We study portal interactions connecting visible and dark sectors, and involving local interactions of a photon, a dark photon and a axionlike particle (ALP) at future e+e-colliders. These interactions, mediated by higher-dimensional effective operators, may arise at one loop by kinetic mixing between dark and ordinary photons, or, for massless dark photons, by direct short-distance contributions. We explore these portal interactions for a heavy ALP with masses between about 10 and 230 GeV, and for a massless dark photon, by investigating the sensitivity of the production e+e-→γγγ to the effective couplings, where the dark photon γ gives rise to missing momentum in the final state. We will show how an appropriate choice of missing-energy and missing-mass cuts can optimize the signal to standard-model background ratio. Exclusion regions for the effective photon-dark-photon-ALP couplings versus the ALP mass are worked out for a few representative values of the collision energy and integrated luminosity, as presently envisaged by future e+e-projects.
