Optimization of Jet Finding Algorithm in High Energy Heavy Ion Collisions with ALICE at LHC 17/10/2009 Dousatsu Sakata University of Tsukuba & RIKEN Takuma Horaguchi University of Tsukuba & JSPS APS/JPS HAW09

Outline  Jet Physics for Nuclear Collisions  Motivation  FastJet  Eventgeneration with PYTHIA  Jet Reconstruction for PYTHIA - d ϕ resolution, dη resolution, measured energy  Jet Efficiency with PYTHIA  HIJING + Jet[PYTHIA]  Jet Reconstruction for HIJING - measured energy  Jet Efficiency with HIJING  Summary & Outlook APS/JPS HAW09

3  Mach Cone  E loss in QGP (jet quenching) - collisional E loss - radiative E loss 3 PHENIX, arXiv: [nucl-ex] Jet Physics for Nuclear Collisions Jet is very useful probe for investigating the hot & dense matter created by ultra relativistic heavy ion collisions APS/JPS HAW09

4 Motivation To bring out mechanism of jetquenching we need to understand performance of Jet Finding algorithms below.  Performance of Jet energy reconstruction  Accuracy of Jet axis determination Jets in heavy ion collisions looks quite different from Jets of p+p collisions It is not clear which jet finding algorithm is best in heavy ion collisions. The purpose of this analysis is to optimize a jet finding algorithm APS/JPS HAW09 PYTHIA[60GeV] PYTHIA+HIJING

FastJet FastJet: sequential clustering algorithms / 5 Parameters - R size (= √d ϕ 2 +d η 2 ) - p T cut of single particle - Jet enrgy threshold Procedure of Jet Finding Calculate particle distance : d ij Calculate Beam distance : d iB =k ti 2p Find smallest distance (d ij or d iB ) If d ij is smallest combine particles If d iB is smallest and the cluster momentum larger than threshold call the cluster a Jet. arXiv: v2 [hep-pn] (2008) APS/JPS HAW09 k T algorithm Cambridge/Aachen algorithm anti-k T algorithm Cone jetK T jet

Event Generation with PYTHIA p+p 5500GeV p T hard : Detector smearing: not included AliRootv4-16-Rev-03 # of event10000 ProcessMB & All QCDon ϕ 0 ~ 2π η-1 ~ APS/JPS HAW09 PYTHIA(60GeV) We use all final state particles to find jet. Status of PYTHIA (tuned for ALICE) PYTHIA: event generator in pp collisions

Jet Reconstruction Δ ϕ resolution Δη resolution measured energy p T hard(pt of scatterd parton) 100GeV/c Jet threshold 20GeV/c k T algorithm anti-k T algorithm Cambridge algorithm Resolution : σ of sharp gaussian Performance of energy reconstruction of Jet with anti-k T algorithm is about 50% better than k T algorithm! Performance of energy reconstruction of Jet with anti-k T algorithm is about 50% better than k T algorithm! 7 We analyze for the closest Jet to parton APS/JPS HAW09 R R R

Definition of Efficiency for Jet ΔEt/parton Et k T anti-k T Camb Efficiency = # of Jets / # of partons |(parton Et – Jet Et)/(parton Et)|<0.3 & In 3σ of broad gaussian for d ϕ & In 3σ of broad gaussian for dη 8 |η of parton|<1. |η of Jet|< APS/JPS HAW09 How many jets dose keep on the property of the partons ?

Jet Efficiency with PYTHIA 9 p T hard = 100 GeV/c threshold = 20 GeV/c R = 0.2 threshold = 20 GeV/c k T algorithm anti-k T algorithm Cambridge algorithm Efficiency of Jet Finding with anti-kt algorithm is about 30% better than kt algorithm Efficiency of Jet Finding with anti-kt algorithm is about 30% better than kt algorithm 60% of Jets has correct energy & correct direction for 100GeV/c Jets at R=1.0 Anti-kt is better in pp collisions! APS/JPS HAW09

HIJING + Jet[PYTHIA] Pb+Pb 5500GeV Centrality : % AliRootv4-16-Rev-03 # of event10000 p T hard min 2GeV/c Jet Quenchingon Shadowingon 10 We embed PYTHIA Jets in HIJING event centrality HIJING PYTHIA Jets (same Jets) Jets APS/JPS HAW09 HIJING+Jet [PYTHIA(60GeV)] HIJING: event generator in heavy ion collisions We use all final state particles to find jet. Status of HIJING (tuned for ALICE)

Energy reconstruction with HIJING anti-kt algorithm p T hard :100GeV/c R :0.2 Jet threshold :20 GeV/c 11 Jets has about 40GeV background at central collision at p T cut=0.5 GeV/c Jets has about 40GeV background at central collision at p T cut=0.5 GeV/c APS/JPS HAW09  Jet Energy If p T cut is small Jet energy is enhanced, If p T cut is large Jet energy is suppressed.  Background Energy Background energy goes down as p T cut is larger.

Jet Efficiency with HIJING anti-kt algorithm p T hard :100GeV/c R :0.2 Jet threshold :20 GeV/c We should set p T cut 1.0GeV/c at most central, 0.5GeV/c for the other centrality APS/JPS HAW09 We optimized pt cut to maximize the efficiency. Efficiency = # of Jets / # of partons |(parton Et – Jet Et)/(parton Et)|<0.3 & In 3σ of broad gaussian for d ϕ & In 3σ of broad gaussian for dη

Summary & Outlook We compare 3type Jet Finding algorithms with PYTHIA.  anti-k T algorithm is well-suited to the pp collisions. We tune p T cut for jets with high multiplicity.  p T cut=1.0GeV/c is the better for most central collisions We will  include detector smearing.  consider effect of Jet energy threshold & cone radius in high multiplicity.  optimize algorithm for low energy Jets. 13 Thank you very much! APS/JPS HAW09

Backup APS/JPS HAW09 14

ALICE-EMcal 4 〜 6 SM J-Cal : EMCal for back-to back jets trigger J-Cal 15 Appendix [J-Cal] APS/JPS HAW09 J-Cal is approved by ALICE!

Area of Jet & background Et in PYTHIA Jet APS/JPS HAW09 16 p T hard = 100 GeV/c threshold = 20 GeV/c R = 0.2 threshold = 20 GeV/c Jet Area background Et = (Jet Area)*(Et JET /d ϕ dη) Et JET :ΣEt(exclude JetArea)

Measured Energy and Energy Correction APS/JPS HAW09 17 E T corrected = E T reconstructed - E T background /dΦdη*Area Jet Back ground : Multiple parton interaction Beam remnant threshold:20GeV/c p T hard GeV/c p T hard 60 – 80 GeV/c Black : Cam/Aachen Blue : k T Red : anti-k T

Number of Jets All on |η|<3 All on |η|<1 initial on |η|<3 final on |η|<3 ini-fin on |η|<3 BR-MPI on |η|<3 All off |η|<3 η cut of particles anti-kt algorithm p T hard = 100GeV/c Jet threshold = 20GeV/c # of event of N-jets / total events 3-Jets 1-Jet 2-Jets4-Jets 5-Jets 70% of total events has less than 2-jets for setting of all radiation on 18 Using particles: StatusCode of PYTHIA =1 (final state particles) APS/JPS HAW09

Radiation effect for d ϕ resolution anti-kt algorithm p T hard = 100GeV/c Jet threshold = 20GeV/c APS/JPS HAW09 All on |η|<3 All on |η|<1 initial on |η|<3 final on |η|<3 ini-fin on |η|<3 BR-MPI on |η|<3 All off |η|<3 η cut of particles Using particles: StatusCode of PYTHIA =1 (final state particles)

Radiation effect for Energy reconstruction anti-kt algorithm p T hard = 100GeV/c Jet threshold = 20GeV/c APS/JPS HAW09 All on |η|<3 All on |η|<1 initial on |η|<3 final on |η|<3 ini-fin on |η|<3 BR-MPI on |η|<3 All off |η|<3 η cut of particles Using particles: StatusCode of PYTHIA =1 (final state particles)

Efficiency & Purity 3σ Efficiency: # of Jet in 3σ of “broad” gaussian / # of parton S/(S+N)(pulity): # of Jet in 3σ of “sharp” gaussian/ # of Jet in 3σ of “broad” gaussian anti-kt algorithm p T hard = 100GeV/c Jet threshold = 20GeV/c anti-kt kt Cambridge APS/JPS HAW09

Energy reconstruction with HIJING anti-k T algorithm R :0.2 p T cut : 0.5GeV/c Jet threshold :20 GeV/c p T hard :100GeV/c R :0.2 Jet threshold :20 GeV/c 22 Jets has about 40GeV background at central collision at p T cut=0.5 Jets has about 40GeV background at central collision at p T cut= APS/JPS HAW09

dE/parton Et distribution APS/JPS HAW09 23 p T hard = 20GeV/c 60GeV/c 100GeV/c R =0.2 R =0.6 R =1.0 PYTHIA threshold = 20GeV/c dE/parton Et anti-kt Camb kt |dE/parton Et|<0.3

Efficiency for Energy APS/JPS HAW09 ΔEt/parton Et k T anti-k T Camb |η of parton|<1. |η of Jet|<1. Efficiency for Energy = # of Jets / # of partons |(parton Et – Jet Et)/(parton Et)|<0.3

Efficiency for direction APS/JPS HAW09 Efficiency for Direction = # of Jets / # of partons In 3σ of broad gaussian for d ϕ & In 3σ of broad gaussian for dη |η of parton|<1. |η of Jet|<1.

Efficiency of HIJING anti-k T algorithm R :0.2 p T cut : 0.5GeV/c Jet threshold :20 GeV/c p T hard :100GeV/c R :0.2 Jet threshold :20 GeV/c We should set p T cut 1.0GeV/c at most central, 0.5GeV/c for the other centrality. Low energy jets are affected by background APS/JPS HAW09