$Z$ Boson Decay into Light and Darkness

Abstract

We study the $Z\to \gamma \bar{\gamma }$ 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.

Same as published PRL version, minor comments added, 5 pages, 3 figures

References in corpus (8)

• Dark Matter and Dark Radiation
• New constraints on light vectors coupled to anomalous currents
• Dark forces coupled to non-conserved currents
• Landau-Yang Theorem and Decays of a Z' Boson into Two Z Bosons
• Dark photons and resonant monophoton signatures in Higgs boson decays at the LHC
• FCNC decays of SM fermions into a dark photon
• Hunting down massless dark photons in kaon physics
• Dark-photon searches via $ZH$ production at $e^{+}{e}^{-}$ colliders

Cited by in corpus (12)

• The Dark Photon
• Neutral Hadrons Disappearing into the Darkness
• Long-lived light neutralinos at future $Z-$factories
• Dark-sector physics in the search for the rare decays $K^{+}\to {\pi }^{+}\bar{\nu }\nu$ and $K_L\to {\pi }^0\bar{\nu }\nu$
• Charged neutron stars and observational tests of a dark force weaker than gravity
• $Z$-boson decays into an invisible dark photon at the LHC, HL-LHC and future lepton colliders
• Phenomenological consequences of an interacting multicomponent dark sector
• Untangling the spin of a dark boson in $Z$ decays
• Magnetic-dipole corrections to $R_K$ and $R_{K^{\ast }}$ in the Standard Model and Dark Photon scenarios
• Dark origin of quark flavor hierarchy and mixing
• Vacuum stability with radiative Yukawa couplings
• Probing Higgs and Top Interactions through the Muon Lens at multi-TeV Muon Colliders