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Search Strategies for New Physics at the Lhc A SEARCH STRATEGIES FOR NEW PHYSICS AT THE LHC A DISSERTATION SUBMITTED TO THE DEPARTMENT OF PHYSICS AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Daniele Spier Moreira Alves August 2011 © 2011 by Daniele Spier Moreira Alves. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/vr734zr1770 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Jay Wacker, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Renata Kallosh, Co-Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Risa Wechsler Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Preface The LHC is in the frontline of experimental searches for New Physics beyond the Stan- dard Model of Particle Physics. Its power is accompanied by no smaller challenges in analyzing and interpreting its results. In this thesis I explore ways to parame- terize new physics phenomena, design search strategies that are sensitive to them, and interpret experimental results in general new physics contexts. In particular, I discuss interpretations of the first ATLAS analysis for supersymmetry with 70/nb of integrated luminosity. I also carry a careful investigation of comprehensive search strategies for new physics with jets and missing energy signatures, and estimate the sensitivity bounds of the 7 TeV LHC to new colored particles decaying to jets and and a neutral particle that escapes detection. Finally, I discuss the implications of the recent LHC excesses hinting to a Higgs boson with mass in the range 142-147 GeV. If confirmed, this range for the Higgs mass will be an important evidence for Split Supersymmetry. I work out the phenomenological predictions of this scenario that will be tested in the very near future by a variety of experiments, including direct and indirect dark matter detection, EDM experiments searching for CP violation and the 7 TeV run of the LHC. iv Acknowledgements I would like to thank my parents for their efforts in providing me with good education and giving me the freedom to choose the path of basic science. I would like to thank Edson Fernando Ferrari for his mentorship and advice when I had to make important decisions. Finally, I would like to thank my advisor Jacob Wacker for his dedication and for his strong influence in my formation and development as a physicist. v Contents Preface iv Acknowledgements v 1 Introduction 1 2 Strong dynamics with multijets and no MET 5 2.1 Simplified Model Definition . .6 2.1.1 Matching the Simplified Model on to Theories . .7 2.2 Relevant Variables/Plots . .7 2.3 Existing Limits . .8 2.4 Possible Reach . .9 3 Multileptons from R-parity violation 10 3.1 Introduction . 10 3.2 Simplified Model Definition . 10 3.2.1 Matching the Simplified Model on to Theories . 11 3.3 Relevant Variables/Plots . 12 3.4 Existing Limits . 13 3.5 Possible Reach . 13 4 ATLAS results with 70 nb−1 of Luminosity 14 4.1 Introduction . 14 4.2 Model and Simulation . 16 vi 4.3 Results and Discussion . 23 5 Jets plus Missing Energy at the 7 TeV LHC 27 5.1 Simplified Models for Colored Octets . 30 5.1.1 Two-body direct decay . 31 5.1.2 Three-body direct decay . 32 5.1.3 One-step cascade decay . 34 5.1.4 Two-step cascade decay . 37 5.2 Backgrounds and Signal Simulation . 39 5.3 Optimal Sensitivities . 43 5.4 Comprehensive Search Strategies . 46 5.4.1 Multiregion Search Strategy in E= T & HT ............ 48 5.4.2 Alternative pT Selection Criteria . 52 5.5 Multiple Decay Modes . 53 5.6 Conclusion . 57 5.7 Appendix . 58 6 Recent Hints of Split Supersymmetry 63 6.1 Introduction . 63 6.2 Gluino Signatures . 66 6.2.1 Short-lived Gluinos . 69 6.2.2 Moderate-lived Gluinos . 71 6.2.3 Long-lived Gluinos . 72 6.3 Dark Matter in Split Supersymmetry . 73 6.3.1 The Higgs Resonance Region . 76 6.3.2 Relic Abundance and Higgsino Mass . 78 6.3.3 Unified Boundary Conditions . 80 6.3.4 Electroweakino Phenomenology . 81 6.3.5 Dark Matter Detection . 83 6.3.6 Electric Dipole Moments . 87 6.4 Discussion . 87 vii Bibliography 89 viii List of Tables 4.1 Searches in [9] used to set limits in this article. The 95% C.L. on the production cross section times efficiency of the cuts, σ(pp × ! g~gX~ ), follow from folding in the uncertainties in the luminosity and background. 19 5.1 Signal efficiencies for the multiple search region of Chapter 5.4.1 for benchmark masses mg~ = 700GeV; mχ0 = 80GeV andg ~ decay modes A = 2-body direct decay and B = 2-step cascade decay. Also included are the expected background cross section in the signal region σbkg . 55 × 5.2 Cross section sensitivity for the benchmark masses mg~ = 700GeV; mχ0 = 80GeV and the twog ~ decay modes A = 2-body direct decay and B = 2-step cascade decay, where, in order to maximize the number of hy- brid events, we take A = B = 50%. The highlighted orange search is B B most effective for the AA topology and the yellow is most sensitive for the BB and AB topologies. The actual sensitivity σactual was computed using (5.24) and the efficiencies displayed in Tab. 5.1. The conserva- tive estimate σcons in case the efficiency for hybrid events is unknown is obtained by taking the lower bound in (5.27) for AB. The last col- umn displays the efficacy of each search region under the conservative estimates σcons. The efficacy, , is defined in (5.18) and quantifies how E close the cross section limits are from the optimal one, σoptimal = 106 fb. 57 ix 5.3 Benchmark simplified models forg ~'s that decay directly to χ0 through a two-body decay. The optimal reach in the cross section for each one of the benchmark models, σopt, is displayed for two luminosity scenarios: 45 pb−1 and 1 fb−1. Also displayed is the reference NLO-QCD cross section forg ~ pair-production. 61 5.4 Benchmark simplified models forg ~'s that decay directly to χ0 through a three-body decay. 61 5.5 Benchmark simplified models forg ~'s that decay through a one-step 0 cascade to χ , with the mass of the intermediate particle mχ± chosen to be m ± = m 0 + 1=4 (mg~ m 0 ). 61 χ χ − χ 5.6 Benchmark simplified models forg ~'s that decay through a one-step 0 cascade to χ , with the mass of the intermediate particle mχ± chosen to be m ± = m 0 + 1=2 (mg~ m 0 ). 62 χ χ − χ 5.7 Benchmark simplified models forg ~'s that decay through a one-step 0 cascade to χ , with the mass of the intermediate particle mχ± chosen to be m ± = m 0 + 3=4 (mg~ m 0 ). 62 χ χ − χ 5.8 Benchmark simplified models forg ~'s that decay through a 2-step cas- cade to χ0, with the masses of the first and second intermediate par- ticles chosen to be m ± = m 0 + 1=2 (mg~ m 0 ) and m 00 = m 0 + χ χ − χ χ χ 1=2 (m ± m 0 ), respectively. 62 χ − χ x List of Figures 4.1 pT spectrum for the 1st, 2nd and 3rd hardest jets in the events (blue, red and green, respectively) when ISR matching is included (solid lines) vs when all radiation is accounted for by the parton shower (dashed lines). An integrated luminosity of = 70 nb−1 is assumed, as well as L masses mg~ = 100 GeV and mχ0 = 90 GeV. 17 4.2 The NLO QCD cross section for pp g~gX~ as a function of the gluino !−1 mass. A cross section of σ = 1=(70 nb ) corresponds to mg~ = 395 GeV −1 and σ = 10=(70 nb ) corresponds to mg~ = 265 GeV. 21 4.3 95% C.L contours of the maximum allowed production cross section σ(pp g~gX~ ) in the mg~ m 0 mass plane, forg ~ directly decaying to ! − χ χ0jj. The contour values are specified in the right color scale. The dark line corresponds to the exclusion boundary for models where the gluino is produced through QCD alone with an NLO cross section (i.e., with all squarks decoupled so that there are no t-channel squark exchange diagrams). The dashed-lines delimit the excluded parameter space of different models where σ(pp g~gX~ ) is given by a simple rescaling of ! the NLO-QCD cross section. The red line is the current estimate of Tevatron limits taken from [27, 28]. The blue line denotes a sample mSUGRA spectrum whereg ~ is the gluino and χ0 is the bino.
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