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Search for the Standard Model Higgs Boson in P-Pbar Interactions with the Decay Mode H -> W+ W- -> Mu+ Nu Mu- Nu at the D0 Experiment

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Search for the Standard Model Higgs Boson in P-Pbar Interactions with the Decay Mode H -> W+ W- -> Mu+ Nu Mu- Nu at the D0 Experiment University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Theses, Dissertations, and Student Research: Department of Physics and Astronomy Physics and Astronomy, Department of 4-2010 Search for the Standard Model Higgs Boson in p-pbar Interactions with the decay mode H -> W+ W- -> mu+ nu mu- nu at the D0 Experiment Dale M. Johnston University of Nebraska at Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicsdiss Part of the Elementary Particles and Fields and String Theory Commons Johnston, Dale M., "Search for the Standard Model Higgs Boson in p-pbar Interactions with the decay mode H -> W+ W- -> mu+ nu mu- nu at the D0 Experiment" (2010). Theses, Dissertations, and Student Research: Department of Physics and Astronomy. 9. https://digitalcommons.unl.edu/physicsdiss/9 This Article is brought to you for free and open access by the Physics and Astronomy, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Theses, Dissertations, and Student Research: Department of Physics and Astronomy by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. SEARCH FOR THE STANDARD MODEL HIGGS BOSON IN pp¯ INTERACTIONS WITH THE DECAY MODE + + H W W − µ νµ−ν AT THE DØ EXPERIMENT → → by Dale Morgan Johnston A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy Major: Physics and Astronomy Under the Supervision of Professor Aaron Dominguez Lincoln, Nebraska April, 2010 SEARCH FOR THE STANDARD MODEL HIGGS BOSON IN pp¯ INTERACTIONS WITH THE DECAY MODE + + H W W − µ νµ−ν AT THE DØ EXPERIMENT → → Dale Morgan Johnston, Ph.D. University of Nebraska, 2010 Advisor: Aaron Dominguez A search for the standard model Higgs boson in pp¯ collisions resulting in two muons and large missing transverse energy is presented. The analysis uses 4.2 1 fb− of integrated luminosity at a center-of-mass energy of √s = 1.96 TeV col- lected between April 2002 and December 2008 with the D0 detector at the Fer- milab Tevatron collider. No significant excess above the background estimation is observed and limits are derived on Higgs boson production. For Leah. ACKNOWLEDGEMENTS There are many people I would like to thank for their support and guidance in this massive undertaking. First and most especially Leah, without your love, patience, understanding, and support throughout the years I never would have made it this far. Special thanks also to my parents, Dean and Alice, for your love and support and also for the good foundation on which to build my life. I have always had the freedom to explore, grow, and learn whatever and wherever I wanted and that means everything to me. Also, I can never express how much I appreciate the endless support of my whole family and the Varney family, who all believed in me and stood by me through the good times and the hardest ones. At UNL, thanks to Aaron Dominguez and Ken Bloom. Your help, guidance, and financial support has been instrumental in my making it through the entire research and thesis writing process. Pushing me to move out to Fermilab definitely proved to be the right move and I owe a lot to that push. Thanks for believing in me and sticking with me until I found the right environment in which to do my research. I would also like to thank Dan Claes and Greg Snow for helping to draw me in to high energy physics in the first place and founding the UNLHEP group that I call home. Also thanks to Kayle DeVaughan, my comrade in arms, for his help and friendship throughout the years. Some of the times would likely have been unbearable without it. Thanks to Shawn Langan, another close friend who has stuck with me through the years and given me his ear for a ridiculous number of complaining sessions. Finally, a special thanks to Ed Schmidt, who gave me a start at UNL and an opportunity to do research when I was just a fledgling graduate student. At D0, I owe a large debt of gratitude to Bj¨orn Penning, who helped me shape this analysis channel into what it is and who stood by me through countless all- nighters when many questioned whether success was possible. Joe Haley was also a good friend through it all and helped me keep my spirits up when times were tough. Also thanks to Marco Verzocchi and Wade Fisher who made me learn what it means to do good research and gave me many hours of their time and many (MANY) words of constructive criticism. Without that motivation I would still be spinning my wheels. Thanks especially to Mike Eads for helping me in the trenches and taking some of the pressure off when I needed it most. Thanks also to Andy Haas and Thomas Gadfort, who got me started in this analysis channel and gave me help in the beginning. Finally, there are many wonderful scientists who contributed to my understanding of physics and quality of work at D0, as well as provided me motivation to keep on top of things as best I could. I want to thank them all, especially Stefan Soldner-Rembold, Aurelio Juste, Gavin Davies, and Don Lincoln. Thanks to the University of Nebraska, Lincoln and the entire D0 Collaboration for their extensive support and for providing wonderful environments to work. To those who I didn’t mention, I greatly appreciate all of the support from everyone over the years. I couldn’t have done it without each of you. Cheers! An updated version of the work described in this dissertation, in concert with other analysis channels, has been published in the following journals: arXiv:1001.4481v3 [hep-ex] FERMILAB-PUB-10-015-E Phys.Rev.Lett.:104,061804(2010) c Copyright by Dale Johnston, 2010. All rights reserved. Contents List of Tables x List of Figures xii 1 Introduction 2 2 The Standard Model Higgs Boson 4 2.1 TheStandardModel ......................... 4 2.2 TheHiggsMechanism ........................ 13 2.3 Restrictions on the Higgs Boson Mass . 16 2.3.1 Theoretical Constraints . 16 2.3.2 Previous Higgs Boson Searches . 22 2.4 Higgs Boson Production and Decay at the Tevatron . 27 2.4.1 HiggsProduction ....................... 27 2.4.2 HiggsDecay.......................... 28 3 Apparatus 32 3.1 TheTevatron ............................. 33 3.2 D0Detector.............................. 36 3.2.1 D0CoordinateSystem . 37 3.2.2 Central Tracking System . 38 3.2.3 Preshower Detectors . 42 3.2.4 Calorimeter .......................... 42 vii viii 3.2.5 MuonSystem ......................... 45 3.2.6 LuminosityMonitor . 47 3.3 Trigger................................. 48 4 Event Reconstruction 51 4.1 Tracks ................................. 51 4.1.1 Track Reconstruction . 52 4.2 PrimaryVertex ............................ 53 4.2.1 Primary Vertex Reconstruction . 54 4.3 Muons................................. 55 4.3.1 MuonReconstruction. 56 4.3.2 Muon Classification . 56 4.4 Jets .................................. 58 4.4.1 Jet Reconstruction . 58 4.4.2 JetEnergyScale ....................... 59 4.5 MissingEnergy ............................ 61 4.5.1E~ /T Reconstruction ...................... 62 4.6 DataProcessing............................ 63 4.6.1 Trigger Requirements . 63 4.6.2 Luminosity and Data Quality . 63 5 Samples 65 5.1 Data.................................. 65 5.2 SimulatedSamples .......................... 66 5.2.1 Normalization......................... 69 6 Event Selection 77 6.1 Preselection .............................. 78 6.2 MonteCarloCorrections . 79 ix 6.2.1 Reweightings . 79 6.2.2 DATA/MC Normalization . 88 6.2.3 Estimation of Multijet Background . 90 6.2.4 Control Plots at Preselection . 90 6.3 FinalSelection ............................ 95 6.3.1 ControlPlotsatCut1 . 98 6.3.2 ControlPlotsatCut2 . 100 6.3.3 ControlPlotsatCut3 . 103 6.3.4 SelectionCutflow . 106 7 Multivariate Discriminant 109 7.1 NeuralNetworks ........................... 109 + 7.2 H W W − NeuralNetwork .................... 113 → 7.2.1 Training ............................ 113 7.2.2 InputVariables . 114 7.2.3 NNOutput .......................... 116 8 Systematic Uncertainties 117 8.1 FlatSystematics ........................... 118 8.2 ShapeDependentSystematics . 120 9 Limits and Conclusions 126 9.1 MassLimits.............................. 126 9.1.1 Limits Without Systematics . 127 9.1.2 Limits Including Systematic Uncertainties . 130 9.2 Conclusion............................... 133 9.2.1 Improvements ......................... 137 9.2.2 Outlook ............................ 138 x A Limit Setting 142 A.1 CLs Method.............................. 142 A.2 Propagation of Uncertainties . 145 B Reweighting 147 B.1 Deriving Reweighting Histograms . 147 B.2 Applying Reweightings . 148 Bibliography 150 List of Tables 5.1 σ BR of generated events for background processes. The mass × ranges listed give the generated invariant mass range of the Z/γ∗ inthesample. ............................ 72 5.2 σ BR of generated events for background processes. The mass × ranges listed give the generated invariant mass range of the Z/γ∗ inthesample. ............................ 73 5.3 σ BR of generated events for background processes. 73 × 5.4 σ BR of generated events for background processes. 74 × 5.5 σ BR of generated events for signal processes. 75 × 5.6 σ BR of generated events for associated production signal pro- × cesses. ................................. 76 6.1 Summary of the event selection criteria. 78 6.2 Number of expected background events and observed events . 107 6.3 Expected number of signal events remaining at Cut3 for all Higgs masses from 115 GeV to 200 GeV. The number listed is the com- bination of all Higgs signals considered (gluon fusion, vector boson fusion,andassociatedproduction). 108 7.1 Input variables for the Neural Network. 115 7.2 Relative importance of the input variables for the Neural Network. 115 8.1 Flat systematic uncertainties in percent. 120 xi xii 9.1 Expected and observed upper limits at 95% C.L. on the cross sec- tion times branching ratio for σ BR(H WW ∗)/SM for the × → µµ final states with respect to the standard model expectation.
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