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

Chapter 5: Monte Carlo Simulation 154
tronic signals from active volumes are simulated such that they mimic the passage
of real particles. From these signals, the full event can then be reconstructed in the
same way as for real events, as described in Chapter 4.
By comparing simulation results with the truth information, we can determine
quantities such as the acceptance of the detector and the smearing of kinematic
distributions due to finite detector resolution.
5.2 Monte Carlo generators used in this analysis
The Monte Carlo datasets we use for our analysis were generated in the Athena [18]
framework using two programs: Pythia [92] and POWHEG [44]. In addition, we
use the FEWZ [67] program to compute the theoretical prediction for the Z → µµ
cross-section with which we compare our measurement. The main features of these
programs are briefly described in this section.
5.2.1 Pythia
Pythia evaluates process cross-sections by using matrix elements that are leading-
order in QCD. For many processes, including the Z → µµ decay, it includes spin
correlations of the decay products. Pythia uses the parton shower technique (see
Chapter 1) to generate initial- and final-state gluon radiation. The Pythia par-
ton shower is accurate to leading-logarithm, and leads to semi-realistic modeling of
multipartonic final states.
Pythia models the underlying event by a set of separate 2 → 2 scatterings using
leading-order matrix elements. Parton-parton interactions occur down to qT =2