Dark Photon Searches at Future $e^+ e^-$ Colliders

The Dark Photon is a hypothetical particle that can be possibly detected in the experiment because of its kinetic mixing with the ordinary, visible photon. In theories where dark matter is explained by the existence of a ’dark sector’, interacting with the visible sector of the Standard Model via gravitation, the photon of the dark sector, the Dark Photon (A_D ), might be the only visible manifestation of the dark sector. This is because this new gauge boson would mix with the (ordinary) photon through kinetic mixing. The strength of the mixing is given by a mixing parameterϵ. This same parameter governs both the production cross-section and the decay of the Dark Photon back to SM particles. Detectors at future e+ e− colliders will have excellent momentum resolution and equally excellent track-finding efficiency. These are the features needed to probe for the expected signal of a Dark Photon, namely a quite small, and quite narrow resonance: If ϵ is large enough to yield a detectable signal, its decay width will be smaller than the detector resolution, but so large that the decay back to SM particles is prompt. This study investigates the sensitivity of future electron-positron colliders as a function of the mixing parameter and the mass of the Dark Photon. In contrast to earlier, theoretical studies, this work takes into account for the first time the full Standard Model background and is based on the detailled simulation of a full detector concept, namely the ILD detector at the ILC.