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

Chapter 3Object ReconstructionThe elementary particles, namely quarks and leptons, emerging from a hard interactionare observed in a detector experiment when they interact with the material ofthe detector, the neutrinos being an exception because they do not interact with thematter and their presence is understood by the missing amount in a balance of energybetween initial and final state. According to the known physics processes [61], particlesinteract with matter and release their energy which subsequently is collected by thedata acquisition (DAQ) system of the detector. The information that comes out ofthe detector includ electronic signals which are used when reconstructing the physicsobjects. Different objects have a distinct behaviour while traversing the detector. Thedetector has been designed in such a way that, for example, the electron objects arestopped in the ECAL and loose almost all their energy before reaching the HCAL. Sinceelectrons are charged particles, they leave hits in the tracker. Therefore an electron isreconstructed by assigning a track in the tracker to a collection of energy deposit inthe ECAL. A more precise description of the electron reconstruction algorithm used inthe CMS collabration, is given in Section 3.1.In case of quarks, as they are not observables before the hadronization process, theypass through the detector as a cascade of hadrons, called jets. Jet reconstruction isnot as simple as reconstructing an electron and much more sophisticated algorithmsare needed. A detailed explanation on how to reconstruct the jet objects is the subjectof Section 3.2. In addition to the electrons and the jets, the reconstruction of theneutrinos is of great importance and is described in Section 3.3. Muons can also bereconstructed in the CMS experiment [62]. As muons are not processed in this analysis,no dedicated part is added to explain their reconstruction.3.1 Electron ReconstructionIn order to reconstruct an electron in CMS, the information from both the trackerand the electromagnetic calorimeter is used. The first step is the clustering of theenergy deposits in the ECAL. It has been shown that most of the energy of a singleelectron or an unconverted photon reaching the ECAL, can be collected in a clusterwhich is made of a few ECAL crystals. For instance, electrons with an energy of 120GeV impinging at the center of an ECAL crystal, deposit about 97% of their energy29