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

Chapter 5: Monte Carlo Simulation 157
production cross-sections for W and Z bosons produced in hadronic collisions to
NNLO accuracy in QCD. As mentioned in Chapter 1, FEWZ takes into account
quark masses, finite width effects, spin correlations of boson decay products and
γ − Z interference in the case of Z bosons.
We use FEWZ to derive the theoretical prediction for the Z → µµ cross-section,
to which we compare the final result of our analysis. In addition, we normalize the
cross-sections of the Pythia-generated W and Z signal and background samples to
the NNLO cross-sections of the corresponding processes obtained from FEWZ. The
MSTW2008NNLO PDF set was used with FEWZ.
5.2.4 Monte Carlo samples used in this analysis
Table 5.1 gives a summary of the Monte Carlo datasets used in our analysis,
together with the cross-sections and the number of events in each sample. For the b ¯ b
and c¯c samples, the leading-order cross-sections from Pythia are used. The cross-
sections of the W and Z samples are normalized to the corresponding NNLO cross-
section values obtained using FEWZ. The t¯t cross-section is taken from [71].
5.2.5 The ATLAS detector simulation
As discussed above, an accurate detector description is crucial for developing an
analysis, and especially for optimizing the various selection criteria. The ATLAS
detector simulation is based on the GEANT4 [60] simulation toolkit, which is used
by a large number of projects in various domains. The ATLAS GEANT4 simulation
contains over a million volumes including active and inactive material.