Vector Portals at Future Lepton Colliders

TL;DR

This paper evaluates the potential of future lepton colliders, including muon and electron-positron colliders, to detect weakly coupled vector dark portals like dark photons and $L_{}-L_{}$ gauge bosons, showing significant sensitivity improvements.

Contribution

It provides a comprehensive analysis of future collider sensitivities to vector dark portals, including the impact of forward muon detectors, with novel focus on both visible and invisible decay channels.

Findings

Future colliders can improve sensitivity to $L_{}-L_{}$ bosons by 1-2 orders of magnitude.

High energy muon and $e^+e^-$ colliders significantly enhance detection prospects for dark photons.

Forward muon detectors further increase sensitivity to these models.

Abstract

We assess the sensitivity of future lepton colliders to weakly coupled vector dark portals (aka ``$ Z' $ bosons'') with masses ranging from tens of GeV to a few TeV. Our analysis focuses on dark photons and $ L_{\mu} - L_{\tau} $ gauge bosons. We consider both visible and invisible decay channels. We demonstrate that both high energy $\mu$ 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_{\mu} - L_{\tau} $ 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 $ \mu $ detectors at the $ \mu $-collider on the sensitivity to both models.