We investigate wide-angle photo- and electroproduction of pions within the handbag mechanism in which the $γ^{(*)} N\to πN'$ amplitudes factorize into hard partonic subprocess amplitudes, $γ^{(*)} q\to πq'$, and form factors representing $1/x$-moments of generalized parton distributions (GPDs). The subprocess is calculated to twist-3 accuracy taking into account the 2- and 3-body Fock components of the pion. Both components are required for achieving gauge invariance in QED and QCD. The twist-2 and twist-3 distribution amplitudes (DAs) of the pion as well as the form factors are taken from our study of $π^0$ photoproduction. Extensive results on photoproduction of charged pions are presented and compared to experiment. Predictions on electroproduction of pions as well as on spin effects are also given. As a byproduct of our analysis we also obtain the complete twist-3 subprocess amplitudes contributing to deeply-virtual electroproduction of pions.
We analyze light meson electroproduction within the handbag model. We study cross sections and spin asymmetries for various mesons. The essential role of the transversity $\tilde H_T$ and $\tilde E_T$ GPDs in electroproduction of pseudoscalar mesons is found. Our results are in good agrement with experiment.
We investigate exclusive electroproduction of $π^-Δ^{++}$ within the handbag approach in which the helicity amplitudes factorize into generalized parton distributions (GPDs) and hard partonic subprocesses. We define the $p-Δ$ transversity GPDs while the helicity non-flip GPDs are taken from the literature. For the numerical estimates of observables we utilize large-$N_C$ results in order to relate a few of the $p-Δ$ GPDs to the proton-proton ones and neglect all other GPDs. In the calculation of the twist-2 and twist-3 subprocess amplitudes we take into account quark transverse momenta in comb…
The wide-angle photo- and electroproduction of pions is investigated within the handbag mechanism in which the $γ^{(*)} N\to πN'$ amplitudes factorize into subprocess amplitudes,$γ^{(*)} q\to πq'$, and form factors representing $1/x$-moments of generalized parton distributions (GPDs). The subprocess is calculated to twist-3 accuracy taking into account both the 2- and 3-body Fock components of the pion. The cross-sections are compared to experiment and spin-effects are discussed.
Exclusive electroproduction of vector mesons is a crucial way to probe QCD factorization properties. Such a factorization is up to now only proven, at the twist 2 level, for a longitudinaly polarized meson. It is crucial to extend our understanding to the case of transversely polarized vector mesons. As a first step in this direction, we evaluate the impact factor of the transition gamma* -> rhoT, which is the relevant part of the amplitude within the kt-factorization approach valid at large energies, taking into account the twist 3 contributions, coming both from quark antiquark and from quar…
We analyze the electroproduction of pseudoscalar mesons within the handbag approach. To investigate these reactions, we consider the leading-twist contribution together with the transversity twist-3 effects that are crucial in the description of experimental data. Our results on the cross section are in agreement with experiment. We present our predictions for spin observables.
We link the recent computation beyond leading twist of the impact factor of the transition gamma*T -> rhoT performed in the light-cone collinear approach, to the dipole picture by expressing the hard part of the process through its Fourier transform in coordinate space. We show that in the Wandzura-Wilczek approximation the impact factor up to twist 3 factorises in the wave function of the photon combined with the distribution amplitudes of the rho-meson and the colour dipole scattering amplitude with the t-channel gluons. We show also that beyond the Wandzura-Wilczek approximation, the hard c…
We present results for the one loop anomalous dimension matrix of flavor non-singlet twist-4 operators of lowest spin that contribute to the leading moment of the $F_2$ structure function in deep inelastic electron-nucleon scattering. We analyze the flavor structure of the anomalous dimension matrix and decompose the leading moment of $F_2$ into separate flavor channels. In addition to building on previous work with higher-twist operators, these results can provide a benchmark for future work that generalizes to include the higher moments as well. We include non-perturbative input from the lat…
Exclusive processes in hard electroproduction are one of the best place for understanding QCD in view of its factorization properties. In particular, in the limit of asymptotic gamma* p center of mass energy, they give an access to the perturbative Regge properties of QCD. The HERA experiment recently provided precise data for rho electroproduction, including all spin density matrix elements. From QCD, it is expected that such a process should factorize between a hard (calculable) coefficient function, and hadronic (p and rho) matrix elements. Such a factorization is up to now only proven for …
We use recent results for the gamma*L -> rhoL and gamma*T -> rhoT impact factors, computed in the impact parameter representation within the collinear factorization scheme, to get predictions for the polarized cross-sections sigmaT and sigmaL of the diffractive leptoproduction of the rho meson at high energy. In this approach the helicity amplitude is a convolution of the scattering amplitude of a color dipole with a target, together with the virtual gamma wave function and with the first moments of the rho meson wave function (in the transverse momentum space), given by the distribution ampli…
We study the single-spin asymmetries of pions produced in semi-inclusive deep-inelastic scattering on the longitudinally polarized nucleon targets. We particularly consider the effects of the twist-3 transverse-momentum dependent distribution functions $f_L^\perp$ and $h_L$ to the asymmetry. We calculate the asymmetric moment $A_{UL}^{\sinϕ_h}$ for $π^+$, $π^-$ and $π^0$ produced off the proton target at HERMES and compare the results with the HERMES data. We also present the prediction of the same asymmetries for different pions at the kinematics of CLAS 5.5 GeV on a proton target, as well as…
We perform detailed calculations of two-photon-exchange QED corrections to the cross section of pion electroproduction. The results are obtained with and without the soft-photon approximation; analytic expressions for the radiative corrections are derived. The relative importance of the two-photon correction is analyzed for the kinematics of several experiments at Jefferson Lab. A significant, over 20%, effect due to two-photon exchange is predicted for the backward angles of electron scattering at large transferred momenta.
In this talk it is reported on an analysis of hard exclusive pion electroproduction within the handbag approach. Particular emphasis is laid on single-spin asymmetries. It is argued that a recent HERMES measurement of asymmetries measured with a transversely polarized target clearly indicate the occurrence of strong contributions from transversely polarized photons. Within the handbag approach such gamma_T --> pion transitions are described by the transversity GPDs accompanied by a twist-3 pion wave function. It is shown that this approach leads to results on cross sections and single-spin asy…
Generalized parton distributions of the nucleon are accessed via exclusive leptoproduction of the real photon. While earlier analytical considerations of phenomenological observables were restricted to twist-three accuracy, i.e., taking into account only terms suppressed by a single power of the hard scale, in the present study we revisit this differential cross section within the helicity formalism and restore power-suppressed effects stemming from the process kinematics exactly. We restrict ourselves to the phenomenologically important case of lepton scattering off a longitudinally polarized…
In this paper we estimate the size of twist-3 corrections to the deeply virtual meson production in neutrino interactions due to the chiral odd transversity Generalized Parton Distribution (GPD). We conclude that in contrast to pion electroproduction, in neutrino-induced reactions these corrections are small. This happens due to large contribution of unpolarized GPDs H, E to the leading-twist amplitude in neutrinoproduction. We provide a computational code, which can be used for evaluation of the cross-sections accounting for these twist-3 corrections with various GPD models. Our results are p…
By extending our previous isobar model to the K^0Lambda isospin channel we investigate the properties of the K^0Lambda photo- and electroproduction at energies near threshold. It is found that the pseudovector (PV) coupling yields significantly larger cross section. Variation of the K_1 coupling constants has significant effect only on the PV model. The electromagnetic form factor of the neutral kaon K^0 is found to have a sizable effect on the longitudinal cross section of the K^0Lambda electroproduction near the threshold.
We discuss various aspects of vector meson production, first analysing the interplay between perturbative and nonperturbative aspects of the QCD calculation. Using a general method adapted to incorporate both perturbative and nonpertubative aspects, we show that nonperturbative effects are important for all experimentally available values of the photon virtuality Q2. We compare the huge amount of experimental information now available with our theoretical results obtained using a specific nonperturbative model without free parameters, showing that quite simple features are able to explain the …
