EFT analysis of leptophilic dark matter at future electron-positron colliders in the mono-photon and mono-Z channels

We consider the possibility that dark matter (DM) only interacts with the Standard Model leptons, but not quarks at tree level, and analyze the future lepton collider prospects of such leptophilic DM in the monophoton and mono-$Z$ (both leptonic and hadronic) channels. Adopting a model-independent effective field theory framework, we consider all possible dimension-six operators of scalar-pseudoscalar (SP), vector-axial vector (VA), and tensor-axial tensor (TAT) types for a fermionic DM and derive the collider sensitivities on the effective cutoff scale $\Lambda$ as a function of the DM mass. As a concrete example, we take the beam configurations of the International Linear Collider with $\sqrt s=1$ TeV and $8$ ab$^{-1}$ integrated luminosity, including the effect of beam polarization, and show that it can probe leptophilic DM at $3\sigma$ level up to $\Lambda$ values of $6.6$, $8.8$, and $7.1$ TeV for the SP-, VA- and TAT-type operators, respectively. This is largely complementary to the direct and indirect searches for leptophilic DM and can potentially provide the best-ever sensitivity in the low-mass DM regime.