DOI 10.1007/s100529900246
Measurement of the spin-density matrix elements
in exclusive electroproduction
of 
mesons at HERA
The ZEUS Collaboration
J. Breitweg - et al.
Argonne National Laboratory, Argonne, IL, USA
Received: 30 July 1999 / Revised version: 20 September 1999 / Published online: 8 December 1999
Abstract
Exclusive electroproduction of
mesons has been measured using the
ZEUS detector at HERA in two Q2 ranges,
0.25<Q2<0.85 GeV2and 3<Q2<30 GeV2. The low-Q2 data span the range
20<W<90 GeV; the high-Q2 data cover the
40<W<120 GeV interval. Both samples extend up to four-momentum transfers
of |t|=0.6 GeV2. The distribution in the azimuthal
angle between the positron scattering plane and the production plane shows a small but significant violation of s-channel
helicity conservation, corresponding to the production
of longitudinally polarised (i.e. helicity zero)
mesons from
transverse photons.
Measurements of the 15 combinations of spin-density matrix elements
which completely define the angular
distributions are presented and discussed.
Copyright Springer-Verlag 2000
