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Study of Diffraction with the ATLAS Detector at The Th`esede doctorat de l’Universit´eParis 11 et de l’Institut de Physique Nucléaire de l’Académie Polonaise des Sciences sp´ecialit´e Champs, Particules, Mati`ere pr´esent´eepar Rafa lSTASZEWSKI pour obtenir les grades de docteur de l’Universit´eParis 11 et de l’Institut de Physique Nucléaire de l’Académie Polonaise des Sciences Study of Diffraction with the ATLAS detector at the LHC Th`esesoutenu le 24 Septembre 2012 devant le jury compos´ede: Etienne AUGE(´ pr´esident) Marco BRUSCHI (rapporteur) Janusz CHWASTOWSKI (directeur de th`ese) Alan MARTIN (rapporteur) Christophe ROYON (directeur de th`ese) Robi PESCHANSKI Antoni SZCZUREK Th`esepr´epar´ee au Service de Physique des Particules du CEA de Saclay et `al’Institut de Physique Nucléaire de l’Académie Polonaise des Sciences de Cracovie The thesis is devoted to the study of diffractive physics with the ATLAS de- tector at the LHC. After a short introduction to diffractive physics including soft and hard diffraction, we discuss diffractive exclusive production at the LHC which is particularly interesting for Higgs and jet production. The QCD mechanism de- scribed by the Khoze Martin Ryskin and the CHIDe models are elucidated in detail. The uncertainties on these models are still large and a new possible exclusive jet measurement at the LHC will allow to reduce the uncertainty on diffarctive Higgs boson production to a factor 2 to 3. An additional measurement of exclusive pion production pp → pπ+π−p allows to constrain further exclusive model relying on the use of the ALFA stations, which are used in the ATLAS Experiment for detection of protons scattered in elastic and diffractive interactions. In the last part of the thesis, the AFP detectors, aiming at measuring the pro- tons scattered in diffractive interactions, are presented. They allow to extend sub- stantially the ATLAS physics programme. In particular, the study of the central diffractive W boson production process makes possible a better determination of the nature of diffractive exchanges. La thèse est consacrée à l’étude de la diffraction en utilisant le détecteur ATLAS aupr`esdu LHC. Apr`esUne courte introduction à la physique diffractive incluant la diffraction ‘dure” et “molle”, nous presentons la production diffractive exclusive qui est particulierement interessante pour produire des jets et ke boson de Higgs. Le mécanisme décrit par le formalisme de Khoze Martin et Ryskin et celui de CHIDe sont d´ecritsen d´etail.Les sources d’incertitudes dans la description théorique sont encore importantes et une nouvelle mesure de la section efficace de production ex- clusive de jets au LHC permettra de reduire l’incertitude de la production diffractive de boson de Higgs `aun facteur 2 `atrois. La mesure de la production exclusive de pions pp → pπ+π−p permet de contraindre les mod`elesde mani`ereplus pr´eciseen utilisant les detecteurs ALFA, qui sont utilisés dans l’exp´erienceATLAS pour la détection de protons diffus´esdans les interactions élastiques et diffractives. Les détecteurs AFP d´ecritsdans la derni`erepartie de la th`ese,mesurant les pro- tons dispersés apr`esinteraction diffractive sont présentés. Ils permettent d’´etendre le programme de physique d’ATLAS, avec en particulier, la production centrale diffractive de boson W , ce qui rend possible une meilleure comprehension de la nature des échanges diffractifs. The Henryk Niewodniczanski´ Institute of Nuclear Physics Polish Academy of Sciences University of Paris-Sud 11 Study of Diffraction with the ATLAS Detector at the LHC Rafa l Staszewski Supervisors: dr hab. Janusz Chwastowski, prof. IFJ dr hab. Christophe Royon, directeur de recherche Contents Contents 3 Introduction 5 Chapter 1. Diffractive Physics 9 1.1 Soft Diffraction . 11 1.2 Hard Diffraction . 16 1.3 Exclusive Production . 22 Chapter 2. Uncertainties of Exclusive Production Models 27 2.1 The FPMC Generator . 27 2.2 KMR and CHIDe Models . 28 2.3 Implementation of Exclusive Production . 32 2.4 Sources of Uncertainty . 38 2.5 Uncertainties at the LHC . 45 Chapter 3. Experimental Apparatus 49 3.1 The Large Hadron Collider . 49 3.2 The ATLAS Detector . 54 3.3 ATLAS Trigger . 62 3.4 Data Processing . 63 Chapter 4. Diffractive Measurements with ALFA Detectors 67 4.1 Theoretical Model . 68 4.2 Dedicated LHC Runs with β∗ = 90 m . 70 4.3 Measurement Using the ATLAS Central Detector . 74 4 Chapter 5. The AFP Detectors 79 5.1 Physics Motivation . 80 5.2 Detector System . 87 5.3 Proton Transport . 92 5.4 Scattered Proton Energy Unfolding . 96 Chapter 6. Central Diffractive W Charge Asymmetry Measurement 103 6.1 W Boson Charge Asymmetry . 104 6.2 Central Diffractive W Production . 106 6.3 W/Z Cross Section Ratio . 109 6.4 ATLAS Simulation and Pile-up Treatment . 111 6.5 Monte Carlo Samples . 112 6.6 Signal Selection . 114 6.7 Results . 122 Summary and Conclusions 133 Bibliography 135 List of Figures 145 List of Tables 155 Introduction iffraction has always been an important part of the studies performed in experiments involving hadron interactions. This is true also for the LHC, Dwhere a large community works on both theoretical and experimental aspects of possible diffractive measurements. At the LHC, contrary to previous studies at HERA and Tevatron, diffraction have a chance to contribute into studies of new physics, both in the Higgs and Beyond Standard Model (BSM) sector. This work presents the author’s contribution into the development of the diffrac- tive programme in the ATLAS experiment at the LHC. The obtained results are important both for the general, motivational aspects concerning the possible mea- surement and their significance, as well as for more technical details crucial for the experimental set-up, its performance and data quality. Chapter 1 of the thesis contains an introduction into the field of diffractive physics. It begins with presenting soft diffraction, contributing to minimum bias interactions, present in pile-up events and thus important to all measurements per- formed at the LHC at high luminosity. Next, hard diffraction and its types are discussed, which are most interesting from the Quantum Chromodynamics (QCD) point of view and the mechanism governing the Pomeron exchange. Finally, exclu- sive production is introduced and its different types are discussed. Chapter 2 discusses the models of exclusive production of jets and the Higgs bo- son and their theoretical uncertainties. Different sources of uncertainty are presented and their impact on the uncertainty of the cross sections is studied. An attempt is made to estimate the overall uncertainty of exclusive processes at the LHC based on measurements from the Tevatron. Finally, the possibility of constraining the uncertainty on Higgs production with early jet measurements is investigated. Chapter 3 briefly introduces the Large Hadron Collider, presenting the main points of its programme, the key aspects of its design and properties and the LHC 6 Introduction experiments. Next, a more detailed description of the ATLAS experiment is given, including sub-detectors, trigger and data processing. The present ATLAS detector allows investigation of diffraction by direct tagging of the forward protons in a very limited way. This can by done with the ALFA detectors, whose primary purpose is to measure elastic scattering, but they allow also studies of single diffractive dissociation processes. Chapter 4 shows that one more process can be measured with ALFA detectors, i.e. the exclusive π+π− production. In order to enhance the ATLAS abilities beyond the ones provided by ALFA, additional dedicated detectors are needed. Chapter 5 presents the project of AT- LAS Forward Proton (AFP) detectors that would allow detection of the diffractively scattered protons. First, the most interesting measurements possible with the AFP detectors are presented, second the detector design is discussed. Then, the proper- ties of the detectors due to the LHC magnetic lattice are shown, in particular the geometrical acceptance of the detectors and the resolutions of the energy reconstruc- tion. In the last chapter, the possibility of the central diffractive W production mea- surement is studied. In particular, the measurement of W charge asymmetry and its role in determining the diffraction mechanism are discussed. The analysis is based on full simulation of the ATLAS detector, including pile-up interactions, and contains the detailed study of signal and backgrounds. The presented results are based on the work performed during the four years of PhD studies. The majority of them have been published or presented during conferences and workshops. Personal Contributions Published Articles • M. Trzebiski, R. Staszewski i J. Chwastowski, LHC High-β∗ Runs: Transport and Unfolding Methods, ISRN High Energy Physics, vol. 2012, Article ID 491460. • R. Staszewski, M. Trzebiski i J. Chwastowski, Dynamic Alignment of the Forward-Proton Detectors at the LHC, Advances in High Energy Physics, vol. 2012, Article ID 428305. • K. Golec-Biernat, C. Royon, L. Schoeffel, R. Staszewski, Electroweak vector bo- son production at the LHC as a probe of mechanisms of diffraction. Published in Phys. Rev. D84 (2011) 114006. • R. Staszewski, P. Lebiedowicz, M. Trzebi´nski,J. Chwastowski, A. Szczurek, Exclusive π+π− Production at the LHC with Forward Proton Tagging. Pub- lished in Acta Phys. Polon. B42 (2011) 1861-1870. 7 • The ATLAS Collaboration (R. Staszewski for the Collaboration), The AFP Project. Published in Acta Phys. Polon. B42 (2011) 1615-1624, presented at Cracow Epiphany Conference 2011 The First Year of the LHC. • A. Dechambre, O. Kepka, C. Royon, R. Staszewski, Uncertainties on exclusive diffractive Higgs and jets production at the LHC. Published in Phys. Rev. D83 (2011) 054013. • R. Staszewski, J. Chwastowski, Transport Simulation and Diffractive Event Reconstruction at the LHC. Published in Nucl. Instrum. Meth. A609 (2009) 136-141. Preprints and Notes • M. Boonekamp, A. Dechambre, V. Juranek, O. Kepka, M. Rangel, C. Royon, R.
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