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

CHAPTER 3: Object Reconstruction 33the parton’s direction that characterizes a jet.Different types of jets are reconstructed at CMS depending on the information of whichsub-detectors is used [67, 68]. As only calorimeter jets are processed in this analysis,there is a dedicated section to describe the details of this kind of jets, followed bydescriptions of the divers jet clustering algorithms currently available at CMS.Calorimeter jets, hereafter called CaloJets, only use the information of the CMS electromagnetic(ECAL) as well as the hadronic (HCAL) calorimeters. The building blocksfor CaloJets are calotowers which are made of the HCAL towers and the correspondingECAL crystals. The energy associated to a calotower is simply obtained by collectingthe energy deposits in those components which pass the so-called “Scheme B” energythresholds [69]. This helps to suppress the electronic noise and not to count them inthe energy of the calotower. In order to further reduce the additional contributionfrom pile-up collisions, calotowers are required to have a transverse energy exceeding0.5 GeV to be used for jet clustering algorithms.3.2.1 Jet Clustering AlgorithmsSeveral methods to define a jet exist [70–72]. Two of them with great importance inthe CMS colaboration are described below. It will become clear that a first method,the iterative cone algorithm, is used in the online triggering program while a secondone, the anti-k T algorithm, is important as it is highly recommended for the offlineanalysis.Iterative Cone AlgorithmThis simple algorithm merges the energy deposits located in a cone around the mostenergetic calotowers. The steps taken to create the iterative cone jets are listed below.• an E T -ordered list of input objects is created;• a cone of size R in (η, φ) space is cast around the seed, the object with the largestE T ;• objects inside the cone are used to calculate the properties of a protojetE T = Σ i E T i , η = Σ i(η i × E T i )E T, φ = Σ i(φ i × E T i )E T;• this protojet is used to seed a new protojet;• the procedure is repeated until the energy and direction of the protojet does notchange between iterations;