Abstract
We study the Z→γγ[over ¯] process in which the Z boson decays into a photon γ and a massless dark photon γ[over ¯], 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[over ¯] decay width can be explored with the large samples of Z bosons foreseen at future colliders.
Highlights
Consider the decay of a massive spin-one particle into two massless spin-one particles
We study the Z → γγprocess 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
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
Summary
Limit of 10−6 (95% C.L.) for the corresponding branching ratio (BR) [9]. Effective dipole moments in a simplified model of the dark sector.—We compute the dipole operators of Eq (1) in a simplified-model framework, where we make as few assumptions as possible on the structure of the dark sector. 1. One-loop vertex diagrams giving rise to the effective dipole operators in Eq (4) between SM fermions and the dark photon γ. The corresponding effective Lagrangian is equal to where the sum runs over all the SM fields, eD is the UDð1Þ dark elementary charge (we assume universal couplings), Λ the effective scale of the dark sector, ψf a generic SM fermion field. In order to reduce the number of dimensionful parameters, we have introduced in Eq (4) a dark-sector effective scale Λ, defined as the common mass of the dark fermion and the lightest messenger scalar. This choice corresponds to the maximal chiral enhancement.
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