Searcharxiv⌕ Search

arXiv subjects

Hai-Yang Cheng

Publications and source records attributed to Hai-Yang Cheng.

At least 181 records · Page 10Linked to original sources
arXiv1998

Nonfactorizable Effects in Spectator and Penguin Amplitudes of Hadronic Charmless B Decays

Nonfactorizable effects in hadronic charmless B -> PP, VP decays can be parametrized in terms of the effective number of colors N_c in the effective parameters $a_i^{eff}$ that are linear combinations of Wilson coefficients. It is shown that N_c(V+A) in the penguin amplitudes induced by the (V-A)(V+A) four-quark operators is different from N_c(V-A) in the decay amplitudes arising from the (V-A)(V-A) operators. Central values of the branching ratios of $B^\pm\toωπ^\pm$ and $B\toππ$ decays favor $N_c(V-A)\approx 2$, in accordance with the nonfactorizable effect observed in $B\to D^{(*)}π(ρ)$. Measurements of the interference effects in $B^-\toπ^-(ρ^-)π^0 (ρ^0)$ decays will provide a more decisive test on the parameter N_c(V-A). However, $N_c(V+A)\sim 2$ is ruled out by $B^\pm\toϕK^\pm$ and $B\toϕK^*$. We find that N_c(V+A) lies in the range $10\geq N_c(V+A)\geq 4.3$, subject to the corrections from W-annihilation and space-like penguin effects. With $N_c(V-A)\approx 2$ we show that the branching ratio of $B\toη' K$ is enhanced considerably at small values of 1/N_c(V+A) so that it is compatible with experiment. In particular, the measurement of $B^0\toη' K^0$ is now well explained without resorting to any new mechanism or new physics beyond the Standard Model. Finally, we point out that it is difficult to understand the observed large branching ratio of $B^\pm\toωK^\pm$ within the present framework. Inelastic final-state interactions may alleviate the difficulty with this decay mode.

hep-ph↗
arXiv1998

Effective Field Theory of Heavy Mesons

In this paper we present a detailed formulation for a recently proposed effective field theory to describe the nonperturbative QCD dynamics of heavy mesons. This effective theory incorporates with heavy quark symmetry (HQS) and the heavy quark effective theory (HQET). Heavy mesons in this theory are constructed as composite particles of a heavy quark bounded with the light degrees of freedom. The heavy meson properties in the heavy quark limit and the $1/m_Q$ corrections can then be explicitly evaluated from this effective theory. All the basic parameters of the HQET, namely, the heavy quark mass $m_Q$, the heavy meson residual mass $\overlineΛ$, and the HQS breaking mass parameters $λ_1$ and $λ_2$, are consistently determined. $λ_1$ is found to be small due to a large cancellation between the heavy quark kinetic energy and the chromo-electric interaction between the heavy quark and light degrees of freedom. We also evaluate the Isgur-Wise function, the decay constant, and the axial-vector coupling constant of heavy mesons.

hep-ph↗
arXiv1998

Remarks on Next-to-Leading Order Analysis of Polarized Deep Inelastic Scattering Data

Since the x dependence of the axial-anomaly effect in inclusive polarized deep inelastic scattering is fixed, the transformation from the $\bar{MS}$ scheme to different factorization schemes are not arbitrary. If the quark spin distribution is demanded to be anomaly-free so that it does not evolve with $Q^2$ and hard gluons contribute to the first moment of $g_1(x)$, then all the moments of coefficient and splitting functions are fixed by perturbative QCD for a given $γ_5$ prescription, contrary to the commonly used Adler-Bardeen (AB) or AB-like scheme. It is urged that, in order to correctly demonstrate the effect of factorization scheme dependence, the QCD analysis of polarized structure functions in next-to-leading order should be performed, besides the $\bar{MS}$ scheme, in the chiral-invariant factorization scheme in which the axial anomaly resides in the gluon coefficient function, instead of the less consistent and ambiguous AB scheme.

hep-ph↗
arXiv1998

A Covariant Light-Front Model of Heavy Mesons within HQET

In this paper we construct a covariant light-front model of heavy mesons within the framework of heavy quark effective theory (HQET). The covariant model consists of the light-front heavy meson bound states constructed in the heavy quark limit with heavy quark symmetry and heavy quark effective theory as its basis. Within this model, the Isgur-Wise function and decay constants in the infinite quark mass limit can be evaluated in a very simple and the most general way. The results are ensured to be consistent with heavy quark symmetry. From the heavy-quark-limit bound states, we can further develop a systematic approach to calculate $1/m_Q$ corrections from the $1/m_Q$ expansion of QCD. This covariant model can serve as a quasi-first-principles description of heavy meson dynamics, namely, a phenomenological covariant bound state in the heavy quark limit which is consistent with heavy quark symmetry, plus a reliable first-principles computation of the $1/m_Q$ corrections in HQET in terms of the $1/m_Q$ expansion of the fundamental QCD theory.

hep-ph↗
arXiv1997

Charmless B decays to eta' and eta

Exclusive charmless B decays to $η'$ and $η$ are studied in the standard effective Hamiltonian approach. The effective coefficients that take into account nonfactorizable effects in external and internal W-emission amplitudes are fixed by experiment. Factorization is applied to the matrix elements of QCD, electroweak penguin operators and next-to-leading corrections to penguin Wilson coefficients, which are renormalization-scheme dependent, are included, but the resultant amplitude is renormalization-scheme and -scale independent. We show that the unexpectedly large branching ratio of $B^\pm\to η' K^\pm$ measured by CLEO can be explained by the Cabibbo-allowed internal W-emission process $b\to c\bar cs$ followed by a conversion of the $c\bar c$ pair into the $η'$ via gluon exchanges characterized by a decay constant of order -50 MeV. Implications are discussed. The mechanism of the transition $c\bar c\toη'$, if correct, will enhance the decay rates of $B\to η' K^{*}$ by more than an order of magnitude. However, it plays only a minor role to the rare decays $B\to η'(η)π(ρ)$. The predicted branching ratio of $B^\pm\to ηπ^\pm$ for positive $ρ$ is marginally ruled out by experiment, indicating that the favored Wolfenstein parameter $ρ$ is negative.

hep-ph↗
arXiv1997

Nonfactorizable Effects in Exclusive Charmless B Decays

Nonfactorizable effects in charmless $B\to PP, VP$ decays can be lumped into the effective parameters a_i that are linear combinations of Wilson coefficients, or equivalently absorbed into the effective number of colors N_c. Naive factorization with N_c=3 fails to explain the CLEO data of $B^\pm \to ωK^\pm$, showing the first evidence for the importance of nonfactorizable contributions to the penguin amplitude. The decays $B^\pm \to ωπ^\pm$ dominated by tree amplitudes are sensitive to the interference between external and internal W-emission diagrams. Destructive interference implied by $N_c=\infty$ leads to a prediction of ${\cal B}(B^\pm \to ωπ^\pm)$ which is about $2σ$ too small compared to experiment. Therefore, the CLEO data of $B^\pm \to ωK^\pm, ωπ^\pm$ suffice to rule out N_c=3 and strongly disfavor $N_c=\infty$ for rare charmless B decays. Factorization based on $N_c\approx 2$ can accommodate the data of $B^\pm \to ωK^\pm$, but it predicts a slightly smaller branching ratio of $B^\pm \to ωπ^\pm$ with ${\cal B}(B^\pm \to ωπ^\pm)/{\cal B} (B^\pm \to ωK^\pm)=0.6$. We briefly explain why the 1/N_c expansion is still applicable to the B meson decay once the correct large-N_c counting rule for the Wilson coefficient c_2(m_b) is applied.

hep-ph↗
arXiv1997

A Phenomenological Analysis of Heavy Hadron Lifetimes

A phenomenological analysis of lifetimes of bottom and charmed hadrons within the framework of the heavy quark expansion is performed. The baryon matrix element is evaluated using the bag model and the nonrelativistic quark model. We find that bottom-baryon lifetimes follow the pattern $τ(Ω_b)\simeqτ(Ξ_b^-)>τ(Λ_b)\simeqτ(Ξ_b^0)$. However, neither the lifetime ratio $τ(Λ_b)/τ( B_d)$ nor the absolute decay rates of the $Λ_b$ baryon and $B$ mesons can be explained. One way of solving both difficulties is to allow the presence of linear $1/m_Q$ corrections by scaling the inclusive nonleptonic width with the fifth power of the hadron mass $m_{H_Q}$ rather than the heavy quark mass $m_Q$. The hierarchy of bottom baryon lifetimes is dramatically modified to $τ(Λ_b)>τ(Ξ_b^-)>τ(Ξ_b^0)>τ( Ω_b)$: The longest-lived $Ω_b$ among bottom baryons in the OPE prescription now becomes shortest-lived. The replacement of $m_Q$ by $m_{H_Q}$ in nonleptonic widths is natural and justified in the PQCD-based factorization approach formulated in terms of hadron-level kinematics. For inclusive charmed baryon decays, we argue that since the heavy quark expansion does not converge, local duality cannot be tested in this case. We show that while the ansatz of substituting the heavy quark mass by the hadron mass provides a much better description of the charmed-baryon lifetime {\it ratios}, it appears unnatural and unpredictive for describing the {\it absolute} inclusive decay rates of charmed baryons, contrary to the bottom case.

hep-ph↗
arXiv1997

Nonleptonic Weak Decays of Bottom Baryons

Cabibbo-allowed two-body hadronic weak decays of bottom baryons are analyzed. Contrary to the charmed baryon sector, many channels of bottom baryon decays proceed only through the external or internal W-emission diagrams. Moreover, W-exchange is likely to be suppressed in the bottom baryon sector. Consequently, the factorization approach suffices to describe most of the Cabibbo-allowed bottom baryon decays. We use the nonrelativistic quark model to evaluate heavy-to-heavy and heavy-to-light baryon form factors at zero recoil. When applied to the heavy quark limit, the quark model results do satisfy all the constraints imposed by heavy quark symmetry. The decay rates and up-down asymmetries for bottom baryons decaying into $(1/2)^++P(V)$ and $(3/2)^++P(V)$ are calculated. It is found that the up-down asymmetry is negative except for $Ω_b \to (1/2)^++P(V)$ decay and for decay modes with $ψ'$ in the final state. The prediction $B(Λ_b \to J/ψΛ)=1.6 \times 10^{-4}$ for $|V_{cb}|=0.038$ is consistent with the recent CDF measurement. We also present estimates for $Ω_c \to (3/2)^++P(V)$ decays and compare with various model calculations.

hep-ph↗
arXiv1997

Remarks on the Strong Coupling Constants in Heavy Hadron Chiral Lagrangians

In the heavy quark limit, there are three independent strong axial coupling constants in chiral Lagrangians for ground-state heavy hadrons: g in the meson sector, and g_1, g_2 in the baryon sector. The coupling g_2 is extracted from recent CLEO measurements of $Σ_c^*\toΛ_cπ$ to be $|g_2|=0.57\pm 0.10$, in agreement with the nonrelativistic quark model expectation but in $2σ$ deviation from the large-N_c argument. The parameter g_1 cannot be determined directly from strong decays of heavy baryons. Nevertheless, some information of g_1 can be learned from the radiative decay $Ξ_c^{'*0}\toΞ_c^0γ$, which is prohibited at tree level by SU(3) symmetry but induced by chiral loops. A measurement of $Γ(Ξ_c^{'*0}\toΞ_c^0γ)$ will yield two possible solutions for g_1. Assuming the validity of the quark model relations among different coupling constants, the experimental value of g_2 implies that $|g|=0.70\pm 0.12$ and $|g_1|=0.93\pm 0.16$.

hep-ph↗
arXiv1996

Can $B\to J/ψK(K^*)$ decays be described by factorization ?

The new measurements of $B\to J/ψK(K^*)$ decays by CDF and CLEO indicate that the production ratio $R$ and the fraction of longitudinal polarization $Γ_L/Γ$ are smaller than the previous results. In conjunction with the new result of parity-odd transverse polarization in $B\to J/ψK^*$, we found a minimal modification to the factorization hypothesis: While the data of $B\to J/ψK^*$ can be accommodated in the factorization approach with nonfactorizable terms $χ_{A_1} = χ_{A_2} = χ_{V} \equiv χ$ of order 15%, the result of $R$ measurement requires that the nonfactorizable effect $χ_{F_1}$ on $B\to J/ψK$ be slightly larger than $χ$. Therefore, the effective parameter $a_2^{\rm eff}$ is not universal even for $B\to J/ψK(K^*)$ decays. We have generalized the considerations to $B\to ψ(2S)K(K^*)$ and $B_s\to J/ψϕ$ and found that the predictions are in agreement with currently available data.

hep-ph↗
arXiv1996

Status of the Proton Spin Problem

The proton spin problem triggered by the EMC experiment and its present status are closely examined. Recent experimental and theoretical progresses and their implications are reviewed. It is pointed out that the sign of the sea-quark polarization generated perturbatively by hard gluons via the anomaly mechanism is predictable: It is negative if the gluon spin component is positive. We stress that the polarized nucleon structure function $g_1(x)$ is independent of the $k_\perp$-factorization scheme chosen in defining the quark spin density and the hard photon-gluon scattering cross section. Consequently, the anomalous gluon and sea-quark interpretations for $Γ_1$, the first moment of $g_1(x)$, are equivalent. It is the axial anomaly that accounts for the observed suppression of $Γ_1^p$.

hep-ph↗
arXiv1996

Baryonic Form Factors in the Nonrelativistic Quark Model

Weak current-induced baryonic form factors at zero recoil are evaluated in the rest frame of the heavy parent baryon using the nonrelativistic quark model. Contrary to previous similar work in the literature, our quark model results do satisfy the constraints imposed by heavy quark symmetry for heavy-heavy baryon transitions. Assuming a dipole $q^2$ behavior, we have applied the quark model form factors to nonleptonic, semileptonic and weak radiative decays of the heavy baryons.

hep-ph↗
arXiv1996

Mesonic Form Factors in the Light-Front Quark Model

Form factors for $P\to P$ and $P\to V$ transitions due to the valence-quark configuration are calculated directly in the physical time-like range of momentum transfer within the light-front quark model. It is pointed out that the Bauer-Stech-Wirbel type of light-front wave function fails to give a correct normalization for the Isgur-Wise function at zero recoil in $P\to V$ transition. Some of the $P\to V$ form factors are found to depend on the recoiling direction of the daughter mesons relative to their parents. Thus, the inclusion of the non-valence contribution arising from quark-pair creation is mandatory in order to ensure that the physical form factors are independent of the recoiling direction.

hep-ph↗
arXiv1996

Mesonic Form Factors and the Isgur-Wise Function on the Light-Front

Within the light-front framework, form factors for $P\to P$ and $P\to V$ transitions ($P$: pseudoscalar meson, $V$: vector meson) due to the valence-quark configuration are calculated directly in the entire physical range of momentum transfer. The behavior of form factors in the infinite quark mass limit are examined to see if the requirements of heavy-quark symmetry are fulfilled. We find that the Bauer-Stech-Wirbel type of light-front wave function fails to give a correct normalization for the Isgur-Wise function at zero recoil in $P\to V$ transition. Some of the $P\to V$ form factors are found to depend on the recoiling direction of the daughter mesons relative to their parents. Thus, the inclusion of the non-valence configuration arising from quark-pair creation is mandatory in order to ensure that the physical form factors are independent of the recoiling direction. The main feature of the non-valence contribution is discussed.

hep-ph↗
arXiv1996

Analysis of Two-Body Decays of Charmed Baryons Using the Quark-Diagram Scheme

We give a general formulation of the quark-diagram scheme for the nonleptonic weak decays of baryons. We apply it to all the decays of the antitriplet and sextet charmed baryons and express their decay amplitudes in terms of the quark-diagram amplitudes. We have also given parametrizations for the effects of final-state interactions. For SU(3) violation effects, we only parametrize those in the horizontal $W$-loop quark diagrams whose contributions are solely due to SU(3)-violation effects. In the absence of all these effects, there are many relations among various decay modes. Some of the relations are valid even in the presence of final-state interactions when each decay amplitude in the relation contains only a single phase shift. All these relations provide useful frameworks to compare with future experiments and to find out the effects of final-state interactions and SU(3) symmetry violations.

hep-ph↗
arXiv1996

CKM Favored Semileptonic Decays of Heavy Hadrons at Zero Recoil

We study the properties of Cabibbo-Kobayashi-Maskawa (CKM) favored semileptonic decays of mesons and baryons containing a heavy quark at the point of no recoil. We first use a diagrammatic analysis to rederive the result observed by earlier authors that at this kinematic point the $B$ meson decays via $b\to c$ transitions can only produce a $D$ or $D^*$ meson. The result is generalized to include photon emissions which violate heavy quark flavor symmetry. We show that photons emitted by the heavy quarks and the charged lepton are the only light particles that can decorate the decays $\bar{B}\to D(D^*) + \ellν$ at zero recoil, and the similar processes of heavy baryons. Implications for the determinations of the CKM parameter $V_{cb}$ are discussed. Also studied in this paper is the connection between our diagrammatic analysis of suppression of particle emission and the formal observation based on weak currents at zero recoil being generators of heavy quark symmetry. We show that the two approaches can be unified by considering the Isgur-Wise function in the presence of an external source.

hep-ph↗
arXiv1995

Sea Quark or Anomalous Gluon Interpretation for $g_1^p(x)$ ?

Contrary to what has been often claimed in the literature, we clarify that the hard photon-gluon cross section $\gg_{\rm hard}(x)$ in polarized deep inelastic scattering calculated in the gauge-invariant factorization scheme does {\it not} involve any soft contributions and hence it is genuinely {\it hard}. We show that the polarized proton structure function $g_1^p(x)$ up to the next-to-leading order of $α_s$ is independent of the factorization convention, e.g., the gauge-invariant or chiral-invariant scheme, chosen in defining $\gg_{\rm hard}(x)$ and the quark spin density. Thereby, it is not pertinent to keep disputing which factorization prescription is correct or superior. The hard-gluonic contribution to $Γ_1^p$, the first moment of $g_1^p(x)$, is purely factorization dependent. Nevertheless, we stress that even though hard gluons do not contribute to $Γ_1^p$ in the gauge-invariant scheme, the gluon spin component in a proton, which is factorization independent, should be large enough to perturbatively generate a negative sea polarization via the axial anomaly. We briefly comment on how to study the $Q^2$ evolution of parton spin distributions to the next-to-leading order of QCD in the chiral-invariant factorization scheme.

hep-ph↗