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

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Chapter 4: Data Collection and Event Reconstruction 106 passed to a vertex fitter which uses a collinearity constraint, taking advantage of the massless nature of the photon 8 . The fitter returns a new conversion vertex candidate with the track parameters redefined with respect to the new vertex. The procedure is repeated until the fit converges. The next step is the rejection of fake conversion vertices. Cuts based on the fit χ 2 and the invariant mass of the reconstructed photon eliminate a large fraction of fakes. During the next level of event reconstruction, the use of calorimeter information for electron track identification further improves the purity of the vertices. This procedure can reconstruct conversion vertices up to a radial distance of 800 mm from the beam axis, beyond which track reconstruction efficiency drops drasti- cally owing to insufficient hits [35]. However, because of the limited spatial resolution of the TRT, the efficiency for conversion track pair reconstruction suffers significantly after a radial distance of 400 mm. Figure 4.5 shows the conversion radii of reconstructed photon conversions in data. The material layers at which conversions take place are mapped out by the photons. Beamline reconstruction The proton beams collide inside the beampipe in a luminous region that is ap- proximately linear along the z-axis. This is referred to as the beamline. A precise knowledge of the position, shape and size of the beamline is important for many physics studies such as b-tagging, as we have seen above. Ideally, the beamline would 8 For the decay of a massive long-lived neutral such as KS → 2π, the mass of the parent particle is used in the fitter. If the mass of the parent is not known, as in the search for long-lived new particles, an unconstrained fit can be used.
Chapter 4: Data Collection and Event Reconstruction 107 Figure 4.5: Radius of conversion of converted photons in 2010 data up to R = 400 mm. The material of the beampipe, the Pixel and SCT layers, and the support material are clearly mapped out by the photons.
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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 6 Event Selection After eve
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Chapter 6: Event Selection 189 Reco
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Chapter 6: Event Selection 193 Firs
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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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