Hard exclusive pseudoscalar meson electroproduction and spin structure of a nucleon

The amplitude for hard exclusive pseudoscalar meson electroproduction off nucleon (nuclear) targets is computed in QCD within the leading $\alpha_s \ln {Q^2/\lambda_{QCD}^{2}}$ approximation. We show that the distribution of recoil nucleons depends strongly on the angle between the momentum of the recoil nucleon and the polarization vector of the target (or outgoing nucleon). This dependence is especially sensitive to the spin flip skewed parton distribution (SPD) $\widetilde E$. We argue also that the scaling for this spin asymmetry sets in at lower Q^2 than that for the absolute cross section. Basing on the chiral quark-soliton model of the nucleon we estimate quantitatively the spin asymmetry. In the case of pi+ production this asymmetry is dominated at small t by the contribution of the pion pole in the isovector SPD $\widetilde E$ as required by PCAC. In the case of K0 production off a proton we find a large enhancement of the cross section as compared to the case of pi0 production. For the forward production of neutral pseudoscalar mesons off a deuteron target we find the cross section should be zero for the zero deuteron helicity (along the $\gamma^*D$ direction). We consider also cross sections of quasielastic processes off nuclei including the feasibility to implant K+,rho-mesons into nuclear volume.