Vector Portals at Future Lepton Colliders

Abstract

We assess the sensitivity of future lepton colliders to weakly coupled vector dark portals (aka ``$Z^{\prime }$ bosons'') with masses ranging from tens of GeV to a few TeV. Our analysis focuses on dark photons and $L_{\hat{I}}¼-L_{\ddot{I}}„$ gauge bosons. We consider both visible and invisible decay channels. We demonstrate that both high energy $\hat{I}¼$ colliders and future $e^{+}{e}^{-}$ colliders, using the FCC-ee $Z$-pole and $ZH$ operation modes as a benchmark, offer significant improvements in sensitivity. We find that both colliders can enhance the sensitivity to $L_{\hat{I}}¼-L_{\ddot{I}}„$ bosons (for both visible and invisible decays) and to invisibly decaying dark photons by 1--2 orders of magnitude across the relevant mass range. Furthermore, we study the impact of forward $\hat{I}¼$ detectors at the $\hat{I}¼$-collider on the sensitivity to both models.

20 pages, 8 figures