Selected Items in Jet Algorithms

The main properties that should be satisfied by any jet definition were already pointed out almost twenty years ago [1, 2]: the jet algorithm should be 1) simple to implement both in experimental analysis and theoretical calculations, 2) well defined and yielding a finite cross section at any order of perturbation theory, 3) relatively insensitive to hadronization. Many different jet definitions have been proposed in recent years but it turns out that some of them do not strictly meet the above features. This could in principle lead to serious problems especially when infrared(IR)-safety (i.e., the second item in the above list) is not correctly implemented, since in this case the matching with fixed-order theoretical results would be spoiled and the whole jet-finding procedure would heavily depend on non-perturbative effects (hadronization, underlying event, pileup). We can group jet algorithms in two broad classes: Iterative Cone (IC) and Sequential Recombination (SR). IC algorithms cluster particles according to their relative distance in coordinate-space and have been extensively used at past lepton and hadron colliders. SR algorithms cluster particles according to their relative distance in momentum-space and are somehow preferred by theorists since they rigorously take into account IR-safety. In what follows I will try to give an overview of recent developments obtained in both the IC and SR classes. For a complete and extensive treatment of these and other aspects of jet algorithms I refer the reader to the recent literature on the subject [3, 4].