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The apparent discrepancy between the Rosenbluth and the polarization transfer method for the ratio of the electric to magnetic proton form factors can be explained by a two-photon exchange correction which does not destroy the linearity of the Rosenbluth plot. Though intrinsically small, of the order of a few percent of the cross section, this correction is kinematically enhanced in the Rosenbluth method while it is small for the polarization transfer method, at least in the range of Q 2 where it has been used until now.

This paper presents the reactions which can be performed at COMPASS to study the Generalized Parton Distributions (GPDs). The high energy muon beam at CERN allows to measure Hard Exclusive Meson Production or Deeply Virtual Compton Scattering (DVCS) in the Bjorken regime in a large range of Q 2 and x Bj (1.5 ≤ Q 2≤ 7.5 GeV2 and 0.03 ≤ x Bj ≤ 0.25). Exploratory measurements dedicated to ρ ° or π° production can be investigated with the present setup. DVCS measurement require an upgrade of the COMPASS setup