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The authors develop a QCD sum rule analysis of the form factor F{sub?{sup *}?{sup *}?°}(q2, Q2) in the region where virtuality of one of the spacelike photons is small q2 ≪ 1 GeV2 while another is large: Q2 ≳ 1 GeV2. They construct the operator product expansion suitable for this kinematic situation and obtain a QCD sum rule for F{sub?{sup *}{gamma}{sup *}?°}(0, Q2). Their results confirm expectation that the momentum transfer dependence of F{sub {gamma}{sup *}{gamma}{sup *}?°}(0,Q2) is close to interpolation between its Q2=0 value fixed by the axial anomaly and Q−2 pQCD behavior for large Q2. Their approach, in contrast to pQCD, does not require additional assumptions about the shape of the pion distribution amplitude {var_phi}{sub {pi}}(x). The absolute value of the 1/Q2 term obtained in this paper favors {var_phi}{sub {pi}}(x) close to the asymptotic form {var_phi}{sub {pi}}{sup as}(x) = 6f{sub {pi}}x(1-x).
Within the QCD sum rule approach, we develop a formalism that enables one to calculate the form factors of the heavy-light mesons in the m(sub)Q -> infinity limit. It is shown that the behaviour of the universal Isgur-Wise form factor is determined by the quark propagation function in imaginary time.
The shape of hadronic distribution amplitudes (DAs) is a critical issue for the perturbative QCD of hard exclusive processes. Recent CLEO data on [gamma][gamma]{sup *} → [pi]° form factor clearly favor a pion DA close to the asymptotic form. The author argues that QCD sum rules for the moments of the pion DA {var_phi}{sub [pi]}(x) are unreliable, so that the humpy shape of {var_phi}{sub {pi}}(x) obtained by Chernyak and Zhitnitsky is a result of model assumptions rather than an unambiguous consequence of QCD sum rules. This conclusion is also supported by a direct QCD sum rule calculation of the [gamma][gamma]{sup *} → {pi}° form factor which gives a result very close to the CLEO data.
In this talk the author reports on recent progress in a few areas closely related to the virtual Compton scattering studies. In particular, he discusses the quark-hadron duality estimate of the?{sup *}p →? transition, QCD sum rule calculation of the??{sup *} →?° form factor, and application of perturbative QCD to deeply virtual Compton scattering at small t.
In this paper the authors present the result of a direct QCD sum rule calculation of the transition form factor??{sup *} →?° in the region of moderately large invariant momentum Q2> 1GeV2 of the virtual photon. In contrast to pQCD, they make no assumptions about the shape of the pion distribution amplitude {var_phi}{sub?}(x). Their results agree with the Brodsky-Lepage proposal that the Q2-dependence of this form factor is given by an interpolation between its Q2=0 value fixed by the axial anomaly and 1/Q2 pQCD behavior for large Q2, with normalization corresponding to the asymptotic form {var_phi}{sub pi}{sup as}(x)=6 f{sub {pi}}x(1-x) of the pion distribution amplitude. Their prediction for the from factor F{sub?}{sup *}{gamma}{sup *}{pi}°(q12 = 0,q22 = -Q2) is in good agreement with new CLEO data.
Within the QCD sum rule approach, we develop a formalism that enables one to calculate the form factors of the heavy-light mesons in the m{sub Q} --> (infinity) limit. It is shown that the behavior of the universal Isgur-Wise form factor is determined by the quark propagation function in imaginary time.
We extend the QCD sum rule analysis of the pion electromagnetic form factor F(sub)pi(Q^2) into the region of moderately large momentum transfers 3GeV^2
The authors analyze the basic hard exclusive processes: [pi][gamma]{sup *}[gamma]-transition, pion and nucleon electromagnetic form factors, and discuss the analytic continuation of QCD formulas from the spacelike q2 0 to the timelike region q2 0 of the relevant momentum transfers. They describe the construction of the timelike version of the coupling constant [alpha]{sub s}. They show that due to the analytic continuation of the collinear logarithms each eigenfunction of the evolution equation acquires a phase factor and investigate the resulting interference effects which are shown to be very small. They found no sources for the K-factor-type enhancements in the perturbative QCD contribution to the hadronic form factors. To study the soft part of the pion electromagnetic form factor, they use a QCD sum rule inspired model and show that there are non-canceling Sudakov double logarithms which result in a K-factor-type enhancement in the timelike region.