  Lepton flavor violated processes are strongly suppressed in the Standard
Model, but can be sizable in some extended models. The current experimental
bounds on decay modes $\ell'^{\pm} \to a \ell^{\pm}$ are much weaker than the
other processes because of the huge irreducible backgrounds $\ell'^{\pm} \to
\ell^{\pm} \bar{\nu}_{\ell} \nu_{\ell'}$. In this paper, we study polarization
effects of both the signals and backgrounds. We find that signals and
backgrounds have distinctive polarization effects, and for the irreducible
backgrounds both longitudinal and transverse polarization effects survive when
the relative momentum of the two neutrinos are integrated out. At low energy
$e^+e^-$ collider, for instance the Belle II experiment, leptons are generated
in pair but with no net polarization. However, we show that polarization
correlation of the lepton pair is a useful observable for probing the signal.
More interestingly, the polarization correlation depends on product of scalar
and pseudo-scalar couplings, and hence are sensitive to their relative sign.
Because kinematical reconstruction or tagging is not necessary, number of the
available signal events increases significantly.
