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

68 CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factorthe mass constraints of both the W boson and the top quark on t h → W h b → q¯qb h andrequiring the reconstructed jets to fulfill the constraints, while allowing the measuredenergies to change within their resolutions, would result in obtaining jets with modifiedenergy. The method of applying mass constraints, which makes use of the LagrangeMultipliers, is called a Kinematic Fit and is introduced in Section 5.2.In addition to the baseline event selection cuts described in the previous chapter, someextra cuts are applied on the selected events which are introduced in Section 5.3. Themethod to estimate the residual jet energy calibration factors is described in Section 5.4and the statistical properties of these estimators are discussed in Section 5.5. Finally,the method is applied on the proton-proton collision data which has been collected bythe CMS experiment during the physics runs in 2010. The plots which compare thesimulation versus data are shown in Section 5.6.5.1 Multi-Variate Analysis Method (MVA)In the final state of the e+jets t¯t decay, t¯t → e¯ν e q¯qb¯b, there are four quarks producedwhich should result in at least four reconstructed jets in the detector. If no tagging toolsare used to distinguish between light jets, originating from the quarks which appear inthe decay products of the hadronic W boson, and b jets, originating from the b quarkswhich appear in the decay products of both the hadronic and the leptonic top quarks,one can not differentiate between them. Therefore in order to label the jets and assignthem to the four quarks, there are 4! = 24 different ways to do the assignment giventhat only four reconstructed jets are taken into account. Since the order of the jetswhich are assigned to the light quarks is not important when reconstructing the Wboson momentum, they can be used interchangably. Hence the number of differentways to assign four reconstructed jets to the four quarks is divided by 2 and reduces to12. The quarks which are produced in the decay products of e+jets t¯t event, initiatefrom heavy particles such as W boson and top quark. Hence the four leading jets thatare the jets with the maximum p T among the reconstructed jet collection, could begood candidates to represent the four quarks and thus are used to do the assignment.It should be noted that there are usually more than four jets reconstructed in the eventdue to radiations, hence the four leading jets are not always the correct jets as willbe discussed in Section 5.1.1. Another important note to be emphasized is that theelectron fakes a jet but is removed from the list of reconstructed jets.To summarize the above discussion, there exist 12 different combinations in order toassociate four leading jets to the four quarks arrising from the e+jets t¯t decay. Among12 possible combinations, only one combination represents the correct assignment.Since the determination of the correct combination of jets is essential for the rest ofanalysis, specific efforts should be made in order to select the correct combination.In order to select the correct jet-parton combination among 12 combinations, when nosimulation information is available, it is possible to take randomly one combination.The probability of selecting the correct jet combination randomly is 1 ∼ 8% as only12one among twelve comibnations is the right candidate in each trial. The probability ofselecting the right jet combination is further improved by considering the kinematicsof some variables that can differentiate between the right and wrong jet combinations.