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

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Chapter 3: Luminosity Measurement at the LHC and in ATLAS 88 3.3.2 LUCID LUCID (LUminosity measurement with a Cherenkov Integrating Detector) is the dedicated luminosity monitor for ATLAS, designed to operate over the full dynamic range of the LHC. It measures relative luminosity by detecting inelastic pp scattering in the forward direction. The detector consists of two aluminum vessels, 1.5 m long and ≈ 30 cm in diameter, placed around the beampipe ≈ 17 m from the IP. Each vessel contains 20 aluminum tubes filled with C4F10 gas, arranged in two concentric circles, and projective to the IP (Figure 3.13). The pseudorapidity coverage is 5.5 < |η| < 6.1. LUCID can operate independently of the Level 1 trigger, and indeed of the trigger and data acquisition system. This implies that it can measure the delivered luminosity in addition to the luminosity recorded by ATLAS. Figure 2. An artists impression of the LUCID detector in place between the beampipe and the conical Figure 3.13: Configuration of the LUCID tubes. Their positioning between the beampipe and the conical beampipe support tube is shown. A charged particle passing through a tube gives off Cherenkov radiation, which reaches the end of the tube after 3 reflections on average. The Cherenkov detection
Chapter 3: Luminosity Measurement at the LHC and in ATLAS 89 threshold is 2.5 GeV for pions and 10 MeV for electrons. The tubes are read out by PMTs. When the instantaneous luminosity increases above 10 33 cm −2 s −1 , the plan is to increase the number of tubes on each side to 168, and to replace the PMTs by scintillation fibers [23]. The fast response time of LUCID (≈ 3 ns) allows it to measure bunch-by-bunch luminosity. Signals due to secondary particles are minimal owing to two factors: firstly, the Cherenkov threshold suppresses low-energy particles; secondly, the projective geometry leads to a shorter path length in the gas for secondary particles than for primary particles from the IP. Since the number of Cherenkov photons is directly proportional to the path length in gas, secondaries produce a relatively small signal. LUCID measurements must be calibrated with absolute luminosity measurements obtained in parallel from some other source. Once LUCID has been calibrated, its measurements can be used over a wide range of beam conditions that may not be suitable for absolute measurement devices such as ALFA. As discussed before, in the 2010 run the calibration to absolute luminosity was done using beam parameters. Upon calibration, the overall accuracy of the luminosity measurement was 11%, the dominant contribution coming from beam current measurement [94]. From 2011 onwards, the calibration is expected to use ALFA measurements, with an achievable accuracy of ≈3%. Over the long term, physics processes with well-measured cross- sections can also be used for the calibration, but the accuracy of this method depends on theoretical uncertainties in the cross-sections, as discussed in Section 1.5.
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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 5: Monte Carlo Simulation 1
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Chapter 6 Event Selection After eve
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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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