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

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10 CHAPTER 1: The Standard Model of Particle Physicstheoretically flavour changing neutral current phenomenon (FCNC) which is not experimentallyobserved. Based on the GIM mechanism [12] which was originally proposedfor two quark generations, the same argument which forbids flavour changingtransitions among down-type quarks, applies for three quark doublets.Also there is lots of evidence for the existence of the top quark. There have been somedetailed studies of the Zb¯b vertex using the data collected at the e − e + colliders LEPand SLC, resulting in the measurement of the isospin of the b quark [13]. It has beenmeasured [14] that the b quark has a left-handed third component of the isospin of − 1 2which implies that the b quark must have a weak isospin partner. In other words, thetop quark must exists as a counterpart of the isospin doublet of the b quark.1.2.2 Indirect Constraints on the Mass of the Top QuarkAll electroweak quantities of the Standard Model depend, at leading order, on threeparameters, namely two gauge coupling constants g Y , g W and vacuum expectation valueof the Higgs boson field v. At higher order of corrections, two extra parameters, beingthe top quark mass m t and the mass of the Higgs boson m H , contribute to the radiativeloop corrections and therefore can affect the values of the electroweak observables.Hence, the precision electroweak measurements can provide indirect constraints on themass of both the top quark and the Higgs boson.While the dependency of the electroweak observables such as the mass of the Z bosonon the mass of the top quark is quadratic, the mass of the Higgs boson appears into theradiative corrections through logarithmic terms. Therefore, the inferred constraints onm t are stronger than those on m H and that is why there has been a good prediction onthe mass of the top quark before its discovery. A global fit of the Standard Model tothe precision data successfully predicts a value for the mass of the top quark as [15, 16]m t = 179.4 12.1−9.2 GeV 3 ,which is in good agreement with the measured value of the top quark [17]m t = 173.2 ± 0.9 GeV.The successful prediction of the mass of the top quark provided by the precision electroweakdata indicates the predictive power of the radiative corrections which persuadesone to obtain constraints on the other parameters of the model, such as Higgs bosonmass. As already mentioned, because of the logarithmic dependence of the radiativecorrections on the mass of the Higgs boson, it is not possible to derive stringent predictionson the Higgs boson mass. In the following, the results of the Standard Model fitto the precision electroweak data in order to extract constraints on m H are discussed.Using the measured value of the top quark mass, the Standard Model fit to the electroweakprecision data is performed in order to infer the mass of the Higgs boson [18].The result of the fit, expressed in terms of ∆χ 2 , is shown in Figure 1.2. The most likelyvalue of the mass of the Standard Model Higgs boson is determined from the minimum3 Throughout the thesis, the natural units are used.
CHAPTER 1: The Standard Model of Particle Physics 11of the ∆χ 2 curve to be m H = 91 +45−32 GeV , which means the data prefers a light Higgsboson. The direct searches for the Higgs boson at LEP have exculded a Higgs bosonwith a mass below 114.4 GeV [19], therefore the preferred value is slightly above theexclusion limit.Figure 1.2: The indirect determination of the mass of the Higgs boson obtained fromthe Standard Model fit to the electroweak data assuming the world average mass ofthe top quark [18]. Also the exclusion region on m H obtained from direct searches forthe Higgs boson at LEP is also shown.Figure 1.3 contains the information of the direct as well as the indirect measurementsof both the mass of the top quark and the W boson. Also the lines indicatingthe various Higgs boson masses are overlaid. The upper limit of 1000 GeV on the massof the Higgs boson comes from the theoretical calculation [20]. The direct searches forthe Standard Model Higgs boson at the Tevatron, result in an exclusion of the mass ofthe Higgs particle in a narrow band specified on the plot [21].As it can be seen from Figure 1.3, the direct and indirect measurements of m t and m Ware in good agreement, confirming that the Standard Model of particle physics is notobviously wrong. It should also be noted that the contours of the W and top quarkmasses are calculated at 68% confidence level. In order to make stronger limits on themeasured values, more data should be provided. Therefore, a more precise determinationof either the mass of the top quark or the mass of the W boson, would makeit possible to accept or reject the existence of the Higgs particle within the StandardModel.1.2.3 Direct Searches of the Top Quark and the Discovery EraAs mentioned before, the top quark was finally discovered by the CDF [22] and D0 [23]experiments at the Tevatron. The latest world average value measured for the mass of
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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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