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Hai-Yang Cheng

Publications and source records attributed to Hai-Yang Cheng.

At least 199 records · Page 11Linked to original sources
arXiv1995

Polarized Parton Distribution Functions Reexamined

The hard-gluonic contribution to the first moment of the polarized proton structure function $g_1^p(x)$ is dependent of the factorization convention chosen in defining the quark spin density and the hard cross section for photon-gluon scattering. Two extremes of interest, namely gauge-invariant and chiral-invariant factorization schemes, are considered. We show that in order to satisfy the positivity constraint for sea and gluon polarizations, the polarized valence quark distributions should fully account for the observed $g_1^p(x)$ at $x\gsim 0.2\,$. This together with the first-moment and perturbative QCD constraints puts a pertinent restriction on the shape of $Δu_v(x)$ and $Δd_v(x)$. The spin-dependent sea distribution in the gauge-invariant factorization scheme is extracted from the data of $g_1^p(x)$. It is shown in the chiral invariant scheme that it is possible to interpret the $g_1^p(x)$ data with anomalous gluonic contributions, yet a best least $χ^2$ fit to the data implies a gluon spin distribution which violates the positivity condition $|ΔG(x)|\leq G(x)$. We then propose a more realistic set of parton spin distributions with sea polarization and with a moderate value of $ΔG$. The polarized parton distributions in this work are presented in the next-to-leading order of QCD at the scale $Q^2=10\,{\rm GeV}^2\,$. Predictions for the polarized structure functions $g_1^n(x)$ of the neutron and $g_1^d(x)$ of the deuteron are given.

hep-ph↗
arXiv1995

U(1) Goldberger-Treiman Relation and Its Connection to the Proton Spin

The U(1) Goldberger-Treiman (GT) relation for the axial charge $g_A^0$ is reexamined. It is stressed that the isosinglet GT relation in terms of the $η_0$ holds irrespective of the quark masses and the axial anomaly. We pointed out that the identification of the $η_0-N$ and $\dk-N$coupling terms with the quark and gluon spin components respectively in a proton is possible but valid only in the chiral-invariant factorization scheme. In general, the two-component U(1) GT relation can be identified in a gauge-invariant way with connected and disconnected insertions. The observation that $(\sqrt{3}f_π/2m_N)\getao$ is related to the connected insertion i.e., the total valence quark contribution to the proton spin enables us to determine the physical coupling constants $\getap$ and $\geta$ from the GT relations for $g_A^0$ and $g_A^8$. We found $\getap=3.4$ and $\geta=4.7\,$.

hep-ph↗
arXiv1995

1/M Corrections to Baryonic Form Factors in the 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 at the symmetric point $v\cdot v'=1$ and are in agreement with the predictions of the heavy quark effective theory for antitriplet-antitriplet heavy baryon form factors at zero recoil evaluated to order $1/m_Q$. Furthermore, the quark model approach has the merit that it is applicable to any heavy-heavy and heavy-light baryonic transitions at maximum $q^2$. 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. It is emphasized that the flavor suppression factor occurring in many heavy-light baryonic transitions, which is unfortunately overlooked in most literature, is very crucial towards an agreement between theory and experiment for the semileptonic decay $Λ_c\toΛe^+ν_e$. Predictions for the decay modes $\b,~Λ_c \to pϕ,~Λ_b\toΛγ$, $Ξ_b\toΞγ$, and for the semileptonic decays of $Λ_b,~Ξ_{b,c}$ and $Ω_b$ are presented. }

hep-ph↗
arXiv1995

Hadronic Weak Decays of Heavy Mesons and Nonfactorization

The parameters $χ_{1,2}$, which measure nonfactorizable soft gluon contributions to hadronic weak decays of mesons, are updated by extracting them from the data of $D,\,B\to PP,~VP$ decays ($P$: pseudoscalar meson, $V$: vector meson). It is found that $χ_2$ ranges from $-0.36$ to $-0.60$ in the decays from $D\to\bar{K}π$ to $D^+\toϕπ^+,~D\to\bar{K}^*π$, while it is of order 10\% with a positive sign in $B\toψK,~Dπ,~D^*π,~Dρ$ decays. Therefore, the effective parameter $a_2$ is process dependent in charm decay, whereas it stays fairly stable in $B$ decay. This implies the picture that nonfactorizable effects become stronger when the decay particles become less energetic after hadronization. As for $D,\,B\to VV$ decays, the presence of nonfactorizable terms in general prevents a possible definition of effective $a_1$ and $a_2$. This is reenforced by the observation of a large longitudinal polarization fraction in $B\toψK^*$ decay, implying $S$-wave dominated nonfactorizable effects. The nonfactorizable term dominated by the $S$-wave is also essential for understanding the decay rate of $B^-\to D^{*0} ρ^-$. It is found that all nonfactorizable effects $A_1^{nf}/A_1^{BK^*},~A _1^{nf}/A_1^{Bρ},~A_1^{nf}/A_1^{BD^*}$ ($nf$ standing for nonfactorization) are positive and of order 10\%, in accordance with $χ_2(B\to D(D^*)π( ρ))$ and $χ_2(B\toψK)$. However, we show that in $D\to\bar{K}^*ρ$ decay nonfactorizable effects cannot be dominated by the $S$-wave. A polarization measurement in the color- and Cabibbo-suppressed decay mode $D^+\toϕρ^+$ is strongly urged in order to test if $A_2^{nf}/A_2$ plays a more pivotal role than $A_1^{nf}/A_1$ in charm decay.

hep-ph↗
arXiv1994

Extraction of $a_1$ and $a_2$ from $B\to ψK(K^*),~ D(D^*)π(ρ)$ Decays

Based on the factorization approach, we show that the CLEO data for the ratio $Γ(B\toψK^*)/Γ(B\to ψK)$ and the CDF measurement of the fraction of longitudinal polarization in $B\toψK^*$ can be accounted for by the heavy-flavor-symmetry approach for heavy-light form factors provided that the form factor $F_0$ behaves as a constant, while the $q^2$ dependence is of the monopole form for $F_1,~ A_0,~A_1$, and of the dipole behavior for $A_2$ and $V$. This $q^2$ extrapolation for form factors is further supported by $B\to K^*γ$ data and by a recent QCD-sum-rule analysis. We then apply this method to $B\to D(D^*)π(ρ)$ decays to extract the parameters $a_1$ and $a_2$. It is found that $a_1(B\to D^{(*)}π(ρ))=1.01\pm 0.06$ and $a_2(B\to D^{(*)}π(ρ))=0.23\pm 0.06\,$. Our result $a_2/a_1=0.22\pm 0.06$ thus significantly improves the previous analysis that leads to $a_2/a_1=0.23\pm 0.11$. We argue that, contrary to what anticipated from the leading $1/N_c$ expansion, the sign of $a_2(B\toψ\K)$ should be positive and $a_2(B\toψ\K)~{\large ^>_\sim}~ a_2(B\to D^{(*)}π(ρ))$.

hep-ph↗
arXiv1994

Vector Dominance Effects in Weak Radiative Decays of the B Mesons

The long distance vector-meson-dominance (VMD) effects on the weak radiative decays $\bb\toργ$ and $\bb^0\to D^{*0}γ$ are studied. For $\bb\toργ$ decays, the VMD contribution is $(10-20)\%$ of the short-distance penguin amplitude. The pole effect is as important as the VMD one in the decay $B^-\toρ^-γ$, but it is suppressed in $\bb^0\to ρ^0γ$. The branching ratio of $\bb\toργ$, estimated to be of order $10^{-6}$, strongly depends on the sign of the Wolfenstein parameter $ρ$. A measurement of any deviation of the ratio $R=Γ(B^-\toρ^- γ)/Γ(\bb^0\toρ^0γ)$ away from the isospin value 2 will not only provide a probe on the long-range contribution but also fix the sign of $ρ$: $R>2$ for $ρ<0$ and $R<2$ for $ρ>0$. The decay $\bb^0\to D^{*0}γ$ does not receive short-distance contributions, and its branching ratio, predicted to be $0.9\times 10^{-6}$, is dominated by $W$-exchange accompanied by a photon emission.

hep-ph↗
arXiv1994

The $Λ_b\to J/ψ+Λ$ Decay Revisited

The $\b$ decay does not receive any nonspectator contributions and hence its theoretical calculation is relatively clean. Two different methods are employed for the study of $\b$. In the first method, both $b$ and $s$ quarks are treated as heavy and leading $1/m_s$ as well as $1/m_b$ corrections are included. The branching ratio and the asymmetry parameter $α$ are found to be $(0.7-1.5)\times 10^{-3}$ and $0.18$, respectively. In the second method, only the $b$ quark is treated as heavy. The prediction of the decay rate is very sensitive to the choice of the $q^2$ dependence for form factors. We obtain $α=0.31$ and ${\cal B}(\b) \simeq 1.6\times 10^{-4}$ for dipole $q^2$ behavior as well as $8.2\times 10^{-4}$ for monopole $q^2$ dependence. We conclude that ${\cal B}(\b)~{\large ^<_{\sim}}~10^{-3}$ and $α$ is of order $0.20\sim 0.30$ with a positive sign.

hep-ph↗
arXiv1994

Effective Lagrangian Approach to Weak Radiative Decays of Heavy Hadrons

Motivated by the observation of the decay $\bar{B}\to \bar{K}^*γ$ by CLEO, we have systematically analyzed the two-body weak radiative decays of bottom and charmed hadrons. There exist two types of weak radiative decays: One proceeds through the short-distance $b\to sγ$ transition and the other occurs through $W$-exchange accompanied by a photon emission. Effective Lagrangians are derived for the $W$-exchange bremsstrahlung processes at the quark level and then applied to various weak electromagnetic decays of heavy hadrons. Predictions for the branching ratios of $\bar{B}^0\to D^{*0} γ,~Λ_b^0\toΣ_c^0γ,~Ξ_b^0\to Ξ_c^0γ$ and $Ξ_b^0\to\xip_c^0γ$ are given. In particular, we found ${\cal B}(\bar{B}^0 \to D^{*0}γ)\approx 0.9\times 10^{-6}$. Order of magnitude estimates for the weak radiative decays of charmed hadrons: $~D^0\to \bar{K}^{*0}γ,~Λ_c^+\toΣ^+γ$ and $Ξ_c^0\toΞ^0γ$ are also presented. Within this approach, the decay asymmetry for antitriplet to antitriplet heavy baryon weak radiative transitions is uniquely predicted by heavy quark symmetry. The electromagnetic penguin contribution to $Λ_b^0\toΛγ$ is estimated by two different methods and its branching ratio is found to be of order $1\times 10^{-5}$. We conclude that weak radiative decays of bottom hadrons are dominated by the short-distance $b\to sγ$ mechanism.

hep-ph↗
arXiv1994

Nonfactorizable Contributions to Nonleptonic Weak Decays of Heavy Mesons

Nonfactorizable contributions to exclusive two-body nonleptonic weak decays of heavy mesons arising from color octet currents, characterized by the parameter $r_2$, are extracted from the data. It is found that $r_2$ is equal to $\sim -0.67\,$, $-(0.9\sim 1.1),~-(1.2\sim 1.3)$, $\sim 0.36$ respectively for $D\to \bar{K}π,~\bar{K}^*π,~\bar{K}^*ρ,~\bar{B}\to Dπ$ decays. As expected, soft-gluon effects become stronger when the decay particles are less energetic, allowing more time for significant final-state strong interactions. As a consequence, the parameter $a_2$ is not universal and is channel or class dependent, contrary to the anticipation of the factorization approach. The leading $1/N_c$ expansion works most successfully for $D\to\bar{K}π$ decays as the subleading $1/N_c$ factorizable contribution is almost compensated by the soft-gluon effect. We argue that, in contrast to what happens in $B^-\to D^{(*)} π(ρ)$ decays, the nonfactorizable term and the subleading $1/N_c$ factorizable term in $\bar{B}\to ψ\bar{K}^{(*)}$ decays are opposite in signs, in accordance with a recent QCD sum rule calculation. Implications are discussed.

hep-ph↗
arXiv1994

Form Factors for $B\toπ$ and $D\toπ$ Transitions

Armed with the information on the form factor $\fdpi(0)$ inferred from recent CLEO measurements of SU(3)-breaking effects in charmed meson decays, we have studied the form factor $\fbpi$. In the heavy quark limit, $\fbpi(q^2)$ is related to $\fdpi(q^2)$ in the kinematic region close to zero recoil. Assuming pole dominance for its $q^2$ dependence, $\fbpi(0)$ is estimated to be $\approx 0.39\,.$ If the requirement of heavy quark symmetry is relaxed so that it applies only to soft pion emissions from the heavy meson, we find that $\fbpi(0)$ is more likely of order $0.55\sim 0.60\,.$

hep-ph↗
arXiv1994

SU(3) Breaking Effects in Charmed Meson Decays

The decay rates of $D^+\toπ^+π^0$ and $D^0\to K^+π^-$ recently measured by CLEO give the ratios $R_1=2|V_{cs}/V_{cd}|^2Γ(D^+\to π^+π^0)/Γ(D^+\to\bar{K}^0 π^+)=3.29\pm 1.16$ and $R_2=|V_{cs}^*V_{ud}/(V_{cd}^*V_{us})|^2Γ(D^0\to K^+π^-)/Γ(D^0\to K^-π^+)=2.92\pm 1.34\,$. Both are about three times of those expected from SU(3) symmetry. We show that, in the large-$N_c$ factorization approach, such large SU(3) violations can be accounted for by the accumulations of several small SU(3)-breaking effects. An important requirement is the relative magnitude of the form factors, $f_+^{Dπ}(0)>f_+^{DK}(0)$.

hep-ph↗
arXiv1994

Corrections to Chiral Dynamics of Heavy Hadrons: SU(3) Symmetry Breaking, (with some minor corrections)

In previous publications we have analyzed the strong and electromagnetic decays of heavy mesons and heavy baryons in a formalism which incorporates heavy-quark and chiral symmetries. There are two possible symmetry-breaking effects on the chiral dynamics of heavy hadrons: the finite-mass effects from light quarks and the $1/ m_Q$ corrections from heavy quarks. In the present paper, chiral-symmetry-breaking effects are studied and applications to various strong and radiative decays of heavy hadrons are illustrated. SU(3) violations induced by chiral loops in the radiative decays of charmed mesons and charmed baryons are compared with those predicted by the constituent quark model. In particular, available data for $D^*$ decays favor values of the parameters in chiral perturbation theory which give predictions for $D^*$ decays close to the quark model results except for the $D^{*+}_s$. Implications are discussed.

hep-ph↗
arXiv1993

Radiative Kaon Decays $K^\pm\toπ^\pmπ^0γ$ and Direct CP Violation

It is stressed that a measurement of the electric dipole amplitude for direct photon emission in $\kpm$ decays through its interference with inner bremsstrahlung is important for differentiating among various models. Effects of amplitude CP violation in the radiative decays of the charged kaon are analyzed in the Standard Model in conjunction with the large $N_c$ approach. We point out that gluon and electromagnetic penguin contributions to the CP-violating asymmetry between the Dalitz plots of $\kpm$ are of equal weight. The magnitude of CP asymmetry ranges from $2\times 10^{-6}$ to $1\times 10^{-5}$ when the photon energy in the kaon rest frame varies from 50 MeV to 170 MeV.

hep-ph↗
arXiv1993

Predictions for Semileptonic Decays $\bar{B}\to D(D^*)π\ell\barν$ and Weak Radiative Decays of Bottom Hadrons

The semileptonic decays $\bar{B}\to D(D^*)π\ell\barν$ are analyzed by heavy quark and chiral symmetries. The branching ratio is of order of a few percent for $\dpi$, but it is much smaller for $\dstar$, of order $10^{-4}-10^{-5}$. The decay mode $B^-\to D^+π^-e^-\barν_e$ provides a nice place for extracting the $D^*Dπ$ coupling constant from the semileptonic decays of a $\bar{B}$ meson. Two types of weak radiative decays of $\bar{B}$ mesons and bottom baryons are studied. The decay $Λ_b^0\to Λ^0γ$ receives the electromagnetic penguin $b\to sγ$ contribution and has a branching ratio of order $2\times 10^{-5}$. The radiative decays $\bar{B}\to D^*γ,~ Λ_b^0\toΣ_c^0γ,~Ξ_b^0\to Ξ_c^0γ$ and $Ξ_b^0\to{Ξ'}_c^0γ$ occur through $W$ exchange accompanied with a photon emission and their branching ratios are estimated to be much smaller. We conclude that the weak radiative decays of bottom hadrons are dominated by the short-distance electromagnetic penguin mechanism.

hep-ph↗
arXiv1993

Analysis of Weak-Interaction Effects in High Energy Hadron-Hadron Collisions

Parity-violating (pv) effects in inclusive hadron and jet productions in high energy hadron-hadron collisions are analyzed. Such effects arise from the interference between strong and weak amplitudes. This interference gives rise to a nonzero value of the pv parameters $A_L$ and $P_L$, where $A_L$ measures the difference in the inclusive cross sections of, for example, $p+p\ri {\rm jet}+X$ ($X$=anything), with one of incident proton beams in a state of $\pm$ helicity, and $P_L$ denotes the longitudinal polarization of a high-energy baryon (e.g., $Λ$) produced in $p+p\riΛ+X$ with the initial proton beams unpolarized. In the present paper, the single helicity asymmetry $A_L$ in one-jet, two-jet and two-jet plus photon productions as well as in the Drell-Yan process $p+p\ri\ell^+\ell^-+{\rm jet}+X$ is probed, and the longitudinal polarization $P_L$ of the $Λ$ produced in unpolarized $pp$ collisions is studied. We conclude that the pv effects in high energy proton-proton collisions are in general only sensitive to the spin dependent valence quark distributions.

hep-ph↗
arXiv1993

Direct CP Violation in $K_L\toπ^+π^-γ$

CP violation due to interference between $K_L$ and $K_S$ decays into $π^+π^-γ$ is analyzed in the Standard Model. The CP-violating parameter $ε'_{+- γ}$, which is the difference between $η_{+-γ}$ and $η_{+-}$, receives dominant contributions from $K^0-\bar{K}^0$ mixing and the gluon penguin diagram; its magnitude is calculated to be $10^{-2}ε$ at a typical region of the Dalitz plot. Genuine direct CP violation in $K_L\toπ^+ π^-γ$ decay originating from the electromagnetic penguin diagram is of order $10^{-4}ε$.

hep-ph↗
arXiv1993

Cabibbo-allowed nonleptonic weak decays of charmed baryons

Cabibbo-allowed nonleptonic weak decays of charmed baryons $\lamc,~\xin,~\xip$ and $Ω_c^0$ into an octet baryon and a pseudoscalar meson are analyzed. The nonfactorizable contributions are evaluated under pole approximation, and it turns out that the $s$-wave amplitudes are dominated by the low-lying $\halfm$ resonances, while $p$-wave ones governed by the ground-state $\halfp$ poles. The MIT bag model is employed to calculate the coupling constants, form factors and baryon matrix elements. Our conclusions are: (i) $s$ waves are no longer dominated by commutator terms; the current-algebra method is certainly not applicable to parity-violating amplitudes, (ii) nonfactorizable $W$ exchange effects are generally important; they can be comparable to and somtimes even dominate over factorizable contributions, depending on the decay modes under consideration, (iii) large-$N_c$ approximation for factorizable amplitudes also works in the heavy baryon sector and it accounts for the color nonsuppression of $\lamc\ri p\bar{K}^0$ relative to $\lamc\riΛπ^+$, (iv) a measurement of the decay rate and the sign of the $α$ asymmetry parameter of certain proposed decay modes will help discern various models; especially the sign of $α$ in $\lamc\riΣπ$ decays can be used to unambiguously differentiate recent theoretical schemes from current algebra, and (v) $p$ waves are the dominant contributions to the decays $\lamc\riΞ^0 K^+$ and $\xin\riΣ^+ K^-$, but they are subject to a large cancellation; this renders present theoretical predictions on these two channels unreliable.

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