A Search for Displaced Leptons in the ATLAS Detector

This thesis presents a search for long-lived particles decaying into displaced electrons and/or muons with large impact parameters. This signature provides unique sensitivity to the production of theoretical lepton-partners, sleptons. These particles are a feature of supersymmetric theories, which seek to address unanswered questions in nature. The signature searched for in this thesis is difficult to identify, and in fact, this is the first time it has been probed at the Large Hadron Collider (LHC). It covers a long-standing gap in coverage of possible new physics signatures. This thesis describes the special reconstruction and identification algorithms used to select leptons with large impact parameters and the details of the background estimation. The results are consistent with background, so limits on slepton masses and lifetimes in this model are calculated at 95% CL, drastically improving on the previous best limits from the Large Electron Positron Collider (LEP).

Auteur

Dr. Lesya Horyn is a particle physics researcher. She grew up in New York, where her parents fostered a love of science and learning.. She began doing physics research at the American Museum of Natural History in New York City. She then received a Bachelor of Science from Tufts University, where she began working on the ATLAS experiment with collaborators both at Tufts and as a Research Experience for Undergraduates student at Duke University. She then completed a Ph. D at University of Chicago, where she performed the work presented in this thesis under the supervision of Professor Young-Kee Kim and Dr. Tova Holmes. She currently works on the CMS experiment with Fermi National Accelerator Laboratory and resides in Geneva, Switzerland. In her spare time, she likes reading, cycling, and traveling near and far.

Contenu

Table of ContentsLIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiiLIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxLIST OF ACRONYMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxviiACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxviiABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxixI INTRODUCTION AND MOTIVATION1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 THEORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.1 Leptons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.2 Quarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.3 Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Electroweak Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Strong Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.4 Higgs Boson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.5 Conserved Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.6 Shortcomings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2 Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.1 Minimal Supersymmetric Standard Model . . . . . . . . . . . . . . . 12Relationship to Standard Model Problems . . . . . . . . . . . . . . . 142.2.2 Gauge Mediated Supersymmetry Breaking . . . . . . . . . . . . . . . 142.2.3 Other Theoretical Models Producing Displaced Leptons . . . . . . . . 18II THE EXPERIMENT3 THE LARGE HADRON COLLIDER . . . . . . . . . . . . . . . . . . . . . . . . . 213.1 A Circular Proton-Proton Collider . . . . . . . . . . . . . . . . . . . . . . . . 213.2 Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.3 Injection Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.4 Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.4.1 Pileup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 THE ATLAS DETECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.1 Coordinate System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.2 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.2.1 The Pixel Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.2.2 The Silicon Microstrip Tracker . . . . . . . . . . . . . . . . . . . . . . 35iv4.2.3 The Transition Radiation Tracker . . . . . . . . . . . . . . . . . . . . 364.2.4 Solenoid Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.3 Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.3.1 Electromagnetic Calorimeter . . . . . . . . . . . . . . . . . . . . . . . 374.3.2 Hadronic Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.3.3 Forward Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.4 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.4.1 Monitored Drift Tubes and Cathode-Strip Chambers . . . . . . . . . 42MDT Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.4.2 Resistive Plate Chambers and Thin Gap Chambers . . . . . . . . . . 444.4.3 Toroid Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.5 Particles in ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.6 Trigger and Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . 464.6.1 Trigger architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.6.2 Triggers Used For This Analysis . . . . . . . . . . . . . . . . . . . . . 505 OBJECT RECONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.1 Track Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.1.1 Tracking Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Kalman Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Hough Transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545.2 Standard Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555.2.1 Large Radius Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . 565.2.2 Primary Vertex Reconstruction . . . . . . . . . . . . . . . . . . . . . 565.3 Muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585.3.1 Standard Reconstruction and Identication . . . . . . . . . . . . . . . 58MS Track Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 58Combined Muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Muon Identication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595.3.2 Modications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605.4 Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615.4.1 Standard Reconstruction and Identication . . . . . . . . . . . . . . . 61Cluster Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 61Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Combined Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 63Identication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 645.4.2 Modications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.5 Overlap Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.6 Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666 DATA AND MONTE CARLO SAMPLES . . . . . . . . . . . . . . . . . . . . . . 686.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686.2 Monte Carlo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69III…