20/05/09EMCal meeting The Seedless Infrared Safe Cone algorithm (SISCone) Swensy Jangal, M. Estienne

OUTLINE 1. Description of the algorithm 2. Background subtraction 3. The SISCone classes for AliROOT 4. SISCone coherence wrt other algorithms 5. Conclusion and perspectives

Description of the algorithm SISCone = Seedless Infrared Safe Cone algorithm. Algorithm written by G. Salam and G. Soyez. Optimization of stable cones finding. Respect main criteria of jet reconstruction algorithms : infrared and collinear safe at all orders and fast. A practical seedless infrared safe cone algorithm, G. Salam, G. Soyez

Description of the algorithm (2) is instable 2 is stable 3 is stable 4 is instable = For each particle, looks for cones with this particle plus another one on the circumference For each cone found, check the stability of 4 configurations : Keep in memory which cone is stable and which isn’t in case he would find it again Stability : axis of cone = axis determined by particles’ energy (barycenter). At the end of this procedure, he’s got all the possible stable cones!

Description of the algorithm (3) Why do we need the 4 possibilities? 2R Ex : pT << pT Looking at the 4 possibilities allows to win time and to take all possible cones into account.

Description of the algorithm (4) Split/merge procedure of protojets (non splitted/merged stable cones) : To which jet do they belong? The less energetic jet shares more than f% of his energy The less energetic jet shares less than f% of his energy f = overlap parameter

Background subtraction With jets area calculus. General idea : measuring the probability for a jet to absorb background. Two kinds of areas : - active area : for diffuse noise. - passive area : for punctual noise. Can be found in fastjet package for all algorithms. See : The catchment area of jets, M. Cacciari, G. Salam, G. Soyez Pileup subtraction using jet areas, M. Cacciari, G. Salam Jet areas and what they are good for, M. Cacciari Jet areas as a tool for background subtraction, G. Soyez

Background subtraction (2) Active area Add randomly a uniform distribution of ghosts particles in (η,Φ) space (gives a number of ghosts by area unit). Looks at the number of ghost absorbed by each jet : gives jet area. (Not infrared safe -> several ghosts distributions to look at and mean of all this.) Passive area Add at different position one single ghost in (η,Φ) space. Looks at the area in (η,Φ) space where the ghost is attributed to a particular jet : gives this jet area. The two kind of areas give similar results in heavy ions environment. As SISCone finds too small jet areas, we need the values of kT/Cam algorithms to draw ρ. Draw ρ (mean pt of jets by area unit) : Correct jet number i :

Background subtraction (3) Jet areas and what they are good for, M. Cacciari

The SISCone classes for AliROOT My 2 new classes : AliSISConeJetFinder and AliSISConeJetHeader. Available in the trunk. For charged particles and neutrals (Magali). Find jets with choice of subtracting background or not. Stores found jets in AOD. Main parameters (AliSISConeJetHeader): Cone radius Overlap parameter Cut on protojets pt min Cut on jets pt min Choice of kind of area Area where to study background Has been tested and now works on the grid!!

Emean (GeV) SISCone coherence wrt other algorithms 50 GeV 75 GeV 100 GeV SISCONE UA1 FASTJET No pT cut on particles pT jet min = 14 GeV In progress now that the production problem in AliROOT as been solved. SISCone : Cut on protojets pT : 5 GeV f = 75% UA1 : seed = 4GeV

Conclusion and perspectives Continue more systematic studies with SISCone Get a first pT spectrum for SISCone, Fastjet and UA1 (pwg4 task force) SISCone : fast, infrared and collinear safe at all orders cone algorithm Can be used with aliroot Is working on the GRID