Discovering an invisible Z' at the muon collider

TL;DR

This paper proposes a method to discover a heavy, invisible Z' gauge boson at a future muon collider through photon-associated production, enabling detection despite its weak interactions and high mass.

Contribution

It introduces a novel detection strategy for an invisible Z' at muon colliders, leveraging radiative return to identify its mass and interactions.

Findings

Invisible Z' can be detected via photon energy peaks at muon colliders.

Production is feasible despite weak coupling through mixing mechanisms.

Method allows precise mass and interaction measurements of the Z'.

Abstract

We show in this letter how a heavy $(\mathcal{O}(TeV))$ invisible $Z'$ gauge boson that will practically be out of reach of the Large Hadron Collider (LHC), can be discovered at the future muon collider. The new force carrier has a relatively stronger coupling with the beyond standard model (BSM) sector, while its interaction with the SM fields is much weaker. This weaker coupling is induced through mixing mechanisms, specifically via gauge kinetic mixing and the $Z-Z'$ mixing. We consider a scenario where the new gauge boson decays mostly to charge-neutral long-lived particles and/or dark matter (DM). We show how producing and detecting this heavier invisible $Z'$, that will be beyond the reach of even the very high luminosity LHC, becomes possible if it is produced in association with an energetic photon at the future muon collider. The on-shell production of the $Z'$ will lead to a peak in the photon energy distribution, following the so-called radiative return phenomena and can lead to the accurate determination of the $Z'$ mass and its interaction with SM particles.