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

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Chapter 6 Event Selection After event reconstruction, the four-vectors of leptons and jets are available, to- gether with a number of other quantities helpful in identifying these objects. Since we want to study Z → µµ events, the primary aim of our event selection is to identify events with isolated muons that have high transverse momenta. In addition to the signal, various types of background events will pass our selection criteria, and we must eliminate as many of these events as possible. In this chapter, we describe the criteria for selecting muons and Z → µµ events, and then discuss the acceptance and efficiencies of these criteria. Background estimation will be described in Chapter 7. 6.1 Z event selection The Z event preselection is divided into two parts. First, we apply collision candidate selection criteria to maximize the likelihood that the events are from pp collisions: • We require each event to have a bunch crossing index which corresponds to bunches that collide in ATLAS. This criterion largely eliminates events from 160
Chapter 6: Event Selection 161 beam halo and beam-gas interactions. • Then we require that the events pass the L1 MU6 trigger. A this stage, the data sample contains 5464740 events. • Events are then required to have at least one reconstructed primary vertex which has at least three inner detector tracks associated with it. In addition, we require that the primary vertex be located within 15 cm from the nominal interaction point along the beam pipe. This requirement takes the finite size of the beam spot into account, at the same time reducing contamination from cosmic events. 5103785 events pass the vertex cut. The ‘high-pT muon event’ selection eliminates low-quality, low-pT and possibly fake muons. We require that the events have at least one combined muon, reconstructed in both the inner detector and the muon spectrometer, having pT greater than 15 GeV and within |η| < 2.4. The later cut is motivated by the fact that the muon trigger coverage ends at |η| =2.4 2.2.4. A detailed study of high-pT muons in collision data [61] showed that requiring a muon spectrometer track extrapolated to the primary vertex to have pT greater than 10 GeV largely rejects muons from pion/kaon decays-in-flight as well as poorly matched tracks between the inner detector and the muon spectrometer. Additionally, requiring the difference in pT between the spectrometer and inner detector tracks, |p MS T −p ID T |, to be less than 15 GeV significantly reduces fake muons while losing very little of the signal. We therefore impose these two criteria on the muon track. 22075 events are left in the data sample at this stage.
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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 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 255 [75] N. E. Adam an
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Bibliography 257 [102] The TOTEM Co
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