Diffractive Dissociation of Protons in 7 Tev Collisions at the ATLAS Detector

$Diffractive Dissociation of Protons in 7 Tev Collisions at the ATLAS Detector$ Diffractive dissociation of protons in 7 TeV collisions at the ATLAS detector. Timothy Alexander David Martin Thesis submitted for the degree of Doctor of Philosophy CERN-THESIS-2012-354 21/11/2012 Particle Physics Group, School of Physics and Astronomy, University of Birmingham. December 21, 2012 ABSTRACT 1 A data sample with integrated luminosity 7.1 µb− of pp collisions was collected with a minimum bias trigger at ps = 7 TeV using the ATLAS detector at the Large Hadron Collider. It is analysed to identify large pseudorapidity gaps. The inelastic cross section is presented differentially in ∆ηF , the largest continuous region of pseudorapidity which extends from the edge of the detector at η = 4:9 and cut ± which contains no final state particles above a threshold pT . The measurement F cut is presented in the region 0 < ∆η < 8 for 200 < pT < 800 MeV. Diffractive topologies are isolated at large gap sizes. The distribution is interpreted using triple Regge models of diffractive scattering, and the dependence of the inelastic cross section on the kinematics of diffraction is studied and compared with other LHC measurements. i DECLARATION OF AUTHORS CONTRIBUTION The design, construction and continued successful operation of the LHC, along with the detectors which adorn it is due to the dedication of many thousand scientists, engineers and technicians over a period of many years. Having been fortunate to have the opportunity to work in the ATLAS collaboration during the beginning of this new exploration in high energy physics research, I have made contributions in different areas of the experiment which will be detailed in this thesis. The scope of this document encompasses an overview of the whole LHC project along with ATLAS specifics. Particular focus and detail is given to areas in which I have contributed directly. These include the ATLAS trigger systems, specifically the Level 1 Calorimeter Trigger (L1Calo) and the minimum bias trigger. My official service position was maintenance and development of the Detector Con- trol Systems for L1Calo and other L1 trigger systems. An overview of technical work is separated from the main document in AppendixA. My involvement with the minimum bias trigger group came from being based at CERN and working on a minimum bias analysis during the first data taking campaign at a 900 GeV centre of mass energy in 2009. I continued to work with this group throughout my PhD. The diffractive physics analysis presented in this thesis was for the large part performed jointly with colleagues from Fyzikáln´ı ústav in Prague. The speed and quality of the analysis was greatly increased through collaboration and through maintaining two independent code bases. In addition to the main physics results, many cross checks and control distributions were produced. Some of these controls were produced by only one group or the other and analysis plots included in the narative of this document which was performed solely by Prague is cited to the joint internal documentation of the analysis, [1]. ii ACKNOWLEDGEMENTS I am grateful to everyone who has helped and inspired me throughout my education. I would like to thank Mari Baker both for being a fantastic maths and physics teacher during my A-levels and for introducing me to the LHC. I am thankful to the whole of the Birmingham group, to Pete and John for being such enthusiastic tutors during my undergraduate, to all my fellow post-grad students for the numerous interesting discussions we've had over the years and to Paul for guiding me through all the steps needed to make a physics measurement. At CERN, I am grateful to the level one calorimeter trigger and minimum bias communities, both for being comprised of such friendly and enthusiastic people and for being pleasures to work with. I give my thanks to my loved ones for the support given whilst preparing this document. Finally, I would like to thank the STFC for funding my studentship. Dedicated to the memory of my father. iii iv v Discere grati¯adiscend¯i vi MCViz Contents 1 NON-TECHNICAL SUMMARY 1 1.1 THE EARLY UNIVERSE .......................... 1 1.2 STUDYING THE MOMENT AFTER INCEPTION ............ 3 1.3 WHEN PROTONS INTERACT ...................... 4 2 INTRODUCTION 9 2.1 DOCUMENT STRUCTURE ........................ 11 3 THE CERN COMPLEX 13 3.1 LINEAR ACCELERATORS ........................ 14 3.2 PROTON AND SUPER PROTON SYNCHROTRONS .......... 15 3.3 LARGE HADRON COLLIDER ....................... 16 3.4 THE ALICE DETECTOR ......................... 20 3.5 THE CMS DETECTOR .......................... 21 3.6 THE LHCb DETECTOR .......................... 22 3.7 THE LHCf EXPERIMENT ......................... 23 3.8 THE TOTEM EXPERIMENT ....................... 23 3.9 THE MoEDAL EXPERIMENT ....................... 24 4 THE ATLAS DETECTOR 27 4.1 ATLAS MAGNETIC SYSTEM ....................... 27 4.2 ATLAS INNER DETECTOR ........................ 29 4.2.1 PIXEL DETECTOR AND SEMICONDUCTOR TRACKER ...... 30 4.2.2 TRANSITION RADIATION TRACKER ............... 32 4.3 ATLAS CALORIMETER SYSTEMS .................... 32 4.3.1 ELECTROMAGNETIC CALORIMETRY ............... 33 4.3.2 HADRONIC AND FORWARD CALORIMETRY ........... 34 4.3.3 CALORIMETER ENERGY RESOLOUTION ............. 35 4.3.4 ZERO DEGREE CALORIMETERS ................. 36 4.4 ATLAS MUON SPECTROMETERS .................... 37 4.5 LUCID DETECTOR ............................ 38 5 TRIGGERING AND DATA ACQUISITION 41 5.1 ATLAS TRIGGER ARCHITECTURE ................... 42 5.2 FIRST LEVEL TRIGGER ......................... 42 5.2.1 LEVEL ONE CALORIMETER TRIGGER .............. 43 5.2.1.1 CLUSTER PROCESSORS .................. 44 vii CONTENTS viii 5.2.1.2 JET/ENERGY PROCESSORS ................ 46 5.2.1.3 L1CALO OUTPUT ..................... 46 5.2.2 LEVEL ONE MUON TRIGGER ................... 48 5.2.3 LEVEL ONE MINIMUM BIAS TRIGGER .............. 49 5.2.4 CENTRAL TRIGGER PROCESSOR ................. 50 5.2.5 REGIONS OF INTEREST ...................... 52 5.2.6 DEADTIME ............................. 54 5.3 SECOND LEVEL TRIGGER & EVENT BUILDING ............ 54 5.3.1 EVENT FILTER ........................... 55 5.3.2 STREAMING ............................ 55 6 ATLAS COMPUTING 57 6.1 ATLAS SOFTWARE FRAMEWORK ................... 58 6.2 EVENT STORE .............................. 59 7 STRONG INTERACTIONS AND DIFFRACTION 63 7.1 THE STANDARD MODEL OF PARTICLE PHYSICS .......... 63 7.2 STRONG INTERACTIONS ........................ 64 7.3 REGGE SCATTERING THEORY AND THE POMERON ........ 67 7.4 THE TOTAL CROSS SECTION ...................... 69 7.5 INELASTIC DIFFRACTION ........................ 70 7.6 DISSECTION OF THE TOTAL CROSS SECTION ............ 72 7.7 THE TRIPLE REGGE MODEL ...................... 73 7.8 KINEMATICS OF DIFFRACTIVE DISSOCIATION ............ 75 7.8.1 DIFFRACTIVE MASS VERSUS RAPIDITY GAP CORRELATION .. 77 7.8.2 GENERATOR LEVEL DIFFRACTIVE MASS RECONSTRUCTION .. 78 7.8.3 ATLAS ACCEPTANCE FOR DOUBLE DIFFRACTION ....... 80 8 MONTE CARLO SIMULATION 83 8.1 THE PYTHIA MONTE CARLO MODELS ................ 83 8.1.1 PYTHIA CROSS SECTIONS .................... 83 8.1.2 DIFFRACTION IN PYTHIA ..................... 85 8.1.3 ALTERNATE POMERON FLUXES IN PYTHIA8 .......... 89 8.1.4 NON-DIFFRACTIVE EVENTS IN PYTHIA ............. 92 8.2 PHOJET MONTE CARLO ........................ 94 8.3 HERWIG++ MONTE CARLO ...................... 95 8.4 TUNING OF MONTE CARLO CROSS SECTIONS ............ 96 8.4.1 MONTE CARLO TUNES ...................... 99 8.5 MONTE CARLO STATISTICS ...................... 99 9 EVENT SELECTION AND CUTS 101 9.1 DATA SAMPLE .............................. 101 9.1.1 TRIGGERING ON MINIMUM BIAS EVENTS ............ 102 9.1.2 EFFICIENCY OF THE MBTS .................... 103 9.1.3 OFF-LINE MBTS SELECTION PROCESS .............. 105 9.1.4 TREATMENT OF BACKGROUNDS AND PILE-UP ......... 105 ix CONTENTS 9.2 DEFINITION OF RECONSTRUCTED QUANTITIES ........... 108 9.2.1 TRACKING SELECTION ...................... 108 9.2.2 CALORIMETER SELECTION .................... 110 9.2.2.1 CALORIMETER NOISE SUPPRESSION ........... 113 9.3 DEFINITION OF GENERATOR LEVEL QUANTITIES .......... 116 9.4 OVERALL PARTICLE DETECTION PROBABILITY ........... 116 9.4.1 LOW MOMENTUM B FIELD TRAPPING .............. 117 10 DIFFRACTIVE PHYSICS ANALYSIS 123 10.1 RECONSTRUCTING RAPIDITY GAPS .................. 123 10.1.1 BINNING .............................. 126 10.2 DATA/MC COMPARISON AT RECONSTRUCTED LEVEL ....... 126 10.3 DATA CORRECTION PROCEDURE ................... 131 10.3.1 TRIGGER CORRECTION ...................... 132 10.3.2 BEAM BACKGROUND SUBTRACTION .............. 134 10.3.3 UNFOLDING ............................ 135 10.3.4 UNFOLDED DISTRIBUTIONS ................... 138 10.3.5 STATISTICAL ERROR ....................... 140 10.4 QUANTIFICATION OF SYSTEMATIC UNCERTAINTY ......... 141 10.4.1 MODEL UNCERTAINTY ...................... 141 10.4.2 UNCERTAINTY ON THE DIFFRACTIVE CONTRIBUTIONS .... 143 10.4.3 ENERGY SCALE UNCERTAINTY .................. 143 10.4.4 MBTS RESPONSE UNCERTAINTY ................ 147 10.4.5 TRACKING UNCERTAINTY .................... 148 10.4.6 LUMINOSITY UNCERTAINTY ................... 149 cut 10.5 EVOLUTION OF UNCERTAINTY WITH pT .............. 149 11 RESULTS 151 cut 11.1 FORWARD GAP CROSS SECTION AT pT = 200 MeV ........ 151 cut 11.2 FORWARD GAP CROSS SECTION AT pT > 200 MeV ......... 154 11.3 CLUSTER HADRONISATION MODELS ................. 155 11.4 BEST FIT FOR POMERON INTERCEPT ................ 156 11.5 ξX DEPENDENCE ON INELASTIC CROSS SECTION .......... 161 12 CONCLUSION 167 A DETECTOR CONTROL SYSTEMS 177 A.1 LAYOUT

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