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.
Z Boson Decay into Light and Darkness
Fabbrichesi, M.;Gabrielli, E.
;
2018-01-01
Abstract
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.File | Dimensione | Formato | |
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