Spin effects in vector meson production at LEP

Spin observables may reveal much deeper properties of non perturbative hadronic physics than unpolarized quantities. We discuss the polarization of hadrons produced in $e^{+}{e}^{-}$ annihilation at LEP. We show how final state $q \bar q$ interactions may give origin to non zero values of the off-diagonal element ${\rho }_{1,-1}^{}$ of the helicity density matrix of vector mesons: some predictions are given for $K^{\ast },\varphi ,D^{\ast }$ and $B^{\ast }$ in agreement with recent OPAL data. We also discuss the relative amount of vector and pseudovector meson states and the probability of helicity zero vector states. Similar measurements in other processes are suggested.