Master Class Notes

OPAL has a *much wider* angular coverage than DELPHI. So in OPAL the e+e- events would include the t-channel Bhabhas,
which would spoil the BR measurement for electrons. Therefore the authors have restricted the accepted e+e- events
to 45 degrees, and later correct back for this restriction.
Of course, it would have been also possible to restrict *everything* to 45 degress, but this was not done. DELPHI has a *much more limited* angular coverage. So they do not need to bother about t-channel Bhabhas, since the background from them is small.

Thus the correction factor for e+e- in OPAL is not, because OPAL has limited angular acceptance. On the contrary: The acceptance is very large, so that eleastic Bhabha scattering without intermediate Z is contributing sizably. So we have made an angular cut (just for e+e-) to get rid of bhabha, and multiply with 1.5 afterwards.