Diffractive physics at HERMES

The measured decay angle distributions of the ρ0 are in agreement with the hypothesis that the reaction is dominated by helicity conserving amplitudes with natural parity exchange, and are used to extract a measure of R-σL/σT. Cross section measurements of exclusive ρ0 production on 1H and 3He targets in the range 0.5 < Q2 < 5.6 GeV2 as well as extrapolated photoproduction cross section are presented in comparison with world data. The Q2--dependence of the cross section is well described by the propagator originating from the Vector Meson Dominance model with a power dependence of 2.5. The observed W--dependence is described by a model based on Regge field theory. A perturbative QCD calculation based on Off-Forward Parton Distributions also gives a good description of the longitudinal component of the cross section for Q2 > 2 GeV2