A Z0 decays into an particle-antiparticle pair that fly off at an angle theta to the direction of the incoming electron-positron pair:
In selecting and events we have allowed pretty much all possible values for the angle theta. However, in selecting we have left out any events for which theta is less than 45 degrees.
(Look carefully at any events you observe. They will all occur at angles which are at least 450 from the beam axis. In contrast, in the and events sometimes the particles are at very small angles to the beam direction.)
The fact that we have left some events out means that the number of 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 events. This correction factor turns out to have the value 1.6.
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