Search for lepton flavor violation at future lepton colliders

Lepton flavor violating (LFV) processes via four-Fermi contact interactions at future International Linear Collider (ILC) are studied. The effective Lagrangian is composed of six-operators, and the LFV effects on both $e^+ e^- \to e^+ \ell^-$ and $e^- e^- \to e^- \ell^-$ ($\ell=\mu$ or $\tau$) processes can be parametrized by three parameters. Taking account of previous experimental results of LFV processes $\mu\to 3e$ and $\tau \to 3e$, we find that the upper limits on the LFV parameters for $\ell=\tau$ could be improved at the ILC experiment using the polarized electron beam. The improvement of the upper limits is more than an order of magnitude smaller than previous ones.