Results on exclusive $\rho^{0}$ production from COMPASS

Study of the $\rho^{0}$ spin density matrices $r^{\alpha}_{\gamma\gamma'}$ using exclusive incoherent $\rho^{0}$ production events is performed by the COMPASS collaboration. An overview of this analysis is presented, and preliminary results from the 2002 data are shown. These results are compatible with the previous experiments but with a better statistical accuracy and cover a higher range in $Q^{2}$. The ratio R of the longitudinal to transverse virtual photon polarization cross sections for the studied process is deduced from $r^{04}_{00}$. The non-zero value of $r^{04}_{1-1}$ indicates a weak violation of the s-channel helicity conservation.