Measurement of angular distributions and
in diffractive electroproduction of
The HERMES Collaboration
K. Ackerstaff - et al.
5 DESY, Deutsches Elektronen Synchrotron, 22603 Hamburg, Germany -
DESY Zeuthen, 15738 Zeuthen, Germany
Received: 15 February 2000 / Revised version: 16 October 2000 /
Published online: 8 December 2000 - © Springer-Verlag 2000
Production and decay angular distributions were extracted from measurements of exclusive electroproduction of the meson over a range in the virtual photon negative four-momentum squared 0.5<Q2<4 GeV2 and the photon-nucleon invariant mass range 3.8<W<6.5 GeV. The experiment was performed with the HERMES spectrometer, using a longitudinally polarized positron beam and a 3He gas target internal to the HERA e storage ring. The event sample combines mesons produced incoherently off individual nucleons and coherently off the nucleus as a whole. The distributions in one production angle and two angles describing the decay yielded measurements of eight elements of the spin-density matrix, including one that had not been measured before. The results are consistent with the dominance of helicity conserving amplitudes and natural parity exchange. The improved precision achieved at 4<W<7 GeV, in combination with other data at W>7 GeV, reveals evidence for an energy dependence in the ratio R of the longitudinal to transverse cross sections at constant Q2.
Copyright Società Italiana di Fisica, Springer-Verlag 2000