Ratios of helicity amplitudes for exclusive ρ electroproduction.

Exclusive ρ-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in the kinematic region 0.5 GeV< Q<7.0 GeV, 3.0 GeV < W<6.3 GeV, and − t′<0.4 GeV. Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T ( $\gamma^{*}_{T} \rightarrow\rho_{T}$), T ( $\gamma^{*}_{T} \rightarrow \rho_{L}$), T ( $\gamma^{*}_{L} \rightarrow\rho_{T}$), and T ( $\gamma^{*}_{-T} \rightarrow\rho_{T}$) to T ( $\gamma^{*}_{L}\rightarrow\rho_{L}$) are extracted from the data. For the unnatural-parity-exchange amplitude U, the ratio | U/ T| is obtained. The Q and t′ dependences of these ratios are presented and compared with perturbative QCD predictions. [ABSTRACT FROM AUTHOR]