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

Chapter 7: Background Estimation 222
Since this is small compared to our total background prediction of 0.244 event (Ta-
ble 7.3) and is an upper limit in any case, we conclude that the cosmic ray background
is negligible for our analysis.
7.4 Summary of backgrounds
The total estimated background to Z → µµ in 331 nb −1 of data is 0.244 ± 0.057,
where the uncertainty quoted includes statistical and systematic components, but
not the uncertainty on the luminosity. All backgrounds are predicted based on the
acceptance for the Z selection in simulation, so that the 5.5% uncertainty on CZ
is included in the systematic. This value includes all contributions to CZ, namely,
the uncertainties on trigger, reconstruction and isolation cut efficiencies as well as on
muon momentum scale and resolution.
sample Number of events
W → µν 0.013 ± 0.003(stat) ± 0.001(sys) ± 0.001(lum)
Z → ττ 0.086 ± 0.004(stat) ± 0.007(sys) ± 0.0010(lum)
t¯t 0.107 ± 0.004(stat) ± 0.010(sys) ± 0.012(lum)
EW Total 0.206 ± 0.006 (stat) ± 0.017 (sys) ± 0.023 (lum)
QCD 0.038 ± 0.015(stat) ± 0.038(sys) ± 0.04(lum)
Total Background 0.244 ± 0.016 (stat) ± 0.055 (sys) ± 0.027 (lum)
Table 7.3: Details of the background calculation. The Monte Carlo event predictions
are normalized to 331 nb −1 , and the trigger and reconstruction efficiency scale factors
derived in Section 6.2 have been applied. See Chapter 5 for a definition of the Monte
Carlo samples.