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

SummaryJets appear in the final state of many processes of interest at the Large Hadron Colliderand a precise measurement of their energies is of great importance when searching fornew physics phenomena. Since the reconstruction of the jet objects at hadron collidersis a challenging task, calibration strategies are developed to compensate for the nonperfectionof the jet energy measurements. Historically, jets at hadron colliders arecalibrated using QCD dijet events. The resolution on the jet energy measurement canbe further improved considering the physics constraints which can be imposed on anobserved top quark pair system. Due to the precise measurement of the top quarkmass at the Tevatron collider, top quarks can be used as a calibration tool to improvethe performance of the diverse analyses. Imposing the top quark and W boson massconstraints on the system containing a top quark decaying to jets, and altering the jetenergies within their uncertainties, one can obtain the residual jet energy calibrationfactors.In this thesis the method to estimate the residual jet energy corrections is appliedon the events where top quark pairs are produced in proton-proton collisions and subsequentlydecay to an isolated electron and at least four reconstructed jets and missingtransverse energy, hence pp → t¯t → W + W − b¯b → eν e q¯qb¯b processes. The leptonic sideof the event topology is mainly used to suppress the QCD multijet background eventsand the hadronic side is used for the calibration purpose. Event selection cuts areapplied to enhance the ratio of signal to background events to about S/B ≃ 2.5, whichis obtained based on a sample of simulated proton collisions corresponding to 100pb −1of integrated luminosity at a center of mass energy of 7 TeV. In order to reconstructthe topology of the hadronic decay of the top quark in e+jets t¯t events, it is requiredto assign the three jets arising from the hadronic top to the three partons. A Multi-Variate Analysis method is used to perform the association of the jets to the quarks. Inabout 17% of e+jets t¯t events, the MVA method is able to return the correct combination,where a matching between jets and partons is found. Applying an event-by-eventkinematic fitting technique with Lagrange multipliers, the masses of the W boson andof the top quark are used to constrain the e+jets t¯t events in the hadronic branch. Thekinematic fit is applied in a two-dimensional range of residual jet energy corrections(∆E l , ∆E b ). The points (∆ε l est , ∆ε b est ) minimizing the χ 2 of the fit are the estimatedresidual jet energy corrections. The precision of the method to estimate the residualjet energy corrections is limited to a systematical uncertainty of about 0.9% for lightquark jets and of about 1.0% for b quark jets. The main source of the systematicaluncertainty on the estimation of the b quark jet energy correction originates from theuncertainty on the measured top quark mass provided by the Tevatron collider.137