Ratios of helicity amplitudes for exclusive ρ0 electroproduction

Exclusive ρ 0-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 GeV2t′<0.4 GeV2. Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T 11 (\(\gamma^{*}_{T} \rightarrow\rho_{T}\)), T 01 (\(\gamma^{*}_{T} \rightarrow \rho_{L}\)), T 10 (\(\gamma^{*}_{L} \rightarrow\rho_{T}\)), and T 1−1 (\(\gamma^{*}_{-T} \rightarrow\rho_{T}\)) to T 00 (\(\gamma^{*}_{L}\rightarrow\rho_{L}\)) are extracted from the data. For the unnatural-parity-exchange amplitude U 11, the ratio |U 11/T 00| is obtained. The Q 2 and t′ dependences of these ratios are presented and compared with perturbative QCD predictions.