Measurement of the spin-density matrix elements in exclusive electroproduction of $\rho^0$ mesons at HERA

The ZEUS Collaboration:    J. Breitweg - et al.

doi:doi:10.1007/s100529900246

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2019 Impact factor 4.389

Particles and Fields

Eur. Phys. J. C 12, 393-410
DOI 10.1007/s100529900246

Measurement of the spin-density matrix elements
in exclusive electroproduction of $\rho^0$ 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 $\rho^0$ 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 $\rho^0$ production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) $\rho^0$ mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.