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17/10/2009 Dousatsu Sakata University of Tsukuba & RIKEN Takuma Horaguchi

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. Outline.

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17/10/2009 Dousatsu Sakata University of Tsukuba & RIKEN Takuma Horaguchi

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  1. 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

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

  3. PHENIX, arXiv:0705.3238 [nucl-ex] 3 Jet Physics for Nuclear Collisions APS/JPS HAW09 Jet is very useful probe for investigating the hot & dense matter created by ultra relativistic heavy ion collisions. • Mach Cone • E loss in QGP (jet quenching) • - collisional E loss • - radiative E loss 0 1 2 3

  4. APS/JPS HAW09 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 PYTHIA[60GeV] 　　　PYTHIA+HIJING 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.

  5. KT jet Cone jet APS/JPS HAW09 FastJet:sequential clustering algorithms http://www.lpthe.jussieu.fr/~salam/fastjet/ FastJet kT algorithm Cambridge/Aachen algorithm anti-kT algorithm Procedure of Jet Finding Calculate particle distance : dij Calculate Beam distance : diB=kti2p Find smallest distance (dij or diB) If dij is smallest combine particles If diB is smallest and the cluster momentum larger than threshold call the cluster a Jet. arXiv:0802.1189v2 [hep-pn] (2008) Parameters - R size (= √dϕ2 +dη2) - pT cut of single particle - Jet enrgy threshold

  6. APS/JPS HAW09 PYTHIA: event generator in pp collisions http://home.thep.lu.se/~torbjorn/Pythia.html Event Generation with PYTHIA Status of PYTHIA (tuned for ALICE) p+p 5500GeV 　pThard : 20-40 　　 40-60 　　 60-80 　　 100-120 　　 120-140 Detector smearing: not included PYTHIA(60GeV) We use all final state particles to find jet.

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

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

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

  10. APS/JPS HAW09 HIJING: event generator in heavy ion collisions http://www-nsdth.lbl.gov/~xnwang/hijing/index.html HIJING + Jet[PYTHIA] Status of HIJING (tuned for ALICE) Pb+Pb 5500GeV Centrality : 0-20 　　 20-40 　　 40-60 　　 60-80 　　 80-100% HIJING+Jet [PYTHIA(60GeV)] We embed PYTHIA Jets in HIJING event centrality 0-20 20-40 HIJING 40-60 + PYTHIA Jets (same Jets) 60-80 10000 Jets 80-100 We use all final state particles to find jet.

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

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

  13. APS/JPS HAW09 Summary & Outlook • We compare 3type Jet Finding algorithms with PYTHIA.  anti-kT algorithm is well-suited to the pp collisions. • We tune pT cut for jets with high multiplicity.  pTcut=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. Thank you very much!

  14. APS/JPS HAW09 Backup

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

  16. APS/JPS HAW09 R = 0.2 threshold = 20 GeV/c Area of Jet & background Et in PYTHIA Jet pT hard = 100 GeV/c threshold = 20 GeV/c Jet Area background Et = (Jet Area)*(EtJET/dϕdη) EtJET:ΣEt(exclude JetArea)

  17. APS/JPS HAW09 ETcorrected = ETreconstructed - ETbackground/dΦdη*AreaJet Measured Energy and Energy Correction Back ground : Multiple parton interaction Beam remnant threshold:20GeV/c Black : Cam/Aachen Blue : kT Red : anti-kT pThard 100-120GeV/c pThard 60 – 80 GeV/c

  18. APS/JPS HAW09 Number of Jets # of event of N-jets / total events Using particles: StatusCode of PYTHIA =1 (final state particles) 1-Jet anti-kt algorithm pThard = 100GeV/c Jet threshold = 20GeV/c All on |η|<1 All on |η|<3 initial on |η|<3 final on |η|<3 2-Jets 4-Jets ini-fin on |η|<3 BR-MPI on |η|<3 All off |η|<3 η cut of particles 3-Jets 5-Jets 70% of total events has less than 2-jets for setting of all radiation on

  19. APS/JPS HAW09 Radiation effect for dϕ resolution Using particles: StatusCode of PYTHIA =1 (final state particles) anti-kt algorithm pThard = 100GeV/c Jet threshold = 20GeV/c All on |η|<1 All on |η|<3 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

  20. APS/JPS HAW09 Radiation effect for Energy reconstruction Using particles: StatusCode of PYTHIA =1 (final state particles) anti-kt algorithm pThard = 100GeV/c Jet threshold = 20GeV/c All on |η|<1 All on |η|<3 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

  21. APS/JPS HAW09 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 Efficiency & Purity 3σ 3σ anti-kt algorithm pThard = 100GeV/c Jet threshold = 20GeV/c anti-kt kt Cambridge

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

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

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

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

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

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