Hard exclusive meson production at COMPASS

The concept of Generalized Parton Distributions (GPDs) combines two-dimensional spatial information given by form factors, with longitudinal momentum information from Parton Distribution Functions. GPDs provide comprehensive description of the nucleon structure involving a wealth of new information. For instance, according to Ji’s sum rule, the GPDs H and E enable access to the total angular momenta of quarks, antiquarks and gluons. While H can be approached using measurements of electroproduction cross sections, asymmetry measurements in hard exclusive meson production off transversely polarized targets can help to constrain the GPD E and chiral-odd GPDs. In 2007 and 2010 the COMPASS experiment at CERN collected data by scattering a 160GeV/c muon beam off a transversely polarized NH3 target. Exclusive vector-meson production μ + p → μ′ + p + V with a ρ0 or ω meson in the final state is studied and five single-spin and three double-spin azimuthal asymmetries are measured.