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Kwei-Chou Yang

Publications and source records attributed to Kwei-Chou Yang.

At least 55 records · Page 3Linked to original sources
arXiv2007

Branching Ratios and CP Asymmetries of B \to a_1(1260) πand a_1(1260) K Decays

We present the studies of the decays $B\to a_1(1260) π$ and $a_1(1260) K$ within the framework of QCD factorization. Due to the G-parity, unlike the vector meson, the chiral-odd two-parton light-cone distribution amplitudes of the $a_1$ are antisymmetric under the exchange of quark and anti-quark momentum fractions in the SU(2) limit. The branching ratios for $a_1 π$ modes are sensitive to tree--penguin interference. The resultant ${\cal B}(B^0 \to a_1^\pm π^\mp)$ are in good agreement with the data. However, using the current Cabibbo--Kobayashi--Maskawa angles, $β=22.0^\circ$ and $γ=59.0^\circ$, our results for the mixing-induced parameter $S$ and $α_{\rm eff}$ differ from the measurements of the time-dependent CP asymmetries in the decay $B^0\to a_1^\pm π^\mp$ at about the $3.7σ$ level. This puzzle may be resolved by using a larger $γ\gtrsim 80^\circ$. For $a_1 K$ modes, the annihilation topologies give sizable contributions and are sensitive to the first Gegenbauer moment of the leading-twist tensor (chiral-odd) distribution amplitude of the $a_1$ meson. The $B\to a_1 K$ amplitudes resemble the corresponding $B\to πK$ ones very much. Taking the ratios of corresponding CP-averaged $a_1 K$ and $πK$ branching ratios, we can extract information relevant to the electroweak penguins and annihilations. The existence of new-physics in the electroweak penguin sector and final state interactions during decays can thus be explored.

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arXiv2007

Charmless B decays to a scalar meson and a vector meson

The hadronic charmless B decays into a scalar meson and a vector meson are studied within the framework of QCD factorization. The main results are: (i) The decay rates for the $f_0(980)K^{*-}$ and $f_0(980)\ov K^{*0}$ modes depend on the $f_0-σ$ mixing angle $θ$. The experimental measurements can be accommodated for $θ\approx 20^\circ$. (ii) If the $a_0(980)$ is a $q\bar q$ bound state, the predicted branching ratios for the channels $a_0^+ρ^-$ and $a_0^0ρ^-$ will be very large, of order $30\times 10^{-6}$ and $23\times 10^{-6}$, respectively. If the observation of or the experimental limit on theses two modes is much smaller than the expectation of $\sim 25\times 10^{-6}$, this could hint at a four-quark nature for the $a_0(980)$. (iii) For the $a_0(1450)$ channels, $a_0^+(1450)ρ^-$ and $a_0^0(1450)ρ^-$ are found to have branching ratios of order $16\times 10^{-6}$ and $22\times 10^{-6}$, respectively. A measurement of them at the predicted level will favor the $q\bar q$ structure for the $a_0(1450)$. (iv) Contrary to the naive expectation that $Γ(B^-\to a_0^0ρ^-)\sim {1\over 2}Γ(\bar B^0\to a_0^+ρ^-)$, we found that they have comparable rates due to additional contributions to the $a_0ρ^-$ mode from the $a_0^0$ emission. (v) The predicted central value of $Br(\bar B^0\to\ov K^{*0}_0(1430)ϕ)$ is larger than experiment, though it can be accommodated within theoretical errors. The decays $B\to K^{*}_0(1430)ρ$ are expected to have rates substantially larger than that of $B\to K^{*}_0(1430)π$ owing to the constructive (destructive) interference between the $a_4$ and $a_6$ penguin terms in the former (latter). Experimentally, it is thus important to check if the $B\to K^{*}_0ρ$ modes are enhanced relative to the corresponding $K_0^*π$ channels.

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arXiv2005

Charmless hadronic B decays involving scalar mesons: Implications to the nature of light scalar mesons

The hadronic charmless B decays into a scalar meson and a pseudoscalar meson are studied within the framework of QCD factorization. Although the light scalar mesons $f_0(980)$ and $a_0(980)$ are widely perceived as primarily the four-quark bound states, in practice it is difficult to make quantitative predictions based on the four-quark picture for light scalars. Hence, predictions are made in the 2-quark model for the scalar mesons. Based on the QCD sum rule method, we have derived the leading-twist light-cone distribution amplitudes of scalar mesons and their decay constants. The short-distance approach suffices to explain the observed large rates of $f_0(980)K^-$ and $f_0(980)\bar K^0$ that receive major penguin contributions from the $b\to ss\bar s$ process. When $f_0(980)$ is assigned as a four-quark bound state, there exist extra diagrams contributing to $B\to f_0(980)K$. Therefore, a priori the $f_0(980)K$ rate is not necessarily suppressed for a four-quark state $f_0(980)$. The calculated $\bar B^0\to a_0^+(980)π^-$ and $a_0^+(980)K^-$ rates exceed slightly the current experimental limits. If the branching ratio of $\bar B^0\to a_0^+(1450)π^-$ is measured at the level of $(4\sim 6)\times 10^{-6}$ while $a_0^+(980)π^-$ is found to be smaller, say, of order $(1\sim 2)\times 10^{-6}$ or even smaller than this, it will be likely to imply a 2-quark nature for $a_0(1450)$ and a four-quark assignment for $a_0(980)$. The penguin-dominated modes $a_0(980)K$ and $a_0(1450)K$ receive dominant weak annihilation contributions.

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arXiv2005

Penguin-induced Radiative Baryonic B Decays Revisited

Weak radiative baryonic B decays $\bar B\to B_1\bar B_2γ$ mediated by the electromagnetic penguin process $b\to sγ$ are re-examined within the framework of the pole model. The meson pole contribution that has been neglected before is taken into account in this work. It is found that the intermediate $K^*$ contribution dominates in the $Σ\bar pγ$ mode and is comparable to the baryon pole effect in $Λ\bar pγ$ and $Ξ\barΛγ$ modes. The branching ratios for $B^-\toΛ\bar pγ$ and $B^-\toΞ^0\barΣ^-γ$ are of order $2.6\times 10^{-6}$ and $6\times 10^{-7}$, respectively. The threshold enhancement effect in the dibaryon mass spectrum is responsible by the meson pole diagram. We also study the angular distribution of the baryon in the dibaryon rest frame. The baryon pole diagrams imply that the antibaryon tends to emerge in the direction of the photon in the baryon-pair rest frame. The predicted angular asymmetry agrees with experiment for $B^-\toΛ\bar pγ$. Measurements of the correlation of the photon with the baryon allow us to discriminate between different models for describing the radiative baryonic B decays. For decays $B\toΞ\barΣγ$, a large correlation of the photon to the $\barΣ$ and a broad bump in the dibaryon mass spectrum are predicted.

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arXiv2005

Weak Annihilation Topologies and Final State Interactions in D -> PP Decays

We study two-body D -> PP decays, assuming that each decay process go through the bare amplitude followed by elastic SU(3) rescattering, where the bare amplitude consists of (i) the color-allowed and color-suppressed factorization amplitudes and (ii) the short-distance weak annihilation amplitudes. We have performed the χ^2 fit on 14 branching ratios of D -> PP decays in the formalism of the above mentioned model. The final state interactions can be well accounted for by the short-distance annihilation topologies and SU(3) rescatterings. The two SU(3) rescattering phase differences are δ\equiv δ_{27}-δ_{8} \simeq -46^\circ and σ\equiv δ_{27}-δ_{1} \simeq -21^\circ, where δ_{27}, δ_{8} and δ_{1}are the rescattering phases of final states corresponding to the representations 27, 8 and 1, respectively. We find that the D^0 -> K^0 \bar{K}^0 decay occurs mainly due to the nonzero short-distance weak annihilation effects, originating from SU(3) symmetry-breaking corrections to the distribution amplitudes of the final-state kaons, but receives tiny effects from other modes via SU(3) rescattering. Our results are in remarkable accordance with the current data.

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arXiv2005

Annihilation in Factorization-suppressed B Decays Involving a 1^1P_1 Meson and Search for New-Physics Signals

The recent measurements of large transverse fractions in B \to ϕK^* decays represent a challenge for theory. It was shown that the QCD penguin annihilation could contribute to the transverse amplitudes such that the observation could be accounted for in the Standard Model. However, if one resorts to new physics for resolutions, then the relevant 4-quark operators should be tensor-like or scalar-like. We show that the same annihilation effects could remarkably enhance the longitudinal rates of factorization-suppressed B\to h_1(1380) K^*, b_1(1235) K^* modes to be (12.0^{+4.1}_{-3.0})\times 10^{-6} and (7.0\pm 3.5)\times 10^{-6}, respectively, but attend to be cancelled in the transverse components. Nevertheless, the transverse fractions of B\to h_1(1380) K^* can become sizable due to new-physics contributions. The measurements of these decays can thus help us to realize the role of annihilation topologies in charmless B decays and offer a theoretically clean window to search for the evidence of new physics.

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arXiv2005

Data for Polarization in Charmless B -> phi K^*: A Signal for New Physics?

The recent observations of sizable transverse fractions of B-> phi K* are strongly contrary to the Standard Model expectation. We analyze all possible new-physics four-quark operators. We find that two classes of new-physics operators could offer resolutions to the B-> phi K* polarization anomaly. The (1-γ_5)\otimes (1-γ_5) and σ(1-γ_5)\otimes σ(1-γ_5) operators in the first class contribute to different helicity amplitudes as H{00}: H{--}: H{++} ~ O(1/mb):O(1):O(1/mb^2), while the contributions arising from (1+γ_5)\otimes (1+γ_5) and σ(1+γ_5)\otimes σ(1+γ_5) operators in the second class to various helicity amplitudes are given as H{00}: H{--}: H{++} ~ O(1/mb):O(1/mb^2):O(1). For each class of new physics, we find that all new physics effects can be lumped into a single parameter. Two possible experimental results of polarization phases, \arg(A_\perp)- \arg(A_\parallel) ~ πor 0, originating from the phase ambiguity in data, could be separately accounted for by our two new-physics scenarios: (i) the first scenario with the first class new-physics operators, and (ii) the second scenario with the second class new-physics operators. The consistency between the data and the new physics analysis, suggests a small new-physics weak phase, together with a large strong phase. We obtain sizable transversely polarized amplitudes Λ_{\parallel\parallel} + Λ_{\perp\perp} = Λ_{00}. In the second scenario, we find Λ_{\parallel\parallel} ~ Λ_{\perp\perp}, while, in the first scenario, we obtain Λ_{\parallel\parallel} ~ Λ_{\perp\perp}/2. We discuss the impact of the new-physics weak phase on observations.

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arXiv2005

$B\to f_0(980)K$ Decays and Subleading Corrections

The decay $B\to f_0(980)K is studied within the framework of QCD factorization and the two-quark scenario for $f_0(980)$. There are two distinct penguin contributions and their interference depends on the unknown mixing angle $θ$ of strange and nonstrange quark contents of $f_0(980)$: destructive for $0<θ<π/2$ and constructive for $π/2<θ<π$. The QCD sum rule method is applied to evaluate the leading-twist light-cone distribution amplitudes and the scalar decay constant of $f_0$. We conclude that the short-distance approach is not adequate to explain the observed large rates of $f_0K^-$ and $f_0\ov K^0$. Among many possible subleading corrections, we study and estimate the contributions from the three-parton Fock states of the $f_0$ and from the intrinsic gluon inside the $B$ meson. It is found that the spectator gluon of the $B$ meson may play an eminent role for the enhancement of $f_0(980)K$. We point out that if $f_0(980)$ is a four-quark state as widely perceived, there will exist extra diagrams contributing to $B\to f_0(980)K$. However, in practice it is difficult to make quantitative predictions based on the four-quark picture for $f_0(980)$ as it involves additional nonfactorizable contributions that are difficult to estimate and the calculations of the decay constant and form factors of $f_0(980)$ are beyond the conventional quark model.

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arXiv2004

Impact of Subleading Corrections on Hadronic B Decays

We study the subleading corrections originating from the 3-parton (q\bar q g) Fock states of final-state mesons in B decays. The corrections could give significant contributions to decays involving an ωor η^{(\prime)} in the final states. Our results indicate the similarity of ωK and ωπ^- rates, of order 5\times 10^{-6}, consistent with the recent measurements. We obtain a_2(B\to J/ψK)\approx 0.27+0.05i, in good agreement with data. Without resorting to the unknown singlet annihilation effects, 3-parton Fock state contributions can enhance the branching ratios of Kη' to the level above 50\times 10^{-6}.

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arXiv2003

Higher Twist Effects in Hadronic B Decays

Within the framework of QCD factorization, we discuss various important corrections arising from higher twist distribution amplitudes of mesons in the hadronic B decays.

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arXiv2003

Indication for Large Rescatterings in Charmless Rare B Decays

The current wealth of charmless B decay data may suggest the presence of final state rescattering. In a factorized amplitude approach, better fits are found by incorporating two SU(3) rescattering phase differences, giving delta ~ 65 degree and sigma ~ 90 - 100 degree. Fitting with unitarity phase phi_3 as a fit parameter gives phi_3 ~ 96 degree, the CP asymmetries A_{pi pi}, S_{pi pi} agree better with BaBar, and the sigma phase is slightly lower. Keeping phi_3 = 60 degree fixed in fit gives S_{pi pi} ~-0.9, which agrees better with Belle. With the sizable delta, sigma rescattering phases as fitted, many direct CP asymmetries flip sign, and B0 --> pi0 pi0, K- K+ rates are of order 10^{-6}, which can be tested soon.

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arXiv2002

Hadronic B Decays to Charmed Baryons

We study exclusive B decays to final states containing a charmed baryon within the pole model framework. Since the strong coupling for $Λ_b\bar B N$ is larger than that for $Σ_b \bar BN$, the two-body charmful decay $B^-\toΣ_c^0\bar p$ has a rate larger than $\bar B^0\toΛ_c^+\bar p$ as the former proceeds via the $Λ_b$ pole while the latter via the $Σ_b$ pole. By the same token, the three-body decay $\bar B^0\toΣ_c^{++}\bar pπ^-$ receives less baryon-pole contribution than $B^-\toΛ_c^+\bar pπ^-$. However, because the important charmed-meson pole diagrams contribute constructively to the former and destructively to the latter, $Σ_c^{++}\bar pπ^-$ has a rate slightly larger than $Λ_c^+\bar pπ^-$. It is found that one quarter of the $B^-\to Λ_c^+\bar pπ^-$ rate comes from the resonant contributions. We discuss the decays $\bar B^0\toΣ_c^0\bar pπ^+$ and $B^-\toΣ_c^0\bar pπ^0$ and stress that they are not color suppressed even though they can only proceed via an internal W emission.

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arXiv2002

Three-Body Charmful Baryonic B Decays $\bar B\to D(D^*)N\bar N$

We study the charmful three-body baryonic B decays $\bar B\to D^{(*)}N\bar N$: the color-allowed modes $\bar B^0\to D^{(*)+}n\bar p$ and the color-suppressed ones $\bar B^0\to D^{(*)0}p\bar p$. While the $D^{*+}/D^+$ production ratio is predicted to be of order 3, it is found that $D^0p\bar p$ has a similar rate as $D^{*0}p\bar p$. It is pointed out that $\bar B^0\to D(D^*)N\bar N$ are dominated by the nucleon vector current or by vector meson intermediate states, whereas $\bar B^0\to D^0p\bar p$ proceeds mainly via the exchange of the axial-vector intermediate state $a_1(1260)$. The study of the $N\bar N$ invariant mass distribution in general indicates a threshold baryon pair production; that is, a recoil charmed meson accompanied by a low mass baryon pair except that the spectrum of $D^0p\bar p$ has a hump at large $p\bar p$ invariant mass $m_{p\bar p}\sim 3.0$ GeV.

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arXiv2002

Nonresonant Three-body Decays of D and B Mesons

Nonresonant three-body decays of D and B mesons are studied. It is pointed out that if heavy meson chiral perturbation theory (HMChPT) is applied to the heavy-light strong and weak vertices and assumed to be valid over the whole kinematic region, then the predicted decay rates for nonresonant charmless 3-body B decays will be too large and especially B^- --> pi^- K^+ K^- greatly exceeds the current experimental limit. This can be understood as chiral symmetry has been applied there twice beyond its region of validity. If HMChPT is applied only to the strong vertex and the weak transition is accounted for by the form factors, the dominant B^* pole contribution to the tree-dominated direct three-body B decays will become small and the branching ratio will be of order 10^{-6}. The decay modes B^- --> (K^- h^+ h^-)_{NR} and bar{B}^0 --> (bar{K}^0 h^+h^-)_{NR} for h = pi, K are penguin dominated. We apply HMChPT in two different cases to study the direct 3-body D decays and compare the results with experiment. Theoretical uncertainties are discussed.

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arXiv2002

Charmless Exclusive Baryonic B Decays

We present a systematical study of two-body and three-body charmless baryonic B decays. Branching ratios for two-body modes are in general very small, typically less than $10^{-6}$, except that $\B(B^-\to p \barΔ^{--})\sim 1\times 10^{-6}$. In general, $\bar B\to N\barΔ>\bar B\to N\bar N$ due to the large coupling constant for $Σ_b\to BΔ$. For three-body modes we focus on octet baryon final states. The leading three-dominated modes are $\bar B^0\to p\bar nπ^-(ρ^-), n\bar pπ^+(ρ^+)$ with a branching ratio of order $3\times 10^{-6}$ for $\bar B^0\to p\bar nπ^-$ and $8\times 10^{-6}$ for $\bar B^0\to p\bar nρ^-$. The penguin-dominated decays with strangeness in the meson, e.g., $B^-\to p\bar p K^{-(*)}$ and $\bar B^0\to p\bar n K^{-(*)}, n\bar n \bar K^{0(*)}$, have appreciable rates and the $N\bar N$ mass spectrum peaks at low mass. The penguin-dominated modes containing a strange baryon, e.g., $\bar B^0\to Σ^0\bar pπ^+, Σ^-\bar nπ^+$, have branching ratios of order $(1\sim 4)\times 10^{-6}$. In contrast, the decay rate of $\bar B^0\toΛ\bar pπ^+$ is smaller. We explain why some of charmless three-body final states in which baryon-antibaryon pair production is accompanied by a meson have a larger rate than their two-body counterparts: either the pole diagrams for the former have an anti-triplet bottom baryon intermediate state, which has a large coupling to the $B$ meson and the nucleon, or they are dominated by the factorizable external $W$-emission process.

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arXiv2002

Final State Rescattering and Color-suppressed \bar B^0-> D^{(*)0} h^0 Decays

The color-suppressed \bar B^0-> D^{(*)0}π^0, D^{(*)0}η, D^0ωdecay modes have just been observed for the first time. The rates are all larger than expected, hinting at the presence of final state interactions. Considering \bar B^0-> D^{(*)0}π^0 mode alone, an elastic D^{(*)}π-> D^{(*)}πrescattering phase difference δ\equiv δ_{1/2} - δ_{3/2} \sim 30^\circ would suffice, but the \bar B^0-> D^{(*)0}η, D^0ωmodes compel one to extend the elastic formalism to SU(3) symmetry. We find that a universal a_2/a_1=0.25 and two strong phase differences 20^\circ \sim θ< δ< δ^\prime \sim 50^\circ can describe both DP and D^*P modes rather well; the large phase of order 50^\circ is needed to account for the strength of {\it both} the D^{(*)0}π^0 and D^{(*)0}ηmodes. For DV modes, the nonet symmetry reduces the number of physical phases to just one, giving better predictive power. Two solutions are found. We predict the rates of the \bar B^0-> D^{+}_s K^-, D^{*+}_s K^-, D^0ρ^0, D^+_s K^{*-} and D^0ϕmodes, as well as \bar B^0-> D^{0}\bar K^0, D^{*0}\bar K^0, D^{0}\bar K^{*0} modes. The formalism may have implications for rates and CP asymmetries of charmless modes.

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arXiv2002

B -> J/psi K^* Decays in QCD Factorization

The hadronic decay B -> J K^* is analyzed within the framework of QCD factorization. The spin amplitudes A_0, A_\parallel and A_\perp in the transversity basis and their relative phases are studied using various different form-factor models for B-K^* transition. The effective parameters a_2^h for helicity h=0,+,- states receive different nonfactorizable contributions and hence they are helicity dependent, contrary to naive factorization where a_2^h are universal and polarization independent. QCD factorization breaks down even at the twist-2 level for transverse hard spectator interactions. Although a nontrivial strong phase for the A_\parallel amplitude can be achieved by adjusting the phase of an infrared divergent contribution, the present QCD factorization calculation cannot say anything definite about the phase phi_\parallel. Unlike B -> J/psi K decays, the longitudinal parameter a_2^0 for B -> J/psi K^* does not receive twist-3 corrections and is not large enough to account for the observed branching ratio and the fraction of longitudinal polarization. Possible enhancement mechanisms for a_2^0 are discussed.

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