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Studies of supersymmetry models for the ATLAS experiment at the Large Hadron Collider Alan James Barr Churchill College A dissertation submitted to the University of Cambridge for the degree of Doctor of Philosophy November 2002 ii Studies of supersymmetry models for the ATLAS experiment at the Large Hadron Collider Alan James Barr Abstract This thesis demonstrates that supersymmetry can be discovered with the ATLAS experiment even if nature conspires to choose one of two rather difficult cases. In the first case where baryon-number is weakly violated, the lightest supersym- metric particle decays into three quarks. This leads to events with a very large multiplicity of jets which presents a difficult combinatorical problem at a hadronic collider. The distinctive property of the second class of model { anomaly-mediation { is the near degeneracy of the super-partners of the SU(2) weak bosons. The heavier charged wino decays producing its invisible neutral partner, the presence of which must be inferred from the apparent non-conservation of transverse momen- tum, as well as secondary particle(s) with low transverse momentum which must be extracted from a large background. Monte-Carlo simulations are employed to show that for the models examined not only can the distinctive signature of the model can be extracted, but that a variety of measurements (such as of sparticle masses) can also be made. The final two chapters present an investigation into part of the experimental hard- ware which will be vital for these analyses. Beam tests of ATLAS semiconductor tracker modules demonstrate that this sub-detector can be expected to perform to its specification, providing the good spatial resolution and efficiency with low noise, even after the equivalent of ten years of irradiation. iii 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. Alan Barr iv Acknowledgements I would like to thank the Particle Physics and Astronomy Research Council for providing financial support during this degree (under PPARC student grant number PPA/S/S/1999/02797). For various parts of the analyses, I have made use of the ATLAS physics analysis framework and tools which are the result of collaboration-wide efforts. I take this opportunity to thank collaborators and others who have given direc- tion and technical help during the work on this thesis; in particular Ben Allanach, Janet Carter, Lars Eklund, Jose Enrique Garcia, James Hetherington, John Hill, Christopher Lester, Gareth Moorhead, Peter Richardson, David Robinson, Mar- cel Vos, Bryan Webber, Steve Wotton and especially my supervisor, Andy Parker, have all made very valuable contributions. I owe a special thanks to Claire for her constructive criticism and artistic consultancy. This thesis is dedicated to my parents. Publications Much of the work in this thesis has already been published. The work on R-Partity violating supersymmetry signatures (chapter 4) was published in two JHEP pa- pers [1, 2]. The study of anomaly mediated supersymmetry breaking (chapter 3) is available as [3,4]. Much of the testbeam analysis, described in chapter 6 has been made available in [5{7], or will be published in future ATLAS notes. v Happy is the man that findeth wisdom, and the man that getteth understanding. For the merchandise of it is better than the merchandise of silver, and the gain thereof than fine gold. Proverbs 3:13-14 vi Contents I Introduction 1 1 Theory and motivation 3 1.1 The standard model . 4 1.1.1 Particles and interactions . 4 1.1.2 The description of mass . 6 1.1.3 The Higgs mass and the technical hierarchy problem . 7 1.2 Supersymmetry . 11 1.2.1 Overview . 11 1.2.2 Grand unified theories . 13 1.2.3 The minimal supersymmetric standard model . 15 1.2.4 R-parity . 19 1.2.5 Supersymmetry breaking . 22 1.2.6 An experimental perspective on theoretical models . 23 1.3 Summary . 25 2 ATLAS 27 2.1 The LHC . 27 2.2 Physics goals . 28 2.3 Detector geometry and terminology . 31 2.4 Inner detector . 33 2.4.1 Pixel detector . 37 2.4.2 Semiconductor tracker . 37 vii Contents 2.4.3 Transition radiation tracker . 38 2.5 Calorimetry . 39 2.5.1 Electromagnetic calorimetry . 41 2.5.2 Hadronic calorimetry . 41 2.6 Muon spectrometer . 43 2.6.1 Precision chambers . 45 2.6.2 Trigger chambers . 46 2.7 Trigger . 46 2.8 Comment . 49 II Simulations of supersymmetry models at ATLAS 51 3 Anomaly mediated supersymmetry 53 3.1 Anomaly mediated supersymmetry . 54 3.2 Benchmark points . 56 3.3 LHC reach for mAMSB . 60 3.3.1 Event simulation . 60 3.3.2 Optimisation of cuts . 63 3.3.3 Results . 66 3.4 Distinguishing wino-like LSPs . 70 3.4.1 Definition of points . 73 3.4.2 The underlying event . 74 3.4.3 Event and track selection . 78 3.4.4 Results . 83 3.5 Other constraints . 86 3.5.1 Cosmological relic density . 86 3.5.2 Muon g 2 ............................ 89 − 3.5.3 B Xsγ ............................. 91 ! 3.6 Conclusions . 92 viii Contents 4 Baryon-violating RPV SUSY 95 4.1 Introduction . 95 4.2 Analysis Strategy . 99 4.3 Standard Model Background . 101 0 0 4.4 Detection of theχ ~1 andχ ~2 . 102 ~ 4.5 Detection of the lR andq ~L . 109 4.6 Other values of λcds00 ............................114 4.7 Extracting the Flavour Structure . 115 4.7.1 Vertex Tagging . 117 4.7.2 Flavour Discrimination from Muons . 120 4.7.3 Statistical Significance . 121 4.8 Conclusion . 123 III Beam tests of ATLAS silicon modules 125 5 Silicon detectors and the SCT 127 5.1 Semiconductor Detectors . 127 5.1.1 Radiation Damage . 130 5.2 The ATLAS SCT . 132 5.2.1 Detectors . 132 5.2.2 Readout . 133 5.3 Performance . 134 6 SCT module beam tests 137 6.1 Introduction . 137 6.2 The beam test environment . 138 6.2.1 Devices Under Test . 139 6.2.2 Calibration . 141 6.3 Analysis Overview . 141 ix Contents 6.3.1 Track-finding and alignment . 142 6.3.2 DST production . 142 6.3.3 Efficiency algorithm . 143 6.4 Results . 146 6.4.1 Residuals . 146 6.4.2 Module Efficiency . 147 6.4.3 Charge collection as a function of bias . 149 6.4.4 Noise occupancy . 153 6.4.5 Signal to noise ratio . 154 6.4.6 Pulse-Shape Analysis . 155 6.4.7 Magnetic field and non-normal incident tracks . 162 6.4.8 Edge measurements . 162 6.4.9 Gap measurements . 167 6.5 Conclusions . 171 7 Conclusions 173 A Abbreviations 175 B The Higgs mechanism 177 C Simulation of particle tracks 181 C.1 Low pT electron identification . 181 C.2 Low pT muon identification . 182 D The variable mT 2 and its generalisation 185 D.1 The properties of mT 2 . 185 D.2 Generalisations of mT 2 . 190 D.3 Uncertainties in p=T and m 0 . 193 χ~1 x List of Figures 1.1 Masses of standard model particles . 8 1.2 Global electroweak fit vs. Higgs mass . 9 1.3 Higgs mass loop corrections . 9 1.4 Gauge coupling unification with and without N = 1 supersymmetry . 14 1.5 Massless N = 1 supermultiplets . 16 1.6 R-parity violating proton decay . 20 1.7 Example sparticle spectra . 22 2.1 Development of proton{(anti-)proton colliders . 28 2.2 The LHC above and below ground . 29 + + 2.3 Event display h ZZ µ µ−e e− . 34 ! ! 2.4 The ATLAS inner detector . 36 2.5 Diagram of the ATLAS calorimeters . 40 2.6 Jet energy resolution . 42 2.7 Magnetic field map in the transition region . 44 2.8 Side view of one quadrant of the muon spectrometer . 45 2.9 Proton{proton cross-sections . 47 3.1 Part of the SPS-9 sparticle spectrum . 56 3.2 Part of the Point d'Aix sparticle spectrum . 57 3.3 AMSB key sparticle masses . 59 + 3.4χ ~1 lifetime and branching ratios . 61 3.5 Background cross-sections . ..

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