Z Boson Decay into Light and Darkness

We study the Z→γγ ̄ process in which the Z boson decays into a photon γ and a massless dark photon γ ̄, 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→γγ ̄ decay width can be explored with the large samples of Z bosons foreseen at future colliders.