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

34 CHAPTER 3: Object Reconstruction• the stable protojet becomes a jet and the corresponding objects which made theprotojet, are removed from the list to look for other protojets.This algorithm has a fast response and is therefore usually used in the triggeringsoftware of the online trigger system. R is a free parameter of the algorithm.Anti-k T AlgorithmThe detailed description of making anti-k T jets is provided here.• starting from an input list of objects, the algorithm looks for the smallest distanceamong all d i and d ij whose definitions areand R ij = √ (η i − η j ) 2 + (φ i − φ j ) 2 ;d i ≡ 12E T id ij ≡ min{ 1 12,2} × ( R ijE T i E T jR )2 ,,• if the smallest is a d i , particle i is removed from the list of particles and called a jet;• if the smallest is a d ij , particles i and j are recombined to form one single newparticle according toE 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;• the procedure is repeated until no particles are left.Also here, R is a free parameter of the algorithm.This algorithm is currently highly recommended within the CMS experiment. The reasonis generally that, with respect to the iterative cone, the anti-k T algorithm is welldefined in the sense that it does not have the defects such as Infrared and Collinearunsafety which means that the algorithm is stable with respect to the soft radiationand collinear decomposition, respectively. In other words the output of the anti-k Tjet finding algorithm does not change if any minor reconfiguration in the input list ofthe objects, such as adding a soft radiation, occures. However, the iterative cone jetfinding algorithm has a fast response and is currently used in the software part of the