On high $Q^{2}$ behavior of the pion form factor for transitions $\gamma^{\ast} \gamma \to \pi^{0}$ and $\gamma ^{\ast} \gamma^{\ast} \to \pi^{0}$ within the nonlocal quark-pion model

The behaviour of the pion transition form factor for the process $\gamma^\star\gamma \to \pi^0$ and $\gamma^\star\gamma^\star\to \pi^0$ at space-like values of photon momenta is estimated within the nonlocal covariant quark-meson model. The nonlocal contributions are important to install the axial anomaly at zero photon virtualities and give the contributions to the twist-4 power correction to the form factor at large virtualities. The leading and next-to-leading order power ...

Using non-local chiral quark model with simple pole ansatz for mass dependence on momentum and non-local currents satisfying Ward-Takahashi identities, we calculate pion to photon transition distribution amplitudes and relevant form factors. For vector amplitude we recover correct normalization fixed by the axial anomaly. We find that due to the non-locality in the vector current, the value of axial form factor at zero momentum transfer is lowered with respect to the vector f...

Recently, the BABAR collaboration published (arXiv:0905.4778) data for the photon-pion transition form factor $F_{\pi\gamma\gamma^{\ast}}(Q^{2})$, which are in strong contradiction to the predictions of the standard factorization approach to perturbative QCD. Immediately afterwards, two mechanisms were suggested (A.E. Dorokhov, arXiv:0905.4577; A.V. Radyushkin, arXiv:0906.0323), that logarithmically enhance the form factor asymptotics and therefore provide a qualitatively sat...

We study the transition of non-perturbative to perturbative QCD in situations with possible violations of scaling limits. To this end we consider the singly- and doubly-virtual pion transition form factor $\pi^0\to\gamma\gamma$ at all momentum scales of symmetric and asymmetric photon momenta within the Dyson-Schwinger/Bethe-Salpeter approach. For the doubly virtual form factor we find good agreement with perturbative asymptotic scaling laws. For the singly-virtual form facto...

We study the pion electromagnetic and $\gamma^* + \pi^0 \to \gamma$ transition form factors at intermediate momentum transfer. We calculate soft, nonperturbative corrections to the leading perturbative amplitudes which arise from the $q\bar q$-component of the pion wave function. We work in Minkowski space and use a Lorentz covariant, gauge-invariant generalized perturbative integral representation for the $q\bar q$ amplitudes. For the transition form factor we find relative ...

Instanton liquid model being effective model of the QCD vacuum describes well the hadrons at low as well at intermediate energies. Thus, contact with perturbative QCD results is possible providing the unique information about the quark-gluon distribution functions in the QCD vacuum and hadrons at low energy normalization point. As an illustrative example we consider the pion transition form factor for the process $\gamma ^*\gamma^*\to \pi^0$ at space-like values of photon mom...

We study the $P\to\gamma\,\gamma^*$ ($P=\pi^0,\eta,\eta'$) transition form factors by means of the local-duality (LD) version of QCD sum rules. For the case of $\eta$ and $\eta'$, the conventional LD model provides a good description of the existing data. However, for the $\pi$ form factor we find disagreement with recent BaBar results for high $Q^2$ even though the accuracy of the LD approximation is expected to increase with $Q^2$. It remains mysterious why the $\eta$ and $...

We derive the minimal Fock-state expansions of the pion and the photon wave functions in light-cone formalism, then we calculate the pion-photon and the photon-pion transition form factors of $\gamma ^{\ast}\pi ^{0}\to \gamma $ and $\gamma ^{\ast}\gamma \to \pi ^{0}$ processes by employing these quark-antiquark wave functions of the pion and the photon. We find that our calculation for the $\gamma ^{\ast}\gamma \to \pi ^{0}$ transition form factor agrees with the experimental...

We investigate the singly virtual transition form factor (TFF) for the $\pi^0\to\gamma^*\gamma$ process in the space-like region using the hard-scattering formalism within the Basis Light-Front Quantization (BLFQ) framework. This form factor is expressed in terms of the perturbatively calculable hard-scattering amplitudes (HSAs) and the light-front wave functions (LFWFs) of the pion. We obtain the pion LFWFs by diagonalizing the light-front QCD Hamiltonian, which is determine...

We present an investigation of the pion's electromagnetic form factor F_{\pi}(Q^2) in the spacelike region utilizing two new ingredients: (i) a double-humped, endpoint-suppressed pion distribution amplitude derived before via QCD sum rules with nonlocal condensates--found to comply at the 1\sigma level with the CLEO data on the $\pi\gamma$ transition--and (ii) analytic perturbation theory at the level of parton amplitudes for hadronic reactions. The computation of F_{\pi}(Q^2...