Coherent electroproduction of vector mesons on spinless targets

The amplitude ratios of vector-meson production by heavy photons on spinless targets are shown to can be explicitly expressed in terms of the spin-density-matrix elements (SDMEs) only if the lepton beam is longitudinally polarized. Making use of the amplitude ratios as free fit parameters instead of the SDMEs reduces the number of the real parameters in data description from 23 to 8. The exact formula for $R = \frac{d \sigma_L}{d t}/\frac{d \sigma_T}{d t}$ in terms of the SDMEs is obtained for spinless targets and the new approximate formula for $R$ is proposed for nucleon targets.