Hard exclusive electroproduction of pseudoscalar mesons and the QCD axial anomaly

Hard exclusive electroproduction of ${\pi }^0,\eta ,{\eta }^{\prime }$ mesons by longitudinal photons is considered. With the help of the QCD factorization theorem we show that at sufficiently large virtualities $Q^2$ amplitudes of hard exclusive processes may be calculated in terms of the target parton distributions and the minimal Fock component of the pseudoscalar meson light cone wave function at zero interquark distances. This result justifies our calculation of the ratios of the ${\pi }^0:\eta :{\eta }^{\prime }$ yields for the electroproduction processes off nucleons and deuterons based on the low energy QCD chiral dynamics, and the QCD anomaly.