Abstract
We study the decay of the $Z$ vector boson into a photon and a massless (invisible) dark photon in high-energy collisions. The photon can be used as trigger for the event, while the dark photon is detected indirectly as missing momentum in the event final state. We investigate the possibility of searching for such a dark photon at the LHC, HL-LHC and future lepton colliders, and compare the respective sensitivities. As expected, the best result is found for the lepton colliders running at the $Z$ mass, FCC-ee and CEPC, with a final sensitivity to branching ratios of order $O(10^{-11})$. We also discuss how to use the photon angular distribution of the events in lepton collisions to discriminate between the dark photon and a pseudo-scalar state like the axion.
Highlights
We study the decay of the Z vector boson into a photon and a massless dark photon in highenergy collisions
We investigate the possibility of searching for such a dark photon at the Large Hadron Collider (LHC), High Luminosity LHC (HL-LHC), and future lepton colliders, and compare the respective sensitivities
We investigate the possibility of searching for such a massless dark photon in Z decays at the Large Hadron Collider (LHC), the High Luminosity LHC (HL-LHC), and future lepton colliders, and compare the respective sensitivities
Summary
The two-body Z-boson decay into an isolated photon γ and a stable (or metastable) neutral particle beyond the Standard Model (SM), effectively coupled to the Z and γ gauge bosons, can give rise to a peculiar experimental signature at high-energy colliders. The kinematical properties of the detected photon and of the neutral particle, which are both monochromatic in the rest frame of the decaying Z, make this process quite attractive in the search for new physics effects—most notably in the case of lepton colliders, for which the monochromatic photon energy is smeared only by bremsstrahlung radiation and detector effects. This feature is lessened at hadron colliders because of the additional challenges in reconstructing the rest frame of the Z boson, due to the characteristic uncertainties in the transverse missing momentum measurement.
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