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Tuning of Event Generators to Measurements of T Production and A simone amoroso TUNINGOFEVENTGENERATORSTOMEASUREMENTS OF t t¯ PRODUCTIONANDAGENERALSEARCHFOR NEWPHYSICSWITHTHEATLASEXPERIMENT CERN-THESIS-2015-151 25/09/2015 TUNINGOFEVENTGENERATORSTO MEASUREMENTSOF t t¯ PRODUCTIONANDA GENERALSEARCHFORNEWPHYSICSWITHTHE ATLASEXPERIMENT Dissertation zur Erlangung des Doktorgrades der Fakultät für Mathematik und Physik der Albert Ludwigs Universität Freiburg Vorgelegt von simone amoroso advisor: prof. dr. gregor herten June, 2015 Simone Amoroso: Tuning of event generators to measurements of tt¯ pro- duction and a general search for new physics with the ATLAS experiment, © June, 2015 datum der mundlichen prufung: 25.09.2015 dekan: Prof. Dr. Dietmar Kröner leiter der arbeit: Prof. Dr. Gregor Herten prüfer: Prof. Dr. Gregor Herten Prof. Dr. Karl Jakobs Prof. Dr. Harald Ita “It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts...” — Sherlok Holmes (from “A Study in Scarlet” by Sir A. C. Doyle) ABSTRACT The start of the Large Hadron Collider provides an unprecedent op- portunity for the exploration of physics at the TeV scale. It is expected to perform precise tests of the structure of the Standard Model and to hint at the structure of the physical laws at a more fundamental level. The first part of this work describes a tune of the initial- and final- state radiation parameters in the Pythia8 Montep Carlo generator, us- ing ATLAS measurements of tt¯ production at s = 7 TeV. The results are compared to previous tunes to the Z boson transverse momentum at the LHC, and to the LEP event shapes in Z boson hadronic decays, testing of the universality of the parton shower model. The tune of Pythia8 to the tt¯ measurements is applied to the next-to-leading or- der generators MadGraph5_aMC@NLO and Powheg, and additional parameters of these generators are tuned to the tt¯ data. For the first time in the context of Monte Carlo tuning, the correlation of the ex- perimental uncertainties has been used to constrain the parameters of the Monte Carlo models. In the second section we report results of a model independent search forp new phenomena with data recorded by the ATLAS de- tector at s = 8 TeV. Event topologies involving isolated electrons, photons, muons, jets, b-jets and missing transverse momentum are investigated. The events are subdivided according to their final states into 697 exclusive analysis channels. For each channel, a search algo- rithm tests the compatibility of the effective mass distribution in data against the distribution in the Monte Carlo simulated background. No significant deviations between data and the Standard Model ex- pectations have been observed. vii ZUSAMMENFASSUNG Seit der Inbetriebnahme des Large Hadron Colliders stehen uns neue Möglichkeiten offen, die physikalischen Phänomene an der TeV Skala zu erforschen. Präzise Tests des Standardmodells der Teilchenphysik können durchgeführt werden um die zugrunde liegenden physikalis- chen Gesetze noch besser zu verstehen. Der erste Teil dieser Arbeit befasst sich mit der Feinabstimmung von Strahlungsparametern des Pythia8 Monte Carlo Generatorsp unter Verwendung von ATLAS Daten aus tt¯ Produktion bei s = 7 TeV. Die Resultate können mit vorherigen LHC Ergebnissen verglichen wer- den, die aus der Kalibration des Transversalimpulses des Z Bosons stammen. Ebenso ist ein Vergleich mit Resultaten aus den hadro- nischen Zerfallsmoden des Z von LEP möglich, was einen Test für die Universalität des Partonmodells darstellt. Diese Feinabstimmung von Pythia8 aus den tt¯ Ereignissen kann auf Generatoren übertra- gen werden, die Strahlungskorrekturen höherer Ordnung verwen- den, wie MADGRAPH5_aMC@NLO oder POWHEG. Auch können zusätzliche Parameter dieser Generatoren auf die tt¯ Ereignisse abges- timmt werden. Im Rahmen dieser Studien wurden zu ersten Mal die Korrelationen experimenteller Unsicherheiten benutzt, um den Parameterraum von Monte Carlo Modellen einzuschränken. Im zweiten Abschnitt werden die Resultate einer modellunabhängi- gen Suchep nach neuer Physik vorgestellt, die mit ATLAS Daten aus 2012 bei s = 8 TeV durchgeführt wurde. Die Eigenschaften von Kollisionsereignissen mit isolierten Elektronen, Myonen, Photonen, Jets, b-jets und fehlender transversaler Energie werden untersucht. Die Ereignisse können anhand dieser Objekte klassifiziert und in 697 exklusive Analysekanäle eingeteilt werden. Für jeden dieser Kanäle wird ein statistischer Suchalgorithmus angewendet um Abweichun- gen vom simulierten Untergrund festzustellen. Dabei wurde keine signifikante Abweichung von der Vorhersage des Standardmodells entdeckt. viii PUBLICATIONS The material presented in this thesis is based on work performed within the ATLAS collaboration, thus relying on the contribution of many people. The author’s personal contribution is here briefly sum- marised. general search for new phenomena The author has been responsible for introducing and developing gen- eral search strategies within ATLAS. General searches have been per- formed using the datasets collected by ATLAS at 7 and 8 TeV of center of mass energy, respectively [1, 2], for which the author has been edi- tor and main analyzer. search for squarks and gluinos with the atlas detec- tor in final statesp with jets and missing transverse mo- mentum using s = 8 tev proton–proton collision data [3] Defined of the trigger strategy for signal and background selection. miss Validated the jet+ET triggers providing an uncertainty on its effi- ciency. search for direct pair production of the top squark in allp-hadronic final states in proton-proton collisions at s = 8 tev with the atlas detector[4] Developed the trigger strategy, and optimized the event selection to be insensitive to pile-up. Computed systematic uncertainties due to the modelling of the top background, dominant in the signal region. Performed studies for constraining nuisance parameters in the global fit to control and signal region. a study of the sensitivity to the pythia8 parton shower tt¯ pp parametersp of production measurements in colli- sions at s = 7 tev with the atlas experiment at the lhc [5] Responsible for implementing handling of uncertainties correlation in the Professor code. Generated the Pythia8 predictions annd tunes. Co-editor of the public note documenting the result. ix ACKNOWLEDGMENTS First of all I would like to thank my advisor, Prof. Gregor Herten, for accepting me into his group, and for the constant support I have been granted. During these four years I have been given the opportunity to participate in many school and conferences, and the rare freedom to pursue some of my own research interests and ideas. A special thank goes also to Sascha Caron, who has been my su- pervisor during the first year in Freiburg, with whom I keep collab- orating on a number of projects. To him goes the merit of disclosing to me the world of General Searches, and of convincing me of their usefulness and originality, I am deeply grateful to Anna Sfyrla, for encouraging me to work in the ATLAS trigger, and to Andreas Hoecker and Monica D’Onofrio, who entrusted me with the responsibility of coordinating the trigger strategy of the SUSY group during LS1. The first part of this thesis originates from work done under an MCnet fellowship. I have to thank Andy Buckley for accepting to supervise me and Stefano Camarda for sharing his deep knowledge of the world of Monte Carlo generators, and for an extremely pleasant collaboration. I would also like to thank all the people with whom I shared my time in Freiburg. Jan-Erik Sundermann for the constant help on computing matters, my student Fabio and all the other italian PhDs: Manuela, Luisa, Valerio, Manfredi, Francesca and Claudia. A final thank goes to my family for their support and encourage- ment in this endeavour. xi CONTENTS introduction1 i theory3 1 the standard model of particle physics5 1.1 History 5 1.2 The matter content 6 1.3 The Higgs mechanism and the electroweak sector 8 1.4 Fermion masses and mixing 10 1.5 The QCD Lagrangian 12 2 the structure of qcd processes 17 2.1 Infrared Safety 17 2.2 Parton Distribution Functions 18 2.3 Jets 20 2.4 Monte Carlo event generators 23 2.4.1 Hard scattering 23 2.4.2 Parton showers 25 2.4.3 Matching and merging 27 2.4.4 Underlying Event and Multiple Parton Interac- tions 28 2.4.5 Hadronization 30 2.4.6 Particle decays 32 2.4.7 Tuning 33 3 theories of new physics 35 3.1 Open problems in the Standard Model 35 3.1.1 Dark matter and dark energy 36 3.1.2 Naturalness and fine-tuning 37 3.1.3 Grand Unification 38 3.2 Supersymmetry 39 3.2.1 The Supersymmetry algebra 39 3.2.2 The Minimal Supersymmetric Standard Model 40 3.2.3 Supersymmetry breaking 42 3.2.4 A natural spectrum 44 3.3 Extra Dimensions 45 3.3.1 Large Extra Dimensions 46 3.3.2 Universal Extra Dimensions 46 3.3.3 Warped extra dimensions 47 3.4 Composite Higgs 47 3.4.1 Little Higgs models 48 ii experiment 49 4 the atlas experiment at the large hadron col- lider 51 4.1 The Large Hadron Collider 51 xiii xiv contents 4.1.1 The accelerator chain 52 4.1.2 Parameters and operations 53 4.2 The ATLAS experiment 55 4.2.1 Coordinate System 56 4.2.2 Magnet system 57 4.2.3 Inner Detector 58 4.2.4 Calorimeter 61 4.2.5 Muon Spectrometer 65 4.2.6 Luminosity and forward detectors 68 4.2.7 Trigger and DAQ 68 5 object reconstruction and selection 71 5.1 Tracking and vertexing 71 5.2 Jets 73 5.2.1 Clustering 74 5.2.2 Jets reconstruction and calibration 74 5.2.3 Jet energy scale and resolution uncertainties 75 5.3 b-jets identification 77
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