$Z$ Boson Decay into Light and Darkness

TL;DR

This paper investigates the rare decay of the Z boson into a photon and a massless dark photon, highlighting how such a process can occur despite the Landau-Yang theorem, and discusses potential experimental detection at future colliders.

Contribution

It presents a theoretical analysis of Z boson decay into a photon and dark photon, including a simplified dark sector model and implications for future collider experiments.

Findings

Branching ratio up to 10^{-6} allowed by LEP bounds.

Dark-photon dipole moments generated via one-loop exchange of dark sector particles.

Potential to observe this decay at future high-luminosity colliders.

Abstract

We study the $Z\rightarrow \gamma \bar \gamma$ process in which the $Z$ boson decays into a photon $\gamma$ and a massless dark photon $\bar \gamma$, when the latter couples to standard-model fermions via dipole moments. This is a simple yet nontrivial example of how the Landau-Yang theorem---ruling out the decay of a massive spin-1 particle into two photons---is evaded if the final particles can be distinguished. The striking signature of this process is a resonant monochromatic single photon in the $Z$-boson center of mass together with missing momentum. LEP experimental bounds allow a branching ratio up to about 10$^{-6}$ for such a decay. In a simplified model of the dark sector, the dark-photon dipole moments arise from one-loop exchange of heavy dark fermions and scalar messengers. The corresponding prediction for the rare $Z\to \gamma \bar \gamma$ decay width can be explored with the large samples of $Z$ bosons foreseen at future colliders.