CP asymmetries induced by particle widths: Application to top-quark decays

An on-shell unstable particle produced in a tree-level process, such as W in t→qW, induces a CP violating signal through its width. This effect arises from an Im(tree)×Re(loop) type of interference. Among t→qq’q¯ processes (e.g., t→bcb¯) proceeding either through the tree-level t→qW or the one-loop t→q’+(virtual g) reactions, the CP asymmetry is largest for the case t→dcd¯ and is of scrO(η/λ²) in Wolfenstein’s parametrization. The resulting effects are found to be small in the standard model, but are much larger than those obtained for radiative top-quark decays. We also briefly discuss B decays where such a mechanism may cause CP violation in, for example, b→se⁺e^-.