays at the LHCb and Belle-II could provide a way to crosscheck the various NP solutions to the "$R_{D^{(*)}}$ puzzle".

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arXiv2018

Study of $B_{c}$$\to$$ J/ψV $ and $B_{c}^{*}$$\to$$η_{c}V$ decays within the QCD factorization

In this paper, we study the non-leptonic $B_{c}\to J/ψV$ and $B_{c}^{*}\toη_{c}V$~$(V=ρ\,,K^{*})$ weak decays in the framework of QCD factorization. In the evaluation, the form factors are calculated by using the Bauer-Stech-Wirbel model and the light-front quark model, respectively. Besides the longitudinal amplitude, the power-suppressed transverse contributions are also evaluated at next-to-leading order. The predictions for the observables of $B_{c}\to J/ψV$ and $B_{c}^{*}\toη_{c}V$ decays are presented. We find that the NLO QCD contribution presents about $ 8\%$ correction to the branching ratios, and the longitudinal polarization fractions of these decays are at the level of $(80\sim90)\%$. In addition, we suggest direct measurements on some useful ratios, $R_{K^{*}/ρ}^{(λ=0)}$ and $\widetilde{R}^{(λ=0)}_{K^{*}/ρ}$, which are very suitable for test the consistence between theoretical prediction and data because their theoretical uncertainties can be well-controlled.

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arXiv2018

Decay constants of pseudoscalar and vector mesons with improved holographic wavefunction

We calculate the decay constants of light and heavy-light pseudoscalar and vector mesons with improved soft-wall holographic wavefuntions, which take into account the effects of both quark masses and dynamical spins. We find that the predicted decay constants, especially for the ratio $f_V/f_P$, based on light-front holographic QCD, can be significantly improved, once the dynamical spin effects are taken into account by introducing the helicity-dependent wavefunctions. We also perform detailed $χ^2$ analyses for the holographic parameters ({\it i.e.} the mass-scale parameter $κ$ and the quark masses), by confronting our predictions with the data for the charged-meson decay constants and the meson spectra. The fitted values for these parameters are generally in agreement with those obtained by fitting to the Regge trajectories. At the same time, most of our results for the decay constants and their ratios agree with the data as well as the predictions based on lattice QCD and QCD sum rule approaches, with only a few exceptions observed.

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arXiv2018

Application of the light-front holographic wavefunction for heavy-light pseudoscalar meson in $B_{d,s}\to D_{d,s}P$ decays

In this paper we extend our analyses of the decay constant and distribution amplitude with an improved holographic wavefunction to the heavy-light pseudoscalar mesons. In the evaluations, the helicity-dependence of the holographic wavefunction is considered; and an independent mass scale parameter is employed to moderate the strong suppression induced by the heavy quark. Under the constraints from decay constants and masses of pseudoscalar mesons, the $χ^2$-analyses for the holographic parameters exhibit a rough consistence with the results obtained by fitting the Regge trajectory. With the fitted parameters, the results for the decay constants and distribution amplitudes are presented. We then show their application in evaluating the $B_{d,s}\to D_{d,s}P$ decays, in which the power-suppressed spectator scattering and weak annihilation corrections are first estimated. Numerically, the spectator scattering and weak annihilation corrections present a negative shift of about $ 0.7\%$ on the branching fractions; while, the predictions are still larger than the experimental data. Such small negative shift confirms the estimation based on the power counting rules.

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arXiv2018

Study of the $B_{s}$ ${\to}$ $ϕf_{0}(980)$ ${\to}$ $ϕ\,π^{+}π^{-}$ decay with perturbative QCD approach

The rare cascade $B_{s}$ ${\to}$ $ϕf_{0}(980)$ ${\to}$ $ϕ\,π^{+}π^{-}$ decay is studied with the perturbative QCD approach based on the formula for the quasi two-body decay, where the two-pion pair originates from the $S$-wave resonant $f_{0}(980)$ state. It is found that with the introduction of the nonperturbative two-pion distribution amplitudes and the Flatté parameterization of the scalar form factor for the $f_{0}(980)$ resonance, the branching ratio in the mass range 400 MeV $<$ $m(π^{+}π^{-})$ $<$ 1600 MeV is ${\cal B}_{\rm theo}(B_{s}^{0}\, {\to}\,ϕ\,f_{0}(980)\,{\to}\,ϕ\,π^{+}\,π^{-})$ $=$ $[1.31_{-0.31}^{+0.40}(a_{ππ}){}_{-0.16}^{+0.19}(m_{b}) {}_{-0.09}^{+0.10}(\text{CKM})]{\times}10^{-6}$, where the uncertainties come from the parameter $a_{ππ}$ of the two-pion distribution amplitudes, the $b$ quark mass $m_{b}$, the Cabibbo-Kobayashi-Maskawa (CKM) factors, respectively. This result agrees with the recent LHCb measurement within uncertainties, ${\cal B}_{\rm exp}(B_{s}^{0}\,{\to}\,ϕ\, f_{0}(980)\,{\to}\,ϕ\,π^{+}\,π^{-})$ $=$ $(1.12{\pm}0.16^{+0.09}_{-0.08}{\pm}0.11){\times}10^{-6}$, where the errors are statistical, systematic and from the normalization, respectively.

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arXiv2017

$Υ(nS)$ ${\to}$ $B_{c}ρ$, $B_{c}K^{\ast}$ decays with perturbative QCD approach

Inspired by the potential prospects of $Υ(nS)$ data samples ($n$ $=$ $1$, $2$, $3$) at LHC and SuperKEKB, $Υ(nS)$ ${\to}$ $B_{c}ρ$, $B_{c}K^{\ast}$ decays are studied phenomenologically with pQCD approach. Branching ratios for $Υ(nS)$ ${\to}$ $B_{c}ρ$ and $B_{c}K^{\ast}$ decays are estimated to reach up to ${\cal O}(10^{-11})$ and ${\cal O}(10^{-12})$, respectively. Given the identification and detection efficiency of final states, searching for these weak decay modes should be fairly challenging experimentally in the future.

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arXiv2017

Study of $J/ψ$ ${\to}$ $D_{s,d}V$ decays with perturbative QCD approach

Inspired by the recent measurements on two-body nonleptonic $J/ψ$ weak decay at BESIII, the charm-changing $J/ψ$ ${\to}$ $D_{s,d}V$ weak decays are studied with perturbative QCD approach, where $V$ denotes $ρ$ and $K^{\ast}$ vector mesons. It is found that branching ratio for $J/ψ$ ${\to}$ $D_{s}ρ$ decay can reach up to ${\cal O}(10^{-9})$, which is within the potential measurement capability of the future high-luminosity experiments.

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arXiv2017

$J/ψ$ ${\to}$ $D_{s,d}π$, $D_{s,d}K$ decays with perturbative QCD approach

Besides the conventional strong and electromagnetic decay modes, the $J/ψ$ particle can also decay via the weak interaction in the standard model. In this paper, nonleptonic $J/ψ$ ${\to}$ $D_{s,d}π$, $D_{s,d}K$ weak decays, corresponding to the externally emitted virtual $W$ boson process, are investigated with the perturbative QCD approach. It is found that branching ratio for the Cabibbo-favored $J/ψ$ ${\to}$ $D_{s}π$ decay can reach up to ${\cal O}(10^{-10})$, which might be potentially measurable at the future high-luminosity experiments.

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arXiv2017

$Υ(nS)$ ${\to}$ $B_{c}^{\ast}D$ decays with perturbative QCD approach

The $Υ(nS)$ ${\to}$ $B_{c}^{\ast}D$ weak decays ($n$ $=$ $1$, $2$, $3$) are investigated with perturbative QCD approach. It is found that the CKM-favored $Υ(nS)$ ${\to}$ $B_{c}^{\ast}D_{s}$ decays have branching ratio of ${\cal O}(10^{-10})$, which might be potentially accessible to the future LHC and SuperKEKB experiments.

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arXiv2017

$B^{\ast}$ ${\to}$ $\bar{D}D$ decays with perturbative QCD approach

The nonleptonic two-body $B^{\ast}$ ${\to}$ $\bar{D}D$ weak decays are studied phenomenologically with the perturbative QCD factorization approach. It is found that the $B_{s}^{{\ast}0}$ ${\to}$ $D_{s}^{-}D_{s}^{+}$, $B_{d}^{{\ast}0}$ ${\to}$ $D_{d}^{-}D_{s}^{+}$, and $B_{u}^{{\ast}+}$ ${\to}$ $\bar{D}_{u}^{0}D_{s}^{+}$ decays have branching ratios ${\gtrsim}$ $10^{-9}$, and might be promisingly measurable at the running LHC and forthcoming SuperKEKB experiments in the future.

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arXiv2017

Study of the $\bar{B}_{q}^{\ast}$ ${\to}$ $DM$ decays with perturbative QCD approach

The $\bar{B}_{q}^{\ast}$ ${\to}$ $DP$, $DV$ weak decays are studied with the perturbative QCD approach, where $q$ $=$ $u$, $d$ and $s$; $P$ and $V$ denote the ground $SU(3)$ pseudoscalar and vector meson nonet. It is found that the branching ratios for the color-allowed $\bar{B}_{q}^{\ast}$ ${\to}$ $D_{q}ρ^{-}$ decays can reach up to $10^{-9}$ or more, and should be promisingly measurable at the running LHC and forthcoming SuperKEKB experiments in the near future.

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arXiv2017

Charmless $B_{c} \to VV$ decays in the QCD factorization approach

In this paper, we studied the charmless $B_{c}\to VV$ ($V$ denotes the light ground $\rm SU(3)$ vector meson) decays within the framework of QCD factorization. In the evaluation, two different schemes for regulating the end-point divergence are adopted. One (scheme I) is to use parameterization model, which is usually employed in the QCD factorization approach; the other (scheme II) is based on the infrared finite gluon propagator of Cornwall prescription. It is found that, in the annihilation amplitudes, the end-point divergence appears only in the power-suppressed corrections related to the twist-3 distribution amplitudes of $V$-meson. The strength of annihilation amplitudes evaluated in scheme II is generally larger than the one in scheme I. Numerically, in the decay modes considered in this paper, the CKM-favored $B_{c} \to ρ^{-}ω, K^{*-}K^{*0} $ decays have the relatively large branching fractions, $\sim {\cal{O}}(10^{-7})$, and hence are likely to be the first observed by the future experiments. In addition, all of the decay modes are dominated by the longitudinal polarization state; numerically, $f_{L}(B_{c} \to VV) \gtrsim 99\%$.

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arXiv2017

Study of the weak annihilation contributions in charmless $B_s\to VV$ decays

In this paper, in order to probe the spectator-scattering and weak annihilation contributions in charmless $B_s\to VV$ (where $V$ stands for a light vector meson) decays, we perform the $χ^2$-analyses for the end-point parameters within the QCD factorization framework, under the constraints from the measured $\bar B_{s}\to$$ρ^0ϕ$, $ϕK^{*0}$, $ϕϕ$ and $K^{*0}\bar K^{*0}$ decays. The fitted results indicate that the end-point parameters in the factorizable and nonfactorizable annihilation topologies are non-universal, which is also favored by the charmless $B\to PP$ and $PV$ (where $P$ stands for a light pseudo-scalar meson) decays observed in the previous work. Moreover, the abnormal polarization fractions $f_{L,\bot}(\bar B_{s}\to K^{*0}\bar K^{*0})=(20.1\pm7.0)\%\,,(58.4\pm8.5)\%$ measured by the LHCb collaboration can be reconciled through the weak annihilation corrections. However, the branching ratio of $\bar B_{s}\toϕK^{*0}$ decay exhibits a tension between the data and theoretical result, which dominates the contributions to $χ_{\rm min}^2$ in the fits. Using the fitted end-point parameters, we update the theoretical results for the charmless $B_s\to VV$ decays, which will be further tested by the LHCb and Belle-II experiments in the near future.

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arXiv2017

Possibility of searching for $B_{c}^{\ast}$ ${\to}$ $B_{u,d,s}V$, $B_{u,d,s}P$ decays

The $B_{c}^{\ast}$ ${\to}$ $B_{u,d,s}V$, $B_{u,d,s}P$ decays are investigated with the QCD factorization approach, where $V$ and $P$ denote the ground $SU(3)$ vector and pseudoscalar mesons, respectively. The $B_{c}^{\ast}$ ${\to}$ $B_{u,d,s}$ transition form factors are calculated with the Wirbel-Stech-Bauer model. It is found that branching ratios for the color-favored and Cabibbo-favored $B_{c}^{\ast}$ ${\to}$ $B_{s}ρ$, $B_{s}π$ decays can reach up to ${\cal O}(10^{-7})$, which might be measurable in the future LHC experiments.

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arXiv2017

Study on the $Υ(1S)$ ${\to}$ $B_{c}D_{s}$ decay

The branching ratio and direct $CP$ asymmetry of the $Υ(1S)$ ${\to}$ $B_{c}D_{s}$ weak decay are estimated with the perturbative QCD approach. It is found that (1) The direct $CP$-violating asymmetry is close to zero. (2) the branching ratio ${\cal B}r(Υ(1S){\to}B_{c}D_{s})$ ${\gtrsim}$ $10^{-10}$ might be measurable at the future experiments.

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