Measurement of the Underlying Event using track-based event shapes in Z -> ℓ+ℓ− events with ATLAS
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Department
Mathematisch-Naturwissenschaftliche Fakultät
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Abstract
Diese Dissertation beschreibt eine Messung von hadronischen Ereignisformvariablen (event shapes) in Protonkollisionen mit einer Schwerpunktsenergie von 7 TeV am Large Hadron Collider (LHC) am CERN (Conseil Europeenne pour la Recherche Nucleaire) bei Genf (Schweiz). Die analysierten Daten mit einer integrierten Luminosität von 1.1 inversen fb wurden im Jahr 2011 mit dem ATLAS Experiment aufgenommen. Für die Analyse wurden solche Ereignisse ausgewählt, in deren harten Streuprozessen ein Z-Boson produziert wurde, welches entweder in ein Elektron-Positron-Paar oder ein Muon-Antimuon-Paar zerfällt. Die Observablen wurden mit sämtlichen rekonstruierten Spuren innerhalb der Akzeptanz des inneren Spurdetektors (Inner Detector) von ATLAS außer denen der Leptonen des Zerfalls des Z-Bosons berechnet. Somit handelt es sich hierbei um die erste Messung dieser Art. Anschließend wurden die Observablen auf Untergrundprozesse mit auf Daten basierenden Methoden korrigiert wobei ein neues Verfahren für die Korrektur des sogenannten Pile-up (Überlagerung mehrerer Proton-Proton Wechselwirkungen) entwickelt und erfolgreich zur Anwedung gebracht wurde. Schließlich wurden die gemessenen Verteilungen entfaltet. Die so erhaltenen Daten sind insbesondere sensitiv auf das sogenannte Underlying Event und können direkt mit Monte-Carlo-Ereignisgeneratoren ohne aufwändige Simulation des ATLAS-Detektors verglichen werden. Abschließend wurde versucht die Modellparameter in den Simulationsprogrammen Pythia8 und Sherpa mithilfe der gewonnenen Daten durch eine bessere Abstimmung (Tuning) zu verbessern. Hierbei zeigte sich, dass das zugrunde liegende Sjostrand-Zijl Modell nicht ausreicht, um eine adäquate Beschreibung der gemessenen Verteilungen zu erreichen.
This thesis describes a measurement of hadron-collider event shapes in proton-proton collisions at a centre of momentum energy of 7 TeV at the Large Hadron Collider (LHC) at CERN (Conseil Europeenne pour la Recherche Nucleaire) located near Geneva (Switzerland). The analysed data (integrated luminosity: 1.1 inverse fb) was recorded in 2011 with the ATLAS-experiment. Events where a Z-boson was produced in the hard sub-process which subsequently decays into an electron-positron or muon-antimuon pair were selected for this analysis. The observables are calculated using all reconstructed tracks of charged particles within the acceptance of the inner detector of ATLAS except those of the leptons of the Z-decay. Thus, this is the first measurement of its kind. The observables were corrected for background processes using data-driven methods. For the correction of so-called pile-up (multiple overlapping proton-proton collisions) a novel technique was developed and successfully applied. The data was further unfolded to correct for remaining detector effects. The obtained distributions are especially sensitive to the so-called Underlying Event and can be compared with predictions of Monte-Carlo event-generators directly, i.e. without the necessity of running time-consuming simulations of the ATLAS-detector. Finally, it was tried to improve the predictions of the event generators Pythia8 and Sherpa by finding an optimised setting of relevant model parameters in a technique called Tuning. It became apparent, however, that the underlying Sjostrand-Zijl model is unable to give a good description of the measured event-shape distributions.
This thesis describes a measurement of hadron-collider event shapes in proton-proton collisions at a centre of momentum energy of 7 TeV at the Large Hadron Collider (LHC) at CERN (Conseil Europeenne pour la Recherche Nucleaire) located near Geneva (Switzerland). The analysed data (integrated luminosity: 1.1 inverse fb) was recorded in 2011 with the ATLAS-experiment. Events where a Z-boson was produced in the hard sub-process which subsequently decays into an electron-positron or muon-antimuon pair were selected for this analysis. The observables are calculated using all reconstructed tracks of charged particles within the acceptance of the inner detector of ATLAS except those of the leptons of the Z-decay. Thus, this is the first measurement of its kind. The observables were corrected for background processes using data-driven methods. For the correction of so-called pile-up (multiple overlapping proton-proton collisions) a novel technique was developed and successfully applied. The data was further unfolded to correct for remaining detector effects. The obtained distributions are especially sensitive to the so-called Underlying Event and can be compared with predictions of Monte-Carlo event-generators directly, i.e. without the necessity of running time-consuming simulations of the ATLAS-detector. Finally, it was tried to improve the predictions of the event generators Pythia8 and Sherpa by finding an optimised setting of relevant model parameters in a technique called Tuning. It became apparent, however, that the underlying Sjostrand-Zijl model is unable to give a good description of the measured event-shape distributions.
Description
Keywords
LHC, ATLAS, QCD, soft, Underlying Event, MPI, LHC, ATLAS, QCD, soft, Underlying Event, MPI
Dewey Decimal Classification
530 Physik
Citation
Schulz, Holger.(2015). Measurement of the Underlying Event using track-based event shapes in Z -> ℓ+ℓ− events with ATLAS. 10.18452/17129