<i>CP</i>asymmetries induced by particle widths: Application to top-quark decays

Abstract

An on-shell unstable particle produced in a tree-level process, such as W in t\ensuremath{\rightarrow}qW, induces a CP violating signal through its width. This effect arises from an Im(tree)\ifmmode\times\else\texttimes\fi{}Re(loop) type of interference. Among t\ensuremath{\rightarrow}qq'q\ifmmode\bar\else\textasciimacron\fi{} processes (e.g., t\ensuremath{\rightarrow}bcb\ifmmode\bar\else\textasciimacron\fi{}) proceeding either through the tree-level t\ensuremath{\rightarrow}qW or the one-loop t\ensuremath{\rightarrow}q'+(virtual g) reactions, the CP asymmetry is largest for the case t\ensuremath{\rightarrow}dcd\ifmmode\bar\else\textasciimacron\fi{} and is of scrO(\ensuremath{\eta}/${\ensuremath{\lambda}}^{2}$) 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\ensuremath{\rightarrow}${\mathit{se}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$.