Correction Factor for \(e^{+}e^{-}\) Events
A \(Z^{0}\) decays into an particle-antiparticle pair that fly off at an angle theta to the direction of the incoming electron-positron pair:
In selecting \(e^{+}e^{-} \rightarrow \mu^{+}\mu^{-}\) and \(e^{+}e^{-} \rightarrow \tau^{+}\tau^{-}\) events we have allowed pretty much all possible values for the angle theta. However, in selecting \(e^{+}e^{-} \rightarrow e^{+}e^{-}\) we have left out any events for which theta is less than 45 degrees.
(Look carefully at any \(e^{+}e^{-} \rightarrow e^{+}e^{-}\) events you observe. They will all occur at angles which are at least 450 from the beam axis. In contrast, in the \(e^{+}e^{-} \rightarrow \mu^{+}\mu^{-}\) and \(e^{+}e^{-} \rightarrow \tau^{+}\tau^{-}\) events sometimes the particles are at very small angles to the beam direction.)
The fact that we have left some \(e^{+}e^{-} \rightarrow e^{+}e^{-}\) events out means that the number of \(e^{+}e^{-} \rightarrow e^{+}e^{-}\) events you observe will be smaller than if we had kept them all. Therefore we have to apply a "correction factor" to get the "total" number of \(e^{+}e^{-} \rightarrow e^{+}e^{-}\) events. This correction factor turns out to have the value 1.6.