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

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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 parton 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
Chapter 5: Monte Carlo Simulation 155 GeV, below which color screening in the hadrons suppresses perturbative interactions. Note that the Pythia underlying event description has been tuned inside the ATLAS event generation framework such that event properties match measurements using the 900 GeV data from 2009 and 7 TeV data in 2010 [93]. As discussed in Chapter 1, hadronization is implemented in Pythia via the Lund string model. External packages can be interfaced with Pythia where particle decay or radiation requires special treatment. These include TAUOLA [65] for τ decays with spin information and PHOTOS [77] for generating final-state photon radiation. PythiaB [87] is a modification of Pythia, used exclusively inside the ATLAS event generation framework, that allows fast generation of events containing b and/or c quarks. Our analysis uses Pythia-generated datasets for the signal channel and four background channels, namely, W → µν, b ¯ b, c¯c and Z → ττ. For the Z → µµ, W → µν and Z → ττ samples, the MRST LO ∗ [86] PDF set was used, and final-state photon radiation was generated with PHOTOS. In the Z → µµ sample, simulated minimum bias interactions were overlaid on top of the hard-scattering event to mimic the effect of event pileup 6.2. The b ¯ b and c¯c samples were generated with PythiaB, also using the MRST LO ∗ PDF set. 5.2.2 POWHEG POWHEG (Positive W eight Hardest Emission Generator) includes full next-to- leading-order QCD corrections in evaluating matrix elements. Additionally, it can be coupled to a parton shower-based Monte Carlo program. Since matrix elements accurately describe the high-pT regime of a given process, and parton shower most
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Measurement of the Z boson cross-se
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Contents Title Page . . . . . . . .
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Contents vi Electron reconstruction
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List of Figures 1.1 Proton structur
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List of Figures x 6.17 Muon pT scal
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List of Tables xii 6.18 Decompositi
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Acknowledgments xiv mate Niels van
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Chapter 1: Introduction and Theoret
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Chapter 2 The Accelerator and the E
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Chapter 2: The Accelerator and the
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Chapter 3 Luminosity Measurement at
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Chapter 3: Luminosity Measurement a
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Chapter 7: Background Estimation 20
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Chapter 8: Properties of Z bosons a
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Chapter 9: Discussion and Outlook 2
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Appendix A: Event list 245 Candidat
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Appendix A: Event list 247 Candidat
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Appendix B: Comparison of muon curv
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Bibliography 251 [13] S. Ask. Statu
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Bibliography 253 [44] S. Frixione,
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Bibliography 255 [75] N. E. Adam an
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Bibliography 257 [102] The TOTEM Co
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