$Z$-boson decays into an invisible dark photon at the LHC, HL-LHC and future lepton colliders

TL;DR

This paper explores the potential to detect a massless dark photon produced in Z boson decays at current and future colliders, highlighting the superior sensitivity of lepton colliders operating at the Z resonance.

Contribution

It introduces a novel search strategy for massless dark photons via Z decays, comparing sensitivities across collider types and proposing angular distribution analysis for discrimination.

Findings

Lepton colliders at Z resonance can reach sensitivities of O(10^{-11}) in branching ratios.

The photon angular distribution helps distinguish dark photons from axion-like particles.

Sensitivity at the LHC and HL-LHC is significantly lower than at lepton colliders.

Abstract

We study the decay of the $Z$ vector boson into a photon and a massless (invisible) dark photon in high-energy collisions. The photon can be used as trigger for the event, while the dark photon is detected indirectly as missing momentum in the event final state. We investigate the possibility of searching for such a dark photon at the LHC, HL-LHC and future lepton colliders, and compare the respective sensitivities. As expected, the best result is found for the lepton colliders running at the $Z$ mass, FCC-ee and CEPC, with a final sensitivity to branching ratios of order $O(10^{-11})$. We also discuss how to use the photon angular distribution of the events in lepton collisions to discriminate between the dark photon and a pseudo-scalar state like the axion.