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

IntroductionThe Standard Model of particle physics describes the elementary particles and theirinteractions, with the exception of gravity which is not formulated within the model.The model has been tested in diverse experiments and its parameters have been measuredwith great precision in high energy physics laboratories. However, there is stilla missing particle which has not yet been detected, being the Standard Model Higgsboson. Also there is evidence which make the Standard Model only an effective theoryand motivate the need to have new physics beyond it. Therefore, efforts are made tobuild the most fundamental theory describing nature. To search for the Higgs particleand test the new physics ideas beyond the Standard Model, the Large Hadron Collider(LHC) was constructed.The LHC, located at the CERN laboratory near Geneva, has been colliding beamsof protons at a center of mass energy of 7 TeV since March 2010. The proton-protoncollisions are detected with the use of detectors installed at the interaction points, suchas the CMS (Compact Muon Solenoid) experiment, which is one of the two generalpurposedetectors at the LHC. The energy and momentum of particles which are producedin the final state of proton-proton interactions, are measured by the detectors.While the energy of leptons can be measured with high precision, the measurement ofthe energy of the quarks, which hadronize to make so-called jets, is quite challenging.While strategies are developed to calibrate the jet energies within the CMS collaboration,searching for new physics benefits from a more precise measurement of the jetenergies than the ones obtained by applying the baseline methods. In this analysis, amethod to calibrate the energy of the jets is applied on the proton collisions containingtwo top quarks in the final state, pp → t¯t. The high rate of the production of thetop quark pairs in proton-proton collisions provides a huge amount of statistics. Thisallows the top quark pairs to be used as a tool to perform the calibration studies.This thesis is organized as follows. In Chapter 1, the Standard Model of particlephysics is reviewed with emphasis on the top quark sector followed by a motivationto construct the LHC. The description of the Large Hadron Collider and the CMSexperiment, is given in Chapter 2. The CMS experiment detects the emerging particlesfrom the proton collisions through electronic signals which are subsequently used toreconstruct the physics objects as described in Chapter 3. The simulation of the protoncollisions is described in Chapter 4 followed by the description of the procedure to selecttop quark events on which the calibration method is based. The method to estimatethe residual jet energy corrections is described in detail in Chapter 5. The estimatedresults of the method are obtained based on both the simulated proton collisions aswell as the real proton collisions collected in 2010 by the CMS experiment. Finally, a1