DOI 10.1007/s100520000536
Measurement of angular distributions and
in diffractive electroproduction of 
mesons
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
Abstract
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
