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

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Chapter 2 Top Quark Physics 2.1 Introduction In 1994 first hints of the top quark were reported by the CDF experiment [9]. Later in 1995 its existence was corroborated by both CDF and D0 experiments [10,11] at the Tevatron. Its discovery completed the three generation structure of the SM and signaled the start of the new field of top quark physics. Some of its properties, like its rather large mass, make it a very interesting object of study, not only for the under- standing of the SM, but also in the search of physics beyond the SM (BSM). This chapter briefly covers the theoretical background relevant for this analysis. In particular, Section 2.3 covers aspects of the top quark, like production and decay, that will be referred to in the following. Section 2.4 reviews FCNCs, not only within the SM, but also in some extensions of the SM. 2.2 The Standard Model The introduction of the SM [12–15] made one of the recurrent dreams of elementary particle physics possible: a fundamental synthesis between electromagnetism and weak interactions 1 . Strong interactions were later described by introducing quantum chromodynamics, and the existence of quarks confirmed in 1974 with the discovery of J/ψ. The following summary is taken from References [16–20]. At present, the SM of particles and their interactions account for practically all the experimental data from high energy physics experiments. Within this model, all particles are built from a number of fundamental, spin 1 2 , particles called fermions (quarks and leptons), summarized in Table 2.1. For each of these fermions, there exists an antiparticle with the same mass but opposite charge. Neutrinos ν have been re- cently observed to have non-zero mass, unlike previously assumed. Furthermore, because they are neutral, 1 Quoting A. Salam from Reference [13]. 3
the question whether each neutrino is its own antiparticle (Majorana particles) or not (Dirac particles) re- mains unanswered. The SM also comprises the interactions among those particles, which are described in terms of particles of spin 1, bosons, which work as mediators of forces between the fermion constituents. The different interaction mediators are summarized in Table 2.2. Strong interactions are responsible for binding the quarks. This interquark force is mediated by a massless particle, the gluon g, which itself carries the corresponding charge, “color”. Also massless, photons γ, mediate the electromagnetic interaction between charged particles. The quanta of weak interaction fields are the charged W-boson and the neutral Z-boson. These last two carry mass, making the weak interaction short-ranged. On the contrary, electromagnetic interaction has infinite range because of the massless mediator. Gluon fields despite being massless are confining, so the strong force is not observed as a long range force. This confinement of quarks means quarks can not exist as free particles. The quarks appear in three families or generations: (u,d), (c,s) and (t,b). Quarks carry color, and they have fractional electrical charge. The leptons also appear in three generations: (e,νe), (µ,νµ) and (τ,ντ). Unlike quarks, they are colorless. Charged leptons are paired with neutral leptons (neutrinos). Because neutrinos are neutral, they only interact weakly. Additionally there exist gravitational interactions which act between all types of particles. However, gravitational forces are, in the scale of particle physics, insignificant (unless there is a TeV scale gravity). In fact, the SM does not take into consideration the gravitational fields, which makes it not a theory of everything. particle flavor Q/|e| leptons quarks 1 st generation 2 nd 3 rd e µ τ −1 νe νµ ντ 0 u c t + 2 3 d s b − 1 3 Table 2.1: The fundamental fermions. interaction mediator spin/parity coupling strenght [α] strong gluon, g 1 − ∼ 1 electromagnetic photon, γ 1 − ∼ 10 −1 weak W ± , Z 1 − , 1 + ∼ 10 −7 Table 2.2: The boson mediators. Note that, the neutral Z-boson is sometimes expressed as Z 0 in different texts. 4
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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: This thesis is organized as follows
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 and 28: 2.3 Top Quark The large mass of the
Page 29 and 30: ATLAS Preliminary Data 2011 Channel
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
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Identification efficiency eff id 1
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(a) (b) Figure 5.4: Electron identi
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ε (P ) µ TL T 1 TL µ ε TL µ ε
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Jets in this analysis are reconstru
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impact parameter significance of we
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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
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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
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ID leptons provides three different
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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
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the fake prediction was scaled down
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two or more, and events with E miss
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Source MC Background Luminosity ±
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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
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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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