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Single spin asymmetries in $\ell p^\uparrow \to hX$ and $e^-e^+ \to q\bar q \to h^\uparrow X$

Spin observables may reveal much deeper properties of non perturbative hadronic physics than unpolarized quantities. We discuss here possible origins of single spin asymmetries in DIS, absent in the elementary lepton-quark interactions, and suggest strategies to isolate and understand some non perturbative spin dependence of distribution and fragmentation functions. We also discuss the polarization of hadrons produced in $e^+e^-$ annihilation at LEP and show how final state $\qq$ interactions may give origin to non zero values of the off-diagonal element $ρ_{1,-1}$ of the helicity density matrix of vector mesons: some predictions are given for $K^{*0}, ϕ$ and $D^*$ in agreement with recent OPAL data. Possible analogous effects in DIS and other processes are suggested.