Measurement of the hard exclusive π0 muoproduction cross section at COMPASS

A new and detailed measurement of the cross section for hard exclusive neutral-pion muoproduction on the proton was performed in a wide kinematic region, with the photon virtuality Q 2 ranging from 1 to 8 (GeV/ c ) 2 and the Bjorken variable x B j ranging from 0.02 to 0.45. The data were collected at COMPASS at CERN using 160 GeV/ c longitudinally polarised μ + and μ − beams scattering off a 2.5 m long liquid hydrogen target. From the average of the measured μ + and μ − cross sections, the virtual-photon–proton cross section is determined as a function of the squared four-momentum transfer between the initial and final state proton in the range 0.08 (GeV/ c ) 2 < | t | < 0.64 (GeV/ c ) 2 . From its angular distribution, the combined contribution of transversely and longitudinally polarised photons are determined, as well as transverse–transverse and longitudinal–transverse interference contributions. They are studied as functions of four-momentum transfer | t | , photon virtuality Q 2 and virtual-photon energy ν . The longitudinal–transverse interference contribution is found to be compatible with zero. The significant transverse–transverse interference contribution reveals the existence of a dominant contribution by transversely polarised photons. This could provide clear experimental evidence for the chiral-odd GPD E ‾ T . In addition, the existence of a non-negligible contribution of longitudinally polarised photons is suggested by the | t | -dependence of the cross section at x B j < 0.1. Altogether, these results provide valuable input for future modelling of GPDs and thus of cross sections for exclusive pseudo-scalar meson production. Furthermore, they can be expected to facilitate the study of next-to-leading order corrections and higher-twist contributions.