Exotic Higgs Decays to Four Taus at Future Electron-Positron Colliders

We study the prospects for the exotic Higgs decay mode $h\to ss\to 4\tau$ at planned electron-positron colliders in the case where the beyond-the-Standard Model particle $s$ is too light to decay into $b\bar{b}$ pairs. We find that with 5 ab${}^{-1}$ of unpolarized collisions at 240 GeV, the branching ratio into this final state can be constrained to be below $\sim 1.3\times {10}^{-4}$ at 95% CL, depending on the mass of $s$ and the assumed tracking efficiency.