Z Boson Decay into Light and Darkness.

Area of Science:

• Particle Physics
• High-Energy Physics
• Beyond Standard Model Physics

Background:

• The Landau-Yang theorem prohibits spin-1 particles decaying into two photons.
• Dark photons are hypothetical particles that interact weakly with Standard Model particles.
• Understanding rare Z boson decays can reveal new physics.

Purpose of the Study:

• To investigate the Z boson decay into a photon and a dark photon (Z→γγ[over ¯]).
• To explore how this decay evades the Landau-Yang theorem.
• To analyze the potential for detecting this process at future colliders.

Main Methods:

• Theoretical analysis of the Z→γγ[over ¯] decay process.
• Modeling dark photon interactions via dipole moments with Standard Model fermions.
• Calculation of the decay width and branching ratio.
• Comparison with existing experimental bounds (LEP).

Main Results:

• The Z→γγ[over ¯] decay is possible if the final photons are distinguishable, evading the Landau-Yang theorem.
• A striking signature of a monochromatic photon and missing momentum is predicted.
• LEP experiments set bounds on the branching ratio up to ~10^{-6}.
• Dark photon dipole moments can arise from one-loop exchange of dark sector particles.

Conclusions:

• The Z→γγ[over ¯] decay provides a testable prediction for dark sector models.
• Future colliders with large Z boson samples can explore this rare decay.
• This study highlights novel decay channels for probing new physics beyond the Standard Model.