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New Physics Searches in the ZZ Sector with the ATLAS Experiment

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New Physics Searches in the ZZ Sector with the ATLAS Experiment New Physics Searches in the ZZ Sector with the ATLAS Experiment Thomas James Barber of King’s College A dissertation submitted to the University of Cambridge for the degree of Doctor of Philosophy February 2011 ii iii Abstract This thesis investigates the prospects of measuring anomalous triple gauge boson cou- plings in the ATLAS detector at the Large Hadron Collider (LHC). The most general V =Z,γ (VZZ,V = Z,γ) vertex is parametrised by four couplings, fi=4,5 , all of which are zero in the Standard Model. Non-zero couplings would manifest themselves as an excess of events in ZZ diboson channels, and, if observed, would be a direct probe of new physics beyond the Standard Model. A set of criteria are outlined to select events recorded by ATLAS in two such channels, 1 ZZ llll(l = e, µ) and ZZ llνν¯. With 1 fb− of integrated luminosity at a centre of → → mass energy of √s = 7 TeV, ATLAS can expect to observe 10 1 events in the ZZ llll +0.9 ± → channel, with 0.5 0.2 background events. In the ZZ llνν¯ channel, 6.2 0.7 signal − +2.0 → ± events are expected, with a background of 1.9 0.2 events. − The expected sensitivity of ATLAS to non-zero anomalous couplings is calculated by per- forming extended, unbinned maximum-likelihood fits to the Z boson transverse momen- 1 tum spectrum. For 1 fb− of integrated luminosity at √s = 7 TeV, ATLAS has the poten- tial to place constraints on the coupling parameters of f Z < 0.06 and f γ < 0.07 at the | i | | i | 95% confidence level. These limits assume a form factor with a cutoff of ΛFF =1.2 TeV. As a prelude to ZZ observation, criteria are defined to select Z ll(l = e, µ) events → 1 in the first 315 nb− of ATLAS pp collision data at √s = 7 TeV. In total 57 events are observed in the electron channel, with 109 in the muon channel, leading to cross section measurements of σ(Z ee)=0.70 0.09 (stat) 0.10 (syst) 0.08 (lumi) pb → ± ± ± and σ(Z µµ)=0.90 0.09 (stat) 0.07 (syst) 0.10 (lumi) pb, both of which are → ± ± ± consistent with the Standard Model predictions. In addition, this thesis presents a summary of developments made to the Data Acqui- sition (DAQ) system of the ATLAS Semiconductor Tracker (SCT). These include the construction of a test system, involving a scaled-down version of the entire SCT read- out chain. The test system was subsequently used to develop a number of new DAQ features, including a hardware-based event simulator and monitoring framework. iv v Declaration This dissertation is the result of my own work, except where explicit reference is made to the work of others, and has not been submitted for another qualification to this or any other university. This dissertation does not exceed the word limit for the respective Degree Committee. Thomas Barber vi vii Acknowledgements I would first of all like to thank the Science and Technology Research Council (STFC) for their financial support under STFC student number (PPA/S/S/2006/04325), includ- ing a very enjoyable two years spent on Long Term Attachment (LTA) based in Geneva. I am also extremely grateful to King’s College for travel grants which have allowed me to present my work and expand my cultural horizons at conferences in Israel and China. I would also like to express my gratitude to my supervisor, Richard Batley, for his support over the last four years and speedy proof-reading in the last four weeks. Thanks also go to Pat Ward for productive and stimulating discussions on all things diboson. During the time I spent at CERN, I am greatly indebted to Dave Robinson for his patience explaining the SCT timing, and Bruce Gallop for his encyclopedic knowledge of the SCT ROD registers. Finally, I would like to dedicate this thesis to my parents, Bridget and Graham, for their unending support over the last 27 years. viii ix Preface This thesis represents the culmination of work I have carried out at the Cavendish Laboratory, Cambridge between October 2006 and February 2011. My research is based on a number of aspects of the ATLAS experiment, one of the detectors designed to record proton-proton collisions in the Large Hadron Collider at The European Organisation for Nuclear Research (CERN) in Geneva. At the start of my studies, the final components of the ATLAS detector were being installed in the Point-1 experimental cavern, 100 m beneath the Franco-Swiss border near Geneva. After extensive commissioning and calibration work, the Large Hadron Collider (LHC) is now accelerating and colliding protons at unprecedented energies of 7 TeV. The field of high energy particle physics has truly moved into a new era. A vital aspect of the ATLAS detector, described in chapter 2, is the measurement of the position and momentum of charged particles produced in LHC collisions. During the two years I spend based at CERN, I was particularly involved with the ATLAS silicon strip detector, the Semiconductor Tracker (SCT). An introduction to the Data Acquisition (DAQ) system required to read out the 6 million channels of the SCT is given in chapter 3. Chapter 4 describes the construction of a scaled-down test system, which was used to develop many new features of the SCT DAQ. The features that I was particularly involved in are documented in chapter 5, and focus on developments made on the SCT Readout Driver (ROD). These include a hardware-based simulation of SCT events and the construction of a new monitoring framework. One of the aims of the ATLAS experiment is to search for as-yet undiscovered physics. Our current understanding of the laws of the Universe is described by the Standard Model (SM), which states that each force is mediated by a set of particles known as gauge bosons. For example, as discussed in chapter 1, the photon, γ, is the force carrier for electromagnetism, while the W and Z bosons mediate the weak force. Interactions between three neutrally charged gauge bosons are forbidden in the Standard Model, and x are hence known as anomalous couplings. Discovery of non-zero anomalous couplings would be a direct indication of new physics. Interactions in which a pair of Z bosons are produced, known as dibosons, are sensi- tive to such anomalous couplings. Chapter 6 outlines a set of criteria for selecting and reconstructing diboson events. Using detailed computer simulations of the ATLAS ex- 1 periment, the expected number of events after 1 fb− of data (approximately one year) is calculated. Anomalous couplings have the effect of increasing the expected number of diboson events, and the momenta of the bosons perpendicular to the beam direction. In chapter 7, I show ATLAS should be able to rule out anomalous couplings with a sensitivity twice that of the current measurements. The LHC began high energy collisions in March 2010, six months before the end of my doctorate. As the majority of this thesis uses simulated collisions, I couldn’t resist the opportunity to get my hands dirty with some real data. Chapter 8 presents the first observation of single Z bosons in the ATLAS detector. I show that the measured cross section (production rate) is in good agreement with theoretical calculations. Contents 1 Anomalous Triple Gauge Couplings 3 1.1 Introduction .................................. 3 1.2 The Electro-weak Interaction ........................ 3 1.2.1 The Weak Interaction ........................ 5 1.2.2 Electro-weak Unification ....................... 7 1.2.3 Spontaneous Symmetry Breaking .................. 8 1.2.4 The Standard Model ......................... 9 1.3 Triple Gauge Couplings ............................ 9 1.3.1 Anomalous Neutral Boson Couplings ................ 10 1.3.2 Origin of Anomalous Couplings ................... 11 1.4 ZZ Diboson Production ........................... 13 1.5 Event Generation ............................... 14 1.5.1 Matrix Element Calculation ..................... 14 1.5.2 Parton Distribution Functions .................... 14 1.5.3 Parton Showering and Hadronisation ................ 15 1.5.4 Monte Carlo Generators ....................... 16 1.5.5 Anomalous Coupling Generators ................... 17 1.6 ZZ Inclusive Cross Section .......................... 18 1.6.1 Gluon-Gluon Fusion ......................... 19 1.6.2 ZZ Decay Modes ........................... 20 1.6.3 Backgrounds to ZZ Production ................... 20 1.6.4 Anomalous Triple Gauge Couplings at the LHC .......... 21 1.7 Current Experimental Results ........................ 24 1.7.1 Results from LEP ........................... 24 1.7.2 Results from the Tevatron ...................... 25 1.7.3 ATLAS Sensitivity Studies ...................... 28 1.7.4 Diboson Background to New Physics ................ 29 1.8 Conclusions .................................. 30 xi xii Contents 2 The ATLAS Experiment 31 2.1 The Large Hadron Collider .......................... 31 2.1.1 Context ................................ 32 2.2 The ATLAS Experiment ........................... 33 2.2.1 Co-ordinate Definition ........................ 34 2.2.2 Prospects for New Discovery ..................... 34 2.2.3 Experimental Requirements ..................... 35 2.3 The ATLAS Detector ............................. 35 2.3.1 The Inner Detector .......................... 36 2.3.2 Calorimeters .............................. 37 2.3.3 Muon Spectrometer .......................... 39 2.3.4 Forward Detectors .......................... 40 2.4 Trigger, Data Acquisition and Control Systems .............. 40 3 The ATLAS Semiconductor Tracker 43 3.1 Introduction .................................. 43 3.2 Silicon Detectors ............................... 44 3.2.1 Physics of Semiconductors ...................... 44 3.2.2 p-n Junctions ............................. 44 3.2.3 Semiconductors as Particle Detectors ................ 45 3.2.4 Radiation Damage .......................... 45 3.3 The ATLAS SCT ............................... 46 3.4 SCT Modules ................................. 48 3.4.1 Silicon Sensors ............................ 48 3.4.2 ABCD3T Chips ............................ 50 3.5 The SCT Data Acquisition System ..................... 51 3.5.1 Hardware ............................... 52 3.5.2 Software ................................ 57 3.6 Calibration of the SCT ...........................
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