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

86 CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factorjet is computed as the width of the Gaussian function fitted on the distribution of thedifference between the value of the X parameter of the reconstructed jet and the valueof the X parameter of the matched parton to that jet. The resolutions on the variouskinematics of the jets, namely transverse energy E T , polar angle θ and azimuthal angleφ as a function of the transverse momentum of the jets are shown in Figure 5.10 andFigure 5.11, for the light quark and the b quark jets, respectively. In each figure, theleft plots represent the typical jets reconstructed in the barrel while the right plots areobtained for typical jets reconstructed in a narrow region in the endcaps. The obtainedresolution plots are fitted with functions which are specified on the plot. For example,the distribution of the resolution on the transverse energy of the jets is fitted with afunction which is parametrized as a+b √ p T +cp T . The free parameters of the functionare determined from the fit and quoted on each plot.From Figure 5.10, it can be seen that the absolute resolution on the transverseenergy for jets with higher transverse momenta is worse, while the relative resolutionis worse for the jets with lower transverse momenta, as discussed in Section 3.2.4. Alsoit can be deduced that the resolution on the angular distributions is better determinedcompared to the resolution on the transverse energy. This is because of the matchingrequirement applied between the reconstructed jets and the partons in (η × φ) space.The resolutions on the spatial coordinates of the reconstructed jets are decreased forjets with higher transverse momenta, because the jets with higher transverse momentaare less affected by the magnetic field. The polar angle of reconstructed jets can bedetermined with higher precision compared to the azimuthal angle. This is due tothe bending of charged particle in the direction of φ. Comparing with the plots inFigure 5.11, it can be understood that the resolution on the transverse energy of bquark jets is worse compared to the resolution on the transverse energy of light quarkjets. This is due to the presence of muons or neutrinos in the decay products of bquark jets which yields to cluster less energy for b quark jets as muons and neutrinosare weakly interacting particles. There is no big difference observed when comparingthe resolutions on the spatial kinematics of b quark and non-b quark jets.5.3 Extra Event Selection CutsSo far, events have been required to pass the basic event selection cuts that are summarizedin Table 4.6. The selected events are subsequently fed to the MVA package. Theoutput of the MVA method is a matching between the reconstructed jets in the finalstate of the e+jets t¯t event and the partons appearing in the hadronic branch of the t¯tsystem t → Wb → q¯qb, as described in Section 5.1. The chosen jet-parton combinationreturned by the MVA method, can be either right or wrong. As already mentioned, theMVA method which has been trained using the variables listed in Section 5.1.4, is notable to choose the correct jet-parton matching in about 50% of the cases. This is partlydue to the ISR/FSR effects which makes it possible to get the radiation jets amongthe four leading jets and then at least one of the four leading jets can not be matchedto the hard-scatter partons. The e+jets t¯t events, for which a wrong jet combinationis recognized by the MVA or the event itself is badly reconstructed, contribute to the