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

7.5 Other Systematic Uncertainties
The measurement of the integrated luminosity has a total uncertainty of 3.7% [72]. This uncertainty is im-
plemented in the analysis by changing the normalization of the MC-driven SM backgrounds and signal. The
uncertainties on the b-tagging efficiencies scale factors used in MC are taken into account by recomputing
the predicted event yields and signal acceptance using the corresponding systematic shift. In both cases,
background and signal, this is found to be 5-6%. The effect of ISR and FSR and top quark mass uncertainties
are evaluated using the MC samples described in Section 4.2. For ISR and FSR, different samples are used,
where each one is varied individually. Half of the maximum deviation between the different samples is taken
as a systematic uncertainty. The same is done for the top mass systematic uncertainty, using the lower and
higher mass samples. The effect of uncertainties in the PDF used for signal generation was evaluated by
comparing the signal acceptance using MSTW2008LO with that from MRST2007 LO* PDFs.
7.6 Summary
The dominant uncertainty for the 2ID+TL analysis is the systematic uncertainty on the fake-TL prediction,
because 85% of the expected background arises from this source. As explained above, this was determined
to be 20% by comparing predicted and observed events with TLs in control regions dominated by fake TL.
Table 7.5 shows the contribution of the different systematic uncertainty sources to the MC-based background,
this is WZ, ZZ, t¯tW and t¯tZ. Table 7.6 presents a summary of each uncertainty and its contribution to the
net background yields and signal acceptance in the 2ID+TL analysis. Notice that most of the MC-related
uncertainties are highly suppressed when normalizing to the net background, since they only contribute
by 15% to it. From those systematic uncertainties, the WZ and ZZ modelling uncertainty is the most
significant. A similar table for the 3ID analysis can be found in Appendix C. The 2ID+TL background
evaluation has a smaller relative background systematic uncertainties in most categories, compared to the
3ID analysis, given that almost 90% of the 2ID+TL background evaluation is data-driven.
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