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

100 CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factor#events250200Entries 3263Mean 78.95RMS 10.22#events180160140120Entries 3263Mean 169.7RMS 15.8150100100806050402000 20 40 60 80 100 120 140 160 180 200m W(GeV)00 50 100 150 200 250 300 350 400m top(GeV)Figure 5.20: The distributions of the reconstructed W boson mass (left) and the topquark mass (right) after applying the estimated corrections on the reconstructed jets.The e+jets t¯t events that pass the extra selection cuts are taken into account.jet energy corrections which are estimated by the method in a first iteration are appliedon the energy of the reconstructed jets and the method is performed for a second time.Ideally the estimated residual jet energy corrections in the second iteration would beequal to zero if the bias in the first iteration was exactly zero. The sum of the estimatedjet energy corrections obtained from the first and the second iterations will be a betterestimate of the residual jet energy corrections. Another thing which can be done toobtain a more precise mass distribution for either the W boson or the top quark, is toapply correction factors which varies in terms of p T of the jets. The method is appliedon the events which are grouped according to the p T of the jets to obtain an estimatedcorrection for the jets contributing to that particular bin of p T . The obtained resultscan then be applied on the reconstructed jets belonging to that p T -bin. In additionto the p T , other kinematic variables such as η can also be used. Applying this kindof differential estimation of the jet energy corrections is expected to result in a massdistribution of both the W boson and the top quark with an improved resolution.5.4.2 Systematic UncertaintiesVarious sources can affect the final estimated results of the jet energy scale correctionfactors. Some of the most important sources that can influence the results, are discussedbelow.The Value of m top as Input Variable for the Kinematic FitThe kinematic fit package requires the event topology to fulfill the mass constraints.The W boson and the top quark mass values that are used in the package may varywithin their uncertainties. While the mass of the W boson is measured with a highprecesion of σ mW = 0.03%, the mass of the top quark is measured with an accuracy ofσ mtop = 0.52%, as discussed in 1.2. To be conservative, a larger uncertainty equivalentto 2.5σ mtop is considered on the value of the top quark mass, hence 1.3%. This resultsin an uncertainty equals to ±2.3 GeV on the top quark mass value of 172.5 GeV which