Abstract:
Production and decay angular distributions were extracted from measurements of exclusive electroproduction of the ρ0(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± storage ring. The event sample combines ρ0 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 ρ0 → π+π- 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.
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