Abstract
Hard exclusive electroproduction of omega mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive pi omega transition form factor.
Highlights
In the framework of quantum chromodynamics (QCD), hard exclusive meson leptoproduction on a longitudinally or transversely polarized proton target provides important information about the spin structure of the nucleon
In contrast to leptoproduction of vector mesons with an unpolarized target, which is mainly sensitive to generalized parton distributions (GPDs) H f, vector-meson leptoproduction off a transversely polarized nucleon is sensitive to the interference between two amplitudes containing H f and E f, respectively, and opens access to E f
Each panel shows as two filled circles the results in two kinematic bins, and as one open square the result in the entire kinematic region
Summary
In the framework of quantum chromodynamics (QCD), hard exclusive meson leptoproduction on a longitudinally or transversely polarized proton target provides important information about the spin structure of the nucleon. Hard exclusive production of vector mesons is described by GPDs H f and E f , where f denotes a quark flavor or a gluon. These “unpolarized”, i.e., parton–helicity–nonflip distributions describe the photon–parton interaction with conservation and flip of nucleon helicity, respectively. By using the quark transverse momenta in the subprocess, the transverse size of the meson is effectively taken into account Using this approach, the GK model describes cross sections, spin density matrix elements (SDMEs), and spin asymmetries in exclusive vector-meson production for values of Bjorken-x below 0.2 [6,7,8]. The measured asymmetry amplitudes are compared to the most recent calculations of the GK model using either possible sign of the π ω transition form factor
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