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Hai-Yang Cheng

Publications and source records attributed to Hai-Yang Cheng.

At least 163 records · Page 9Linked to original sources
arXiv2000

Exclusive Hadronic D Decays to $η'$ or $η$: Addendum on Resonant Final-State Interactions

For hadronic two-body decays of charmed mesons involving $η$ or $η'$, resonance-induced final-state interactions (FSI) that mimic the W-exchange or the W-annihilation topology can play an essential role. In particular, the decays $D^0\to\bar K^0η'$ and $D^+\toπ^+η$, which are largely suppressed in the absence of FSI, are enhanced dramatically by resonant FSI. It is stressed that the effect of resonant FSI is negligible for $ρ^+(η,η')$ final states because of the mismatch of the G parity of $ρ^+(η,η')$ and the J=0, I=1 meson resonance. We argue that a possible gluon-mediated process in which two gluons couple directly to the gluonic component of the $η'$, e.g. the gluonium, rather than to the flavor-singlet $η_0$, can enhance both modes $D_s^+\toρ^+η'$ and $D_s^+\toρ^+η$, especially the former; that is, this new mechanism can account for the unexpectedly large branching ratio of $ρ^+η'$ without suppressing $ρ^+η$.

hep-ph↗
arXiv2000

The Proton Spin Puzzle: A Status Report

The proton spin puzzle inspired by the EMC experiment and its present status are closely examined. Recent experimental progress is reviewed. Various factorization schemes due to the ambiguity arising from the axial anomaly are discussed. Some misconceptions in the literature about the $\MS$ factorization scheme are clarified. It is stressed that the polarized nucleon structure function $g_1(x)$ is independent of the factorization scheme chosen in defining the quark spin density. Consequently, the anomalous gluon and sea-quark interpretations for the deviation of the observed first moment of $g_1^p(x)$ from the Ellis-Jaffe sum rule are equivalent. While it is well known that the total quark spin in the chiral-invariant (CI) factorization scheme (e.g. the improved parton model) can be made to be close to the quark model expectation provided that the gluon spin is positive and large enough, it is much less known that, contrary to the gauge-invariant scheme (e.g. the MSbar scheme), the quark orbital angular momentum in the CI scheme deviates even farther from the relativistic quark model prediction. Recent developments in the NLO analysis of polarized DIS data, orbital angular momentum and lattice calculations of the proton spin content are briefly sketched.

hep-ph↗
arXiv2000

Implications of Recent Measurements of Hadronic Charmless B Decays

Implications of recent CLEO measurements of hadronic charmless B decays are discussed. (i) Employing the Bauer-Stech-Wirbel (BSW) model for form factors as a benchmark, the $B\toπ^+π^-$ data indicate that the form factor $F_0^{Bπ}(0)$ is smaller than that predicted by the BSW model, whereas the data of $B\toωπ, K^*η$ imply that the form factors $A_0^{Bω}(0), A_0^{BK^*}(0)$ are greater than the BSW model's values. (ii) The tree-dominated modes $B\toπ^+π^-, ρ^0π^\pm, ωπ^\pm$ imply that the effective number of colors N_c(LL) for (V-A)(V-A) operators is preferred to be smaller, while the current limit on $B\toϕK$ shows that N_c(LR)>3. The data of $B\to Kη'$ and $K^*η$ clearly indicate that $N_c(LR)\gg N_c(LL)$. (iii) In order to understand the observed suppression of $π^+π^-$ and non-suppression of $Kπ$ modes, both being governed by the form factor $F_0^{Bπ}$, the unitarity angle $γ$ is preferred to be greater than $90^\circ$. By contrast, the new measurement of $B^\pm\toρ^0π^\pm$ no longer strongly favors $\cosγ<0$. (iv) The observed pattern $K^-π^+\sim \ov K^0π^-\sim {2\over 3}K^-π^0$ is consistent with the theoretical expectation: The constructive interference between electroweak and QCD penguin diagrams in the $K^-π^0$ mode explains why ${\cal B}(B^-\to K^-π^0)>{1\over 2}{\cal B}(\ov B^0\to K^-π^+)$. (v) The observation $\nc(LL)<3<\nc(LR)$ and our preference for $\nc(LL)\sim 2$ and $\nc(LR)\sim 6$ are justified by a recent perturbative QCD calculation of hadronic rare B decays in the heavy quark limit.

hep-ph↗
arXiv1999

Factorization Approach for the $ΔI=1/2$ Rule and $ε'/ε$ in Kaon Decays

The $ΔI=1/2$ rule and direct CP violation $ε'/ε$ in kaon decays are studied within the framework of the effective Hamiltonian approach in conjunction with generalized factorization for hadronic matrix elements. We identify two principal sources responsible for the enhancement of A0/A_2: the vertex-type as well as penguin-type corrections to the matrix elements of four-quark operators, which render the physical amplitude renormalization-scale and -scheme independent, and the nonfactorized effect due to soft-gluon exchange, which is needed to suppress the $ΔI=3/2$ $K\toππ$ amplitude. Contrary to the chiral approach which is limited to light meson decays and fails to reproduce the A2 amplitude, the aforementioned approach for dealing with scheme and scale issues is applicable to heavy meson decays. We obtain A0/A2=13-15 if $m_s$(1 GeV) lies in the range (125-175)MeV. The bag parameters $B_i$, which are often employed to parametrize the scale and scheme dependence of hadronic matrix elements, are calculated in two different renormalization scehemes. It is found that $B_8^{(2)}$ and $ B_6^{(0)}$, both of order 1.5 at $μ=1$ GeV, are nearly $γ_5$ scheme independent, whereas $B^{(0)}_{3,5,7}$ as well as $B_7^{(2)}$ show a sizable scheme dependence. Moreover, only $B_{1,3,4}^{(0)}$ exhibit a significant $m_s$ dependence, while the other $B$-parameters are almost $m_s$ independent. For direct CP violation, we obtain $ε'/ε=(0.7-1.1)\times 10^{-3}$ if $m_s(1 {\rm GeV})=150$ MeV and $ε'/ε=(1.0-1.6)\times 10^{-3}$ if $m_s$ is as small as indicated by some recent lattice calculations.

hep-ph↗
arXiv1999

The Delta I=1/2 Rule in Kaon Decays: A New Look

The $K\toππ$ decay amplitudes are studied within the framework of generalized factorization in which the effective Wilson coefficients are gauge-invariant, renormalization-scale and -scheme independent while factorization is applied to the tree-level hadronic matrix elements. Nonfactorized contributions to the hadronic matrix elements of (V-A)(V-A) four-quark operators, which are needed to account for the suppression of the $ΔI=3/2 K\toππ$ amplitude $A_2$ and the enhancement of the $ΔI=1/2 A_0$ amplitude, are phenomenologically extracted from the measured $K^+\toπ^+π^0$ decay and found to be large. The $A_0/A_2$ ratio is predicted to lie in the range 15-17 for $m_s(1 GeV)=(127-150)$ MeV. Vertex and penguin-type radiative corrections to the matrix elements of four-quark operators and nonfactorized effects due to soft-gluon exchange account for the bulk of the $ΔI=1/2$ rule. Comparison of the present analysis with the chiral-loop approach is given.

hep-ph↗
arXiv1999

Charmless Hadronic Two-body Decays of B_u and B_d Mesons

Two-body charmless nonleptonic decays of B_u and B_d mesons are studied within the framework of generalized factorization in which the effective Wilson coefficients $c^{eff}_i$ are renormalization-scale and -scheme independent while factorization is applied to the tree-level hadronic matrix elements. Contrary to previous studies, our $c_i^{eff}$ do not suffer from gauge and infrared problems. Nonfactorizable effects are parametrized in terms of N_c(LL) and N_c(LR), the effective numbers of colors arising from (V-A)(V-A) and (V-A)(V+A) four-quark operators, respectively. Tree and penguin transitions are classified into six different classes. The data of $B^-\toρ^0π^-$ and $B^-\toϕK^-$ clearly indicate that $N_c(LR)\neq N_c(LL)$: The first measurement of the b -> u mode $B^-\toρ^0π^-$ and the experimental information on the tree-dominated mode $B^-\toωπ^-$ all imply that Nc(LL) is less than 3, whereas the CLEO measurement of $B^-\toϕK^-$ shows Nc(LR)>3. For given input parameters, the prediction of ${\cal B}(B\toη' K)$ is largely improved by setting Nc(LL)~2 and Nc(LR)>Nc(LL); in particular, the charm content of the eta' contributes in the right direction. The decay rate of $B\toϕK^*$ is very sensitive to the form-factor ratio A_2/A_1; the absence of $B\toϕK$ events does not necessarily invalidate the factorization approach. If the branching ratio of $B^-\toωK^-$ is experimentally found to be significantly larger than that of $B^-\toρ^0 K^-$, we argue that inelastic final-state rescattering may account for the disparity between $ωK^-$ and $ρ^0 K^-$.

hep-ph↗
arXiv1999

Gauge Invariant and Infrared Finite Theory of Nonleptonic Heavy Meson Decays

We show that the controversies on the gauge dependence and the infrared singularity emerged in the generalized factorization approach for nonleptonic heavy meson decays within the framework of the operator product expansion can be resolved by perturbative QCD factorization theorem. Gauge invariance of the decay amplitude is maintained under radiative corrections by assuming on-shell external quarks. For on-shell external quarks, infrared poles in radiative corrections have to be extracted using the dimensional regularization. These poles, signifying nonperturbative dynamics of a decay process, are absorbed into bound-state wave functions. Various large logarithms produced in radiative corrections are summed to all orders into the Wilson and Sudakov evolution factors. The remaining finite part gives a hard subamplitude. A decay rate is then factorized into a convolution of the hard subamplitude, the Wilson coefficient, and the Sudakov factor with the bound-state wave functions, all of which are well-defined and gauge invariant.

hep-ph↗
arXiv1999

Phenomenological Analysis of D Meson Lifetimes

The QCD-based operator-product-expansion technique is systematically applied to the study of charmed meson lifetimes. We stress that it is crucial to take into account the momentum of the spectator light quark of charmed mesons, otherwise the destructive Pauli-interference effect in $D^+$ decays will lead to a negative decay width for the $D^+$. We have applied the QCD sum rule approach to estimate the hadronic matrix elements of color-singlet and color-octet 4-quark operators relevant to nonleptonic inclusive $D$ decays. The lifetime of $D_s^+$ is found to be longer than that of $D^0$ because the latter receives a constructive $W$-exchange contribution, whereas the hadronic annihilation and leptonic contributions to the former are compensated by the Pauli interference. We obtain the lifetime ratio $τ(D_s^+)/τ(D^0)$ $\approx 1.08\pm 0.04$, which is larger than some earlier theoretical estimates, but still smaller than the recent measurements by CLEO and E791.

hep-ph↗
arXiv1999

Updated Analysis of a_1 and a_2 in Hadronic Two-body Decays of B Mesons

Using the recent experimental data of $B\to D^{(*)}(π,ρ)$, $B\to D^{(*)} D_s^{(*)}$, $B\to J/ψK^{(*)}$ and various model calculations on form factors, we re-analyze the effective coefficients a_1 and a_2 and their ratio. QCD and electroweak penguin corrections to a_1 from $B\to D^{(*)}D_s^{(*)}$ and a_2 from $B\to J/ψK^{(*)}$ are estimated. In addition to the model-dependent determination, the effective coefficient a_1 is also extracted in a model-independent way as the decay modes $B\to D^{(*)}h$ are related by factorization to the measured semileptonic distribution of $B\to D^{(*)}\ell \barν$ at $q^2=m_h^2$. Moreover, this enables us to extract model-independent heavy-to-heavy form factors, for example, $F_0^{BD}(m_π^2)=0.66\pm0.06\pm0.05$ and $A_0^{BD^*}(m_π^2)=0.56\pm0.03\pm0.04$. The determination of the magnitude of a_2 from $B\to J/ψK^{(*)}$ depends on the form factors $F_1^{BK}$, $A_{1,2}^{BK^*}$ and $V^{BK^*}$ at $q^2=m^2_{J/ψ}$. By requiring that a_2 be process insensitive (i.e., the value of a_2 extracted from $J/ψK$ and $J/ψK^*$ states should be similar), as implied by the factorization hypothesis, we find that $B\to K^{(*)}$ form factors are severely constrained; they respect the relation $F_1^{BK}(m^2_{J/ψ})\approx 1.9 A_1^{BK^*}(m^2_{J/ψ})$. Form factors $A_2^{BK^*}$ and $V^{BK^*}$ at $q^2=m^2_{J/ψ}$ inferred from the measurements of the longitudinal polarization fraction and the P-wave component in $B\to J/ψK^*$ are obtained. A stringent upper limit on a_2 is derived from the current bound on $\ov B^0\to D^0π^0$ and it is sensitive to final-state interactions.

hep-ph↗
arXiv1998

Charmless Hadronic Two-body Decays of the B_s Mesons

Two-body charmless nonleptonic decays of the B_s meson are studied within the framework of generalized factorization in which factorization is applied to the tree level matrix elements while the effective Wilson coefficients are $μ$ and renormalization scheme independent, and nonfactorizable effects are parametrized in terms of N_c(LL) and N_c(LR), the effective numbers of colors arising from (V-A)(V-A) and (V-A)(V+A) four-quark operators, respectively. Branching ratios of $B_s\to PP,PV,VV$ decays are calculated as a function of N_c(LR) with two different considerations for N_c(LL). We find that (i) the electroweak penguin contributions account for about 85% (for N_c(LL)=2) of the decay rates of $B_s\to ηπ,η'π,ηρ,η'ρ,ϕπ,ϕρ$, which receive contributions only from tree and electroweak penguin diagrams; a measurement of them will provide a clean determination of the electroweak penguin coefficient a_9, (ii) electroweak penguin corrections to $B_s\toω\etapp,ϕη,ωϕ,K^{(*)}ϕ,ϕϕ$ are in general as significant as QCD penguin effects and even play a dominant role; their decay rates depend strongly on N_c(LR), (iii) the branching ratio of $B_s\to ηη'$ is of order 2x10^{-5}, (iv) the contribution from the $η'$ charm content is important for $B_s\toη'η'$, but less significant for $B_s\toηη'$, and (v) the decay rates for the final states $K^{+(*)}K^{-(*)}$ follow the pattern: $Γ(B_s\to K^+K^-)>Γ(B_s\to K^+K^{*-})\geqΓ(B_s\to K^{*+}K^{*-}) >Γ(B_s\to K^{+*}K^-)$, as a consequence of various interference effects between the penguin amplitudes governed by the coefficients a_4 and a_6.

hep-ph↗
arXiv1998

Exclusive Hadronic D Decays to eta' and eta

Hadronic decay modes $D^0\to(\bar K^0, \bar K^{*0})η,η'$ and $(D^+,D_s^+)\to(π^+,ρ^+)η,η'$ are studied in the generalized factorization approach. Form factors for $(D,D_s^+)\to(η,η')$ transitions are carefully evaluated by taking into account the wave function normalization of the eta and eta'. The predicted branching ratios are generally in agreement with experiment except for $D^0\to\bar K^0η', D^+\toπ^+η$ and $D_s^+\toρ^+η'$; the calculated decay rates for the first two decay modes are too small by an order of magnitude. We show that the weak decays $D^0\to K^-π^+$ and $D^+\to K^+\bar K^0$ followed by resonance-induced final-state interactions (FSI), which are amenable technically, are able to enhance the branching ratios of $D^0\to\bar K^0η'$ and $D^+\toπ^+η$ dramatically without affecting the agreement between theory and experiment for $D^0\to\bar K^0η$ and $D^+\toπ^+η'$. We argue that it is difficult to understand the observed large decay rates of $D_s^+\to ρ^+η'$ and $ρ^+η$ simultaneously; FSI, W-annihilation and the production of excess eta' from gluons are not helpful in this regard. The large discrepancy between the factorization hypothesis and experiment for the ratio of $D_s^+\toρ^+ η'$ and $D_s^+\toη' e^+ν$ remains as an enigma.

hep-ph↗
arXiv1998

Nonfactorizable Effects in Charmless B Decays and B Meson Lifetimes

Status of nonfactorizable effects in hadronic charmless B decays is reviewed. Implications of new CLEO measurements on $B^0\toπ^+π^-$ and $B\toη' K$ are discussed. Nonfactorizable effects due to color ocet 4-quark operators are calculated using renormalization group improved QCD sum rules. The resultant B-meson lifetime ratio $τ(B^-)/τ(B_d)$ agrees with experiment.

hep-ph↗
arXiv1998

Nonspectator Effects and B Meson Lifetimes from a Field-theoretic Calculation

The B meson lifetime ratios are calculated to the order of $1/m_b^3$ in the heavy quark expansion. The predictions of those ratios are dependent on four unknown hadronic parameters $B_1$, $B_2$, $ε_1$ and $ε_2$, where $B_1$ and $B_2$ parametrize the matrix elements of color singlet-singlet four-quark operators and $ε_1$ and $ε_2$ the matrix elements of color octet-octet operators. We derive the renormalization-group improved QCD sum rules for these parameters within the framework of heavy quark effective theory. The results are $B_1(m_b)=0.96\pm 0.04$, $B_2(m_b)=0.95\pm 0.02$, $ε_1(m_b)=-0.14\pm 0.01$, and $ε_2(m_b)=-0.08\pm 0.01$ to zeroth order in $1/m_b$. The resultant $B$ meson lifetime ratios are $τ(B^-)/τ(B_d)=1.11\pm 0.02$ and $τ(B_s)/τ(B_d)\approx 1$ in SU(3) symmetry limit.

hep-ph↗