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

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Chapter 4: Data Collection and Event Reconstruction 114 characterized as EM, hadronic or noise clusters based on their position and shape. They are then weighted with a calibration function depending on their location and energy. Next, a correction for energy loss in dead material in or near the cluster is applied. Finally, a correction is made for signal losses due to the clustering procedure. Note that all tower and topological clusters are defined as pseudo-particles with four-momenta, computed using the reconstructed energy and the cluster direction. This is important, for example, for jet finding algorithms, discussed below. Jet reconstruction The goal of jet reconstruction is to first find a calorimeter-level jet, then estimate the energy and shape of the particle-level (i.e. , hadron-level) jet, and eventually reproduce the kinematics of the original parton at the interaction-level. Because of the widely varying nature of the physics processes that result in jets, there is no optimal way to reconstruct jets from the hadronic final states of all interesting topologies. In general, any jet finding mechanism should be infrared-safe, that is, the presence of soft particles between two particles belonging to the same jet should not influence the inclusion of the two particles into the jet. It should also be collinear- safe, i.e. , it should reconstruct a jet correctly independent of the fact that a certain amount of energy may be carried by one particle or by two collinear particles. From a detector viewpoint, jet finding should be independent of detector technologies, and should not be affected by changing conditions such as underlying event activity and instantaneous luminosity. Two jet finders are commonly used in ATLAS, namely, the seeded fixed cone jet
Chapter 4: Data Collection and Event Reconstruction 115 finder and the sequential recombination algorithm. Both algorithms can use sliding window clusters or topological clusters as well as reconstructed tracks. The main features of each technique are discussed below. The seeded fixed cone jet finder This technique attempts to reconstruct jets using a fixed-size cone around a seed cluster. First, all input clusters are arranged in descending order of pT . If the cluster with the highest pT is above a seed threshold of 1 GeV, all clusters within an η − φ cone of fixed size is combined with the seed cluster. The size of the cone is given by Rcone = √ ∆η 2 +∆φ 2 ; the algorithm can be run with various cone options from Rcone =0.4 to Rcone =0.7. A new direction is computed from the four-momenta of the clusters in the first cone, and a new cone is formed. Clusters are then recollected in this cone, and the direction updated. The procedure continues until the cone direction is stable, at which point it is called a jet. The next seed cluster is then taken from the seed list and a new cone jet formed. The process continues until all seeds have been used up. The jets thus formed can share clusters. Since the cone size is fixed, this procedure is not infrared-safe. For example, a jet from a single hadron can get split into two jets with a large overlap. To remedy this problem, a split-and-merge step is performed after the jet finding. If the clusters shared between two jets contain more than 50% of the pT of the less energetic jet, the two jets are merged. Otherwise, they are kept as separate jets. However, the method is still not collinear-safe, since it uses seeds 12 . 12 An algorithm which starts by forming seed objects passing a threshold can miss some jets. Suppose a quark emits a gluon at a small angle such that the two jets end up in adjacent calorimeter clusters. If the energy in each cluster is below the seed threshold, neither jet will be reconstructed.
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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 189 Reco
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Chapter 6: Event Selection 193 Firs
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Chapter 6: Event Selection 199 the
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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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