Measurement of the Z boson cross-section in - Harvard University ...

Chapter 7: Background Estimation 217
Measured bb fraction (%)
10
8
6
4
2
0
0 2 4 6 8 10
Input bb fraction (%)
Figure 7.6: The measured QCD fraction as a function the input fraction from 8
ensemble tests. The slope of the first-degree polynomial fit gives the correction factor
for the QCD fraction obtained from a template fit to data.
sections of these samples, and use it as a systematic on the Monte Carlo prediction
of the QCD background.
In 331 nb −1 of data, we predict 0.038±0.015(stat)±0.038(sys)±0.04(lum) events
using simulated b ¯ b and c¯c samples.
7.2 Electroweak backgrounds
7.2.1 t¯t background
t¯t decays can fake the signal in two ways. One W can decay into a muon, which
can combine with a muon in a jet to give a Z → µµ decay topology, or both W ’s
can decay into muons which, being high-pT and isolated, together resemble the signal
channel. In the former case, the muon from the jet will generally be non-isolated, and