$Z$ Boson Decay into Light and Darkness

Understand

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.