München 2009 – wissenschaftliches Programm

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T: Fachverband Teilchenphysik

T 28: QCD 2

T 28.6: Vortrag

Mittwoch, 11. März 2009, 18:00–18:15, A017

Electroproduction of single π⁺ mesons on transversely polarised protons — •Ivana Hristova — DESY, 22603 Hamburg, Germany

The exclusive electroproduction of π⁺ mesons was studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positrons and electrons off a transversely nuclear-polarised hydrogen target. Making use of the high degree of target-gas polarisation as well as the excellent lepton-hadron separation and particle identification of the detector, the first measurement was carried out of the single-spin azimuthal asymmetry A_UT for exclusive production of π⁺. The precision of this measurement was limited by the unfeasibility to detect the recoiling neutron in the process e p → e n π⁺ and the lack of data at different beam energies. The results are presented as a function of the Mandelstam variable t, the Bjorken scaling variable x_B, and the virtuality of the exchanged photon Q². The leading contribution to the asymmetry was found to be consistent with zero and with a recent model calculation, while one subleading contribution was found to be large and positive. Overall, the data disagree with leading-twist model calculations based on generalised parton distributions. These distributions give a three-dimensional representation of the hadron structure at the partonic level.