Search for invisible dark photon in [Formula omitted] scattering at future lepton colliders

For the first time, the production of a massive dark photon (DP) in the [Formula omitted] scattering at the future lepton colliders ILC, CLIC, and CEPC is examined. The invisible decay mode of the DP is addressed. We have studied both the unpolarized scattering and the collision of the Compton backscattered photons with the polarized electron. The missing energy distributions are shown. We have considered the wide range 1-1000 GeV of the DP mass [Formula omitted]. The excluded regions at the 95% CL in the plane [Formula omitted], where [Formula omitted] is the kinetic mixing parameter, are obtained. In particular, in the low mass region, 1-10 GeV our bounds for the 90 GeV CEPC are several times stronger than experimental limits obtained by the BaBar collaboration. For the polarized scattering, the excluded bounds for all three colliders are approximately 20% stronger as compared with the unpolarized case.