Exclusive electro-production of pions and vector mesons at HERMES

A review is given of measurements of hard exclusive pi(+) production on a hydrogen target, as well as of the production of exclusive pi(+)pi(-) pairs and light vector mesons on hydrogen and deuterium targets at HERMES using the 27.6 GeV HERA positron beam. First preliminary results for the unpolarized total pi(+) production cross section in the kinematic range 1.5 < Q(2) < 10.5 GeV2 and for 0.02 < x < 0.8 are presented and compared to GPD calculations. Furthermore, the first Legendre moment of the angular distributions of pi(+) mesons in the CM frame of the exclusively produced pion pairs is presented. Its dependence on the pi(+)pi(-) invariant mass can be understood as being due to the interference between relative P-wave (isovector) and S, D-wave (isoscalar) states of the two pions. From the exclusive light vector meson production preliminary results on the ratio of the longitudinal to transverse components of the rho and phi production cross section, and the longitudinal component of the corresponding cross section are reported.