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

Chapter 6ConclusionThe Standard Model of particle physics is able to describe successfully the elementaryparticles and their interactions. The parameters of the model have been widely measuredin diverse experiments which made the Standard Model an excellent candidateto be a reference theory for the description of the nature at the energy scale of Electroweakinteractions. Despite all the successes of the model, there is a part for whichno experimental evidence has been observed yet, being the Higgs mechanism which isresponsible for giving mass to the elementary particles. This was discussed in detail inChapter 1.A possible solution would be provided by the Large Hadron Collider (LHC) as itmainly aimes to search for the missing particle predicted by the mechanism, namelythe Higgs particle. Since March 2010, the LHC has been colliding beams of protons ata center of mass energy of 7 TeV and the collision data has been recorded by detectorsinstalled at four points around the LHC main ring, such as the CMS experiment(Compact Muon Solenoid). The full description of the LHC and the CMS experimentwere given in Chapter 2.When protons collide, various objects can be produced at the final state, such asisolated leptons and quarks. According to the theory, quarks carry color-charges andtherefore can not be observed in free forms. They hadronize and make a cascade ofhadrons, which is referred to as jets. One of the main challenges at hadron collidersis how to define a jet and how to measure its energy. A simple definition of a jet isto collect all the particles produced in the hadronization process within a cone aroundthe initial quark. This works as a first guess but one should note that due to theexistence of the magnetic field, the charged particles produced in the cascade are bentand move away from the direction of the initial quark. This yields to a measurement ofthe energy of the jets lower than its true energy, as the energy deposits of the escapingparticles are not taken into account in the jet area considered in the jet reconstructionalgorithm. Therefore, jets need to be calibrated so that their energies are correctedback to their true values. The various jet definitions and calibration strategies whichare introduced within the CMS collaboration, were explained in Chapter 3.Since jets are produced in final states of many channels of interest, it is desired todetermine their energy as precise as possible. Starting with the baseline calibrationrecipe, the energy of the jets can be further calibrated with the use of constraints127