Pion pole and transversity effects in hard exclusive meson   leptoproduction
  We investigate exclusive electroproduction of vector and pseudoscalar mesons at large photon virtuality $Q^2$. These reactions were analyzed within the handbag approach where amplitudes factorize into hard subprocesses and generalized parton distributions (GPDs).   The essential role of transversity effects were found in pseudoscalar and light vector meson leptoproduction. These contributions are important at low $Q^2$ and determined by twist-3 effects accompanied by transversity GPDs. The transversity contributions lead to large transverse cross sections for most reactions of pseudoscalar meson leptoproduction. The transversity effects in vector meson production are visible in spin observables.   We consider spin effects in the $\omega$ and $\rho^0$ leptoproduction reactions. It is shown that the pion pole contribution is very important in the $\omega$ production. Such effects in the $\rho^0$ channel are much smaller. Our results on spin asymmetries and spin density matrix elements in these reactions were found to be in good agreement with HERMES data. 
S. Goloskokov (BLTP, JINR, Dubna)
hep-ph