CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

ID leptons provides three different scenarios:
a. Opposite-charge ID leptons
b. Two positively-charged ID leptons
c. Two negatively-charged ID leptons
In case a., the fake TL can have either charge so no charge discrimination is used in the numerator of
the fake matrix. In case b. the fake TL must be negatively charged, and a fake matrix with only negatively-
charged TLs in the numerator is used. Similarly, case c. requires a positively-charged TL and a fake matrix
with only positively-charged TLs in the numerator is used. Note that in all cases the same-flavor requirement
is automatically satisfied because the TL is taken to have the same flavor as the ID lepton with which it is
paired. The distributions shown in Figure 6.1 correspond to the full fake matrix used for case a. above.
To account for the reconstructed mass cuts done after the χ 2 minimization described in Section 5.6.1, a
correction is made to the fakes prediction described above: the predicted fakes are multiplied by the fraction
of 2ID+fake TLs that pass the χ 2 mass cuts. Because this technique does predict the 4-momenta vector of
the fake TL, needed for the invariant mass calculations, this fraction is taken from a MC sample with the
expected mixture of Z+jets, t¯t, W+jets, and single top events, and found to be (31.2 ± 10.2)%.
The parent sample with two ID leptons contains all sources of background that can enter the signal
region with a fake TL: Z+jets, dilepton decays of t¯t, and single top. The two ID leptons selected in the
parent sample include both real and fake ID leptons, so the multijet background is also included (although
expected to be very small). Thus the procedure predicts the full background contribution with a fake TL.
Furthermore, 2% of the total fake lepton background (at the final selection level) is added to account for
events with one real ID lepton, one real TL, and one fake ID lepton. This is evaluated in MC by measuring
the fraction of selected events with one fake ID lepton, over events with one fake TL. A conservative 100%
systematic uncertainty has been set on this number.
The predicted number of events at the final selection level with two ID leptons plus a fake TL is given in
Table 6.1. All corrections listed above are included. The quoted systematic uncertainty, 20%, is explained
in Chapter 7.
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