2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 18, 303-316
DOI 10.1007/s100520000536

Measurement of angular distributions and $R= \sigma_L/\sigma_T$
in diffractive electroproduction of $\rho^0$ mesons

The HERMES Collaboration

K. Ackerstaff$^{\rm 5}$ - 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 $\rho^0(770)$ 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$^\pm$ storage ring. The event sample combines $\rho^0$ 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 $\rho^0
\rightarrow \pi^+\pi^-$ 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