Decoherence and More Coherence in the Radiative Decay of the $Z$ Boson

Abstract

Final state radiation in collider processes can be interpreted as interactions with unobserved degrees of freedom and is often discussed within the context of decoherence of an entangled state. We consider the radiative decay of the $Z$ boson as a representative example and perform a detailed analytical study of the $τ^-τ^+γ$ spin state, as determined by the chiral interactions of the Standard Model, over the complete three-body phase space. We explore various quantum information observables to quantify how the $τ^-τ^+$ spin state changes with the photon radiation. We find some striking features, for example that the emitted photon could either lead to decoherence or monotonic enhancement of entanglement for the fermion pair.