Measurement of the spin-density matrix elements
in exclusive electroproduction
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
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