Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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12 CHAPTER 1: The Standard Model of Particle Physics80.5July 2011LEP2 and TevatronLEP1 and SLD68% CLm W[GeV]80.480.3 m H[GeV] ∆α114 300 1000155 175 195m t[GeV]Figure 1.3: The direct (dotted ellipse) and indirect (solid ellipse) measurements of m Wand m t determinde from the data collected at the Tevatron and LEP colliders. Thesolid lines represent the various configurations of the Higgs boson mass.the top quark can be found in Figure 1.4. The indirect measurements from precisiondata are also shown in the same plot. Besides the good agreement between the directand indirect measurements of the mass of the top quark, it can be also seen that themass of the top quark is measured with an uncertainty of 0.5% on the mean valuewhich represents a precise measurement of the mass of the top quark. Even with moredata available from the LHC, it is possible to improve the accuracy on the quoted valuefor the mass of the top quark.In addition to the mass, the top quark has other quantum numbers which shouldbe measured directly from the data. The electric charge, for example, is one of theimportant quantum numbers that must be checked. Since the center of mass energyprovided by the LEP collider was around 200 GeV, much less than the energy neededto produce a pair of top quarks via the interaction e + e − → γ ∗ → t¯t, it was not possibleto use the collected data from the electron-positron collider to measure the electriccharge associated to the top quark. At a proton-proton collider it is still possible tomeasure the electric charge. Because the electric charge measurement requires the topquark to interact with a photon, then it was suggested to search for the interactions inwhich a top is produced together with a radiative photon [24]. According to the studiesperformed based on the data accumulated from the Tevatron, it has been shown thepossibility that the top quark is an exotic charge Q t = − 4 quark can be ruled out at3about 95% confidence level [25]. Hence the top quark which is produced at hadroncolliders and decays to a W boson and a b quark t → Wb, cannot only have Q t = − 4,3but also it is required to have Q t = + 2.3
CHAPTER 1: The Standard Model of Particle Physics 13Top-Quark Mass [GeV]CDF 172.5 ± 1.00D∅ 174.9 ± 1.4Average 173.2 ± 0.90χ 2 /DoF: 6.1 / 10LEP1/SLD 172.6 + 13.5 172.6 − 10.4LEP1/SLD/m W/Γ W179.7 + 11.7 179.7 − 8.7160 170 180 190m t[GeV]July 2011Figure 1.4: The latest results of the direct measurement on the mass of the top quarkprovided by both experiments at the Tevatron collider together with world average ofthe measured value of m t . The indirect measurement from the Standard Model fit tothe electroweak data is also quoted.1.2.4 Top Production and Decay at Hadron CollidersAt hadron colliders, top quarks are produced predominantly via the strong interaction.Depending on the center of mass energy, either gluon-gluon fusion or quark-antiquarkannihilation is the main channel which is responsible for top quark pair production.Due to its huge mass, the top quark has a very short life time, shorter than the timescale of the hadronization process. As a result, the top quark decays before gettinghadronized. According to the Cabibbo-Kobayashi-Maskawa (CKM) matrix [26, 27]which accounts for the possibility of quark mixing states, the probability of a topquark decaying into its counterpart is close to 100% [28]. Hence top quarks alwaysdecay to a W boson and a b quark, t → Wb. The W boson can subsequently decayto quarks or leptons, which mark the various decay channels of a t¯t system. This isexplained in more detail in Section 4.2.1.1.3 Problems of the Standard ModelThe Standard Model of particle physics is a self-consistent gauge field theory which isin good agreement with the observed experimental data. However, as there are so manyfree parameters within the theory, whose values cannot in principle be determined, thismakes it hard to state that the Standard Model is the most fundamental theory whichdiscribes nature. In other words, in order to understand why the masses of the quarks,leptons and the W and Z gauge bosons have their observed values, there might be a
Page 1: Vrije Universiteit BrusselFaculteit
Page 5 and 6: IntroductionThe Standard Model of p
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Chapter 5Estimating of the Jet Ener
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2χ / ndf18.76 / 7p0 9.492e-08 ±9.
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Chapter 6ConclusionThe Standard Mod
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CHAPTER 6: Conclusion 129The method
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Bibliography[1] M. E. Peskin and D.
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BIBLIOGRAPHY 133[40] The ALICE Coll
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BIBLIOGRAPHY 135[76] The CMS Collab
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SummaryJets appear in the final sta
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Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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