CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

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the Tevatron). Figure 2.4 shows the Feynman diagram for these processes. (a) (b) (c) (d) Figure 2.4: Feynman diagrams for top production processes at lowest level: (a), (b), and (c) gluon-gluon scattering diagrams, (d) quark-antiquark diagram. The t¯t production cross section depends on the exact value of the top quark mass, mt, and the collision energy. Figure 2.5 summarizes the results of the t¯t cross-section measurement by both ATLAS and CMS experiments, in different decay channels. These were done with 2011 collision data, at √ s = 7 TeV. The combinations of the different channels yields σt¯t = 177 ± 3(stat.) +8 −7 (syst.) ± 7(lumi.) pb for ATLAS [21] and σ t¯t = 166 ± 2(stat.) ± 11(syst.) ± 8(lumi.) pb for CMS [22]. Measurements for the <strong>2012</strong> collision energy, √ s = 8 TeV, are on-going. Additionally, top quarks are produced via single-top quark production mech- anisms. These, however have smaller production rates compared to the top pairs. Three subprocesses contribute to single-top quark pair production: the exchange of a virtual W-boson in the t-channel, or in the s-channel, and the associated production of a top quark and an on-shell W-boson. The t-channel mode is the process with highest cross section at the Tevatron and at the LHC. The top quarks decay almost exclusively to t → Wb since the CKM matrix element Vtb is close to unity. The Feynman diagram for this decay is depicted in Figure 2.6(a). This means that the final state topo- logy of processes involving top quarks is determined by the decay mode of the W-boson. As mentioned in Section 2.2.3 the hadronic final states have larger branching fractions, but in an environment with copious multijet production, such as the LHC, the leptonic modes provide cleaner signatures. In addition to the SM favored decay, there are other decay channels, predicted to be smaller by several orders of magnitude in the SM. The second most likely decays are the CKM non-diagonal decays t → Ws and t → Wd. The branching ratios (BR) are in the order of BR(t → Ws) ∼ 1.6 × 10 −3 and BR(t → Wd) ∼ 15
ATLAS Preliminary Data 2011 Channel & Lumi. Single lepton Dilepton All hadronic 1 1.02 fb 1 New measurements 50 100 150 200 [pb] 250 300 350 σ tt 15 May <strong>2012</strong> Theory (approx. NNLO) for mt = 172.5 GeV stat. uncertainty total uncertainty σ ± (stat) ± (syst) ± (lumi) tt 0.70 fb 179 ± 4 ± 9 ± 7 pb 1 + 14 + 8 11 7 0.70 fb 173 ± 6 pb 167 ± 18 ± 78 ± + 8 Combination 177 ± 3 ± 7 pb τhad + jets τhad + lepton All hadronic 1 4.7 fb 1 7 6 pb 1.67 fb 200 ± 19 ± 42 ± 7 pb 1 2.05 fb 186 ± 13 ± 20 ± 7 pb + 60 168 ± 12 57 ± 6 pb (a) (b) Figure 2.5: Summary of measurements of the t¯t cross section by the (a)ATLAS and the (b) CMS experiment. In both cases, 2011 pp collision data at a center of mass energy of √ s = 7 TeV were used. The combination of the different channels is σ t¯t = 177 ± 3(stat.) +8 −7 (syst.) ± 7(lumi.) pb for ATLAS [21] and σ t¯t = 166 ± 2(stat.) ± 11(syst.) ± 8(lumi.) pb for CMS [22]. (a) SM decay, t→Wb. (b) FCNC decay, t→Zq. (c) FCNC decay, t→γq. (d) FCNC decay, t→gq. Figure 2.6: Feynman diagrams of Top quark decays. The FCNC deays in the SM are not present at tree level, thus the diagrams presented here are just for illustration. 1 × 10 −4 , respectively. In what follows, for a generic decay channel X, the BR is defined as: BR(t → X) = Γ(t → X) . (2.33) Γ(t → Wb)SM Other rare decays include those that occur via flavor changing neutral currents. The last three diagrams in Figure 2.6 show such decays: t → Zq, t → γq and t → gq. In the SM they occur at one loop level, and are highly suppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism [23], as explained below. 16
Page 1 and 2: CERN-THESIS-2012-153 26/07/2012 c
Page 3 and 4: Abstract A search for flavor-changi
Page 5 and 6: Acknowledgments I first want to tha
Page 7 and 8: Table of Contents List of Tables .
Page 9 and 10: List of Tables 2.1 The fundamental
Page 11 and 12: List of Figures 2.1 The potential V
Page 13 and 14: 5.15 Distributions of the 3ID analy
Page 15 and 16: This thesis is organized as follows
Page 17 and 18: the question whether each neutrino
Page 19 and 20: gauge invariant field strength tens
Page 21 and 22: for strong interactions. Although W
Page 23 and 24: particles. Because of the introduct
Page 25 and 26: which is introduced for mass genera
Page 27: 2.3 Top Quark The large mass of the
Page 31 and 32: as the masses of the quarks involve
Page 33 and 34: LEP HERA Tevatron LHC BR(t → qγ)
Page 35 and 36: to Wtb couplings, where limits on a
Page 37 and 38: tion at the collision point, and F
Page 39 and 40: The positive x-direction is defined
Page 41 and 42: large air-core toroids (one barrel
Page 43 and 44: 3.3.3 Calorimetry The calorimetry i
Page 45 and 46: of ∼ 1% for the H → γγ and H
Page 47 and 48: esolution of 1 cm×1 ns with digita
Page 49 and 50: Luminosity measurement Accurate det
Page 51 and 52: least one hit on the A-side, at lea
Page 53 and 54: caps. The calorimeter selections ar
Page 55 and 56: ut instead fragment to color neutra
Page 57 and 58: Decays A large fraction of the part
Page 59 and 60: The ALGPEN program with HERWIG show
Page 61 and 62: Normalized to unit / 1.96429 GeV No
Page 63 and 64: Chapter 5 Event Selection 5.1 Intro
Page 65 and 66: Variable Cut pT >20 GeV Type MuID t
Page 67 and 68: Identification efficiency eff id 1
Page 69 and 70: (a) (b) Figure 5.4: Electron identi
Page 71 and 72: ε (P ) µ TL T 1 TL µ ε TL µ ε
Page 73 and 74: Jets in this analysis are reconstru
Page 75 and 76: impact parameter significance of we
Page 77 and 78: event is also discarded. Events wit
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event kinematics, however no b-jet
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Events / 10 GeV trackleptons / 10
Page 83 and 84:
analysis, is 22%. Figure 5.13(a) sh
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transverse momentum of the leading
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Events / 10 GeV Events / 10 GeV 14
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two background sources, diboson and
Page 91 and 92:
ID leptons provides three different
Page 93 and 94:
3ID Final Selection ZZ and WZ 9.5
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data, has systematic uncertainties
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Jets The jet energy scale (JES) and
Page 99 and 100:
the fake prediction was scaled down
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two or more, and events with E miss
Page 103 and 104:
Source MC Background Luminosity ±
Page 105 and 106:
Chapter 8 Limit Evaluation 8.1 Intr
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means: Q = L( X,s + b) L( , (8.5)
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computed from Equation 8.12 replaci
Page 111 and 112:
Chapter 9 Conclusions A search for
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There are a total of 666236 γ+jets
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Appendix B Backgrounds to the 3ID a
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Appendix C Systematic Uncertainties
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References [1] J. L. Borges, Tres v
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[40] G. Lu, F. Yin, X. Wang and L.
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[82] ATLAS Collaboration, Expected
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