Meson twist-4 parton distributions in terms of twist-2 distribution amplitudes at large Nc

We show that in the large N_c limit four-quark twist-4 distributions in the pion can be expressed in terms of twist-2 pion distribution amplitude. This allows us to compute the isospin-2 structure function of the pion F_2^{I=2}(x_B) in the large N_c limit. The method can be easily applied to other mesons as well.Comment: 6 pages, one figur

Perturbative QCD factorization of πγ∗→γ(π)\pi \gamma^*\to \gamma(\pi) and B→γ(π)lνˉB\to \gamma(\pi)l\bar \nu

We prove factorization theorem for the processes $\pi\gamma^*\to\gamma$ and $\pi\gamma^*\to\pi$ to leading twist in the covariant gauge by means of the Ward identity. Soft divergences cancel and collinear divergences are grouped into a pion wave function defined by a nonlocal matrix element. The gauge invariance and universality of the pion wave function are confirmed. The proof is then extended to the exclusive $B$ meson decays $B\to\gamma l\bar\nu$ and $B\to\pi l\bar\nu$ in the heavy quark limit. It is shown that a light-cone $B$ meson wave function, though absorbing soft dynamics, can be defined in an appropriate frame. Factorization of the $B\to\pi l\bar\nu$ decay in $k_T$ space, $k_T$ being parton transverse momenta, is briefly discussed. We comment on the extraction of the leading-twist pion wave function from experimental data.Comment: 21 pages in Latex file, version to appear in Phys. Rev.

The gluon/charm content of the eta' meson and instantons

Motivated by recent CLEO measurements of the B-->\eta' K decay, we evaluate gluon/charm content of the eta' meson using the interacting instanton liquid model of the QCD vacuum. Our result is very large due to the strong field of small-size instantons. We show that it provides quantitative explanations of the CLEO data on the B-->\eta' K decay rate (as well as inclusive process B-->\eta' X), via a virtual Cabbibo-unsuppressed decay into \bar c c pair which then becomes eta'. If so, a significant charm component should be present in other hadrons also. In particular, we found a large contribution of the charmed quark in the polarised deep-inelastic scattering on a proton.Comment: 6 pages, LATE

The B(Bs)→D(s)PB(B_s)\to D_{(s)} P, D(s)VD_{(s)} V, D(s)∗PD_{(s)}^{*}P and D(s)∗VD_{(s)}^{*}V decays in the perturbative QCD approach

Two-body non-leptonic charmed decays $B_{(s)} \to D_{(s)}P$, $D_{(s)}^*P$, $D_{(s)}V$ and $D_{(s)}^*V$ are analyzed in perturbative QCD approach, where $P$ and $V$ denote the light pseudoscalar meson and vector meson, respectively. We test the $D$ meson wave function by a $\chi^2$ fit with experimental data of six $B\to DP$ channels. We give the branching ratios of all the charmed B decay channels, most of which agree with experiments amazingly well. The predicted $B_s$ decays can be confronted with the future experimental data. By straightforward calculations, our pQCD approach gives the right relative strong phase of $a_2/a_1$ with experiments. We also predict the percentage of transverse polarizations in $B_{(s)} \to D^* V$ decay channels.Comment: 25 pages, 4 figure

Light-Cone Quantization and Hadron Structure

In this talk, I review the use of the light-cone Fock expansion as a tractable and consistent description of relativistic many-body systems and bound states in quantum field theory and as a frame-independent representation of the physics of the QCD parton model. Nonperturbative methods for computing the spectrum and LC wavefunctions are briefly discussed. The light-cone Fock state representation of hadrons also describes quantum fluctuations containing intrinsic gluons, strangeness, and charm, and, in the case of nuclei, "hidden color". Fock state components of hadrons with small transverse size, such as those which dominate hard exclusive reactions, have small color dipole moments and thus diminished hadronic interactions; i.e., "color transparency". The use of light-cone Fock methods to compute loop amplitudes is illustrated by the example of the electron anomalous moment in QED. In other applications, such as the computation of the axial, magnetic, and quadrupole moments of light nuclei, the QCD relativistic Fock state description provides new insights which go well beyond the usual assumptions of traditional hadronic and nuclear physics.Comment: LaTex 36 pages, 3 figures. To obtain a copy, send e-mail to [email protected]

Renormalization-Scale-Invariant PQCD Predictions for R_e+e- and the Bjorken Sum Rule at Next-to-Leading Order

We discuss application of the physical QCD effective charge $\alpha_V$, defined via the heavy-quark potential, in perturbative calculations at next-to-leading order. When coupled with the Brodsky-Lepage-Mackenzie prescription for fixing the renormalization scales, the resulting series are automatically and naturally scale and scheme independent, and represent unambiguous predictions of perturbative QCD. We consider in detail such commensurate scale relations for the $e^+e^-$ annihilation ratio $R_{e^+e^-}$ and the Bjorken sum rule. In both cases the improved predictions are in excellent agreement with experiment.Comment: 13 Latex pages with 5 figures; to be published in Physical Review

Large corrections to asymptotic FηcγF_{\eta_c \gamma} and FηbγF_{\eta_b \gamma} in the light-cone perturbative QCD

The large-$Q^2$ behavior of $\eta_c$-$\gamma$ and $\eta_b$-$\gamma$ transition form factors, $F_{\eta_c\gamma}(Q^2)$ and $F_{\eta_b\gamma}(Q^2)$ are analyzed in the framework of light-cone perturbative QCD with the heavy quark ($c$ and $b$) mass effect, the parton's transverse momentum dependence and the higher helicity components in the light-cone wave function are respected. It is pointed out that the quark mass effect brings significant modifications to the asymptotic predictions of the transition form factors in a rather broad energy region, and this modification is much severer for $F_{\eta_b\gamma}(Q^2)$ than that for $F_{\eta_c\gamma}(Q^2)$ due to the $b$-quark being heavier than the $c$-quark. The parton's transverse momentum and the higher helicity components are another two factors which decrease the perturbative predictions. For the transition form factor $F_{\eta_c\gamma}(Q^2)$, they bring sizable corrections in the present experimentally accessible energy region ($Q^2 \leq 10 GeV^2$). For the transition form factor $F_{\eta_b\gamma}(Q^2)$, the corrections coming from these two factors are negligible since the $b$-quark mass is much larger than the parton's average transverse momentum. The coming $e^+ e^-$ collider (LEP2) will provide the opportunity to examine these theoretical predictions.Comment: 8 pages, RevTex, 5 PostScript figure

Gluon Virtuality and Heavy Sea Quark Contributions to the Spin-Dependent g_1 Structure Function

We analyze the quark mass dependence of photon gluon fusion in polarized deep inelastic scattering for both the intrinsic and extrinsic gluon distributions of the nucleon. We calculate the effective number of flavors for each of the heavy and light quark photon gluon fusion contributions to the first moment of the spin-dependent structure function $g_1(x)$.Comment: LaTex, 19 page

The Two-Loop Scale Dependence of the Static QCD Potential including Quark Masses

The interaction potential V(Q^2) between static test charges can be used to define an effective charge $\alpha_V(Q^2)$ and a physically-based renormalization scheme for quantum chromodynamics and other gauge theories. In this paper we use recent results for the finite-mass fermionic corrections to the heavy-quark potential at two-loops to derive the next-to-leading order term for the Gell Mann-Low function of the V-scheme. The resulting effective number of flavors $N_F(Q^2/m^2)$ in the $\alpha_V$ scheme is determined as a gauge-independent and analytic function of the ratio of the momentum transfer to the quark pole mass. The results give automatic decoupling of heavy quarks and are independent of the renormalization procedure. Commensurate scale relations then provide the next-to-leading order connection between all perturbatively calculable observables to the analytic and gauge-invariant $\alpha_V$ scheme without any scale ambiguity and a well defined number of active flavors. The inclusion of the finite quark mass effects in the running of the coupling is compared with the standard treatment of finite quark mass effects in the $\bar{MS}$ scheme.Comment: 27 pages, 13 figure

A multiloop improvement of non-singlet QCD evolution equations

An approach is elaborated for calculation of "all loop" contributions to the non-singlet evolution kernels from the diagrams with renormalon chain insertions. Closed expressions are obtained for sums of contributions to kernels $P(z)$ for the DGLAP equation and $V(x,y)$ for the "nonforward" ER-BL equation from these diagrams that dominate for a large value of $b_0$, the first $\beta$-function coefficient. Calculations are performed in the covariant $\xi$-gauge in a MS-like scheme. It is established that a special choice of the gauge parameter $\xi=-3$ generalizes the standard "naive nonabelianization" approximation. The solutions are obtained to the ER-BL evolution equation (taken at the "all loop" improved kernel), which are in form similar to one-loop solutions. A consequence for QCD descriptions of hard processes and the benefits and incompleteness of the approach are briefly discussed.Comment: 13 pages, revtex, 2 figures are enclosed as eps-file, the text style and figures are corrected following version, accepted for publication to Phys. Rev.