Abstract:
The gravitational form factor of the pion is evaluated in two chiral quark models and confronted to the recent full-QCD lattice data. We find good agreement for the case of the Spectral Quark Model, which builds in the vector-meson dominance for the charge form factor. We derive a simple relation between the gravitational and electromagnetic form factors, holding in the considered quark models in the chiral limit. The relation implies that the gravitational mean squared radius is half the electromagnetic one. We also analyze higher-order quark generalized form factors of the pion, related to higher moments in the symmetric Bjorken X-variable of the generalized parton distribution functions, and discuss their perturbative QCD evolution, needed to relate the quark-model predictions to the lattice data. The values of the higher-order quark form factors at t=0, computed on the lattice, also agree with our quark model results within the statistical and method uncertainties.

Abstract:
The vector and axial pion-photon transition distribution amplitudes are analyzed in the Spectral Quark Model. We proceed by the evaluation of double distributions through the use of a manifestly covariant calculation based on the alpha representation of propagators. As a result polynomiality is incorporated and calculations become rather simple. Explicit formulas, holding at the low-energy quark-model scale, are obtained. The corresponding form factors for the anomalous decay pi0 -> gamma gamma* and the radiative pion decays are also evaluated and confronted with the data.

Abstract:
We present a construction of the pion electromagnetic form factor where the transition from large-Nc Regge vector meson dominance models with infinitely many resonances to perturbative QCD is built in explicitly. The construction is based on an appropriate assignment of residues to the Regge poles, which fulfills the constraints of the parton-hadron duality and perturbative QCD. The model contains a slowly falling off non-perturbative contribution which dominates over the perturbative QCD radiative corrections for the experimentally accessible momenta. The leading order and next-to-leading order calculations show a converging pattern which describes the available data within uncertainties, while the onset of asymptotic QCD takes place at extremely high momenta, Q ~ 10^3-10^4 GeV. The method can be straightforwardly extended to study other form factors where the perturbative QCD result is available.

Abstract:
The Bethe-Salpeter equation restores exact elastic unitarity in the s- channel by summing up an infinite set of chiral loops. We use this equation to show how a chiral expansion can be undertaken by successive approximations to the potential which should be iterated. Renormalizability of the amplitudes in a broad sense can be achieved by allowing for an infinite set of counter-terms as it is the case in ordinary Chiral Perturbation Theory. Within this framework we calculate the $\pi \pi$ scattering amplitudes both for s- and p-waves at lowest order in the proposed expansion where a successful description of the low-lying resonances ($\sigma$ and $\rho$) and threshold parameters is obtained. We also extract the SU(2) low energy parameters ${\bar l}_{1,2,3,4}$ from our amplitudes.

Abstract:
The gravitational form factor of the pion is evaluated in two chiral quark models and confronted to the recent full-QCD lattice data. We find good agreement for the case of the Spectral Quark Model, which builds in the vector-meson dominance for the charge form factor. We derive a simple relation between the gravitational and electromagnetic form factors, holding in the considered quark models in the chiral limit. The relation implies that the gravitational mean squared radius is half the electromagnetic one. We also analyze higher-order quark generalized form factors of the pion, related to higher moments in the symmetric Bjorken X-variable of the generalized parton distribution functions, and discuss their perturbative QCD evolution, needed to relate the quark-model predictions to the lattice data. The values of the higher-order quark form factors at t=0, computed on the lattice, also agree with our quark model results within the statistical and method uncertainties.

Abstract:
We explore the BaBar puzzle within the Regge approach. After reviewing the chiral quark models in applications to PDF and PDA of the pion, we argue that variants of these models, fulfilling the chiral anomaly, may in fact violate the second Terazawa-West unitarity bound, which is based on unverified assumptions for the real part of the amplitude. Consequently, the transition form factor need not vanish at large values of the photon virtuality. Then we show that the experimental data may be properly explained with incomplete vector-meson dominance in a simple model with one state, as well as in more sophisticated radial Regge models including infinitely many states. We also consider the experimental constraint from the rare Z -> pi0 gamma decay, which is comfortably satisfied in our approach. Finally, we point out that the photon momentum asymmetry parameter may noticeably influence the precision fits to the data.

Abstract:
Valence double parton distribution functions of the nucleon are evaluated in the framework of a simple model, where the conservation of the longitudinal momentum is taken into account. The leading-order DGLAP QCD evolution from the low quark-model scale to higher renormalization scales is carried out via the Mellin moments of the distributions. Results of the valence quark correlation function show that in general the double distributions cannot be approximated as a product of the single-particle distributions.

Abstract:
We compute the off-forward diagonal non-singlet generalized parton distribution of the pion in two distinct chiral quark models: the Nambu-Jona-Lasinio model with the Pauli-Villars regulator and the Spectral Quark Model. The analysis is carried out in the impact-parameter space. Leading-order perturbative QCD evolution is carried out via the inverse Mellin transform in the index space. The model predictions agree very reasonably with the recent results from transverse-lattice calculations, reproducing qualitatively both the Bjorken-x and the impact-parameter dependence of the data.

Abstract:
The Kwiecinski equations for the QCD evolution of the unintegrated parton distributions in the transverse-coordinate space (b) are analyzed with the help of the Mellin-transform method. The equations are solved numerically in the general case, as well as in a small-b expansion which converges fast for b Lambda_QCD sufficiently small. We also discuss the asymptotic limit of large bQ and show that the distributions generated by the evolution decrease with b according to a power law. Numerical results are presented for the pion distributions with a simple valence-like initial condition at the low scale, following from chiral large-N_c quark models. We use two models: the Spectral Quark Model and the Nambu--Jona-Lasinio model. Formal aspects of the equations, such as the analytic form of the b-dependent anomalous dimensions, their analytic structure, as well as the limits of unintegrated parton densities at x -> 0, x -> 1, and at large b, are discussed in detail. The effect of spreading of the transverse momentum with the increasing scale is confirmed, with growing asymptotically as Q^2 alpha(Q^2). Approximate formulas for for each parton species is given, which may be used in practical applications.