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 & RIKENTakuma Horaguchi University of Tsukuba & JSPS

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09.10.17APS/JPS HAW09

OutlineJet Physics for Nuclear CollisionsMotivationFastJetEventgeneration with PYTHIAJet Reconstruction for PYTHIA - dϕ resolution , dη resolution , measured energy Jet Efficiency with PYTHIAHIJING + Jet[PYTHIA]Jet Reconstruction for HIJING - measured energyJet Efficiency with HIJINGSummary & Outlook

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Mach Cone E loss in QGP (jet quenching) - collisional E loss - radiative E loss

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PHENIX, arXiv:0705.3238 [nucl-ex]PHENIX, arXiv:0705.3238 [nucl-ex]

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Jet Physics for Nuclear Collisions

Jet is very useful probe for investigating the hot & dense matter created by ultra relativistic heavy ion collisions.


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MotivationTo 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.


PYTHIA[60GeV] PYTHIA+HIJING

FastJetFastJet: sequential clustering algorithmshttp://www.lpthe.jussieu.fr/~salam/fastjet/

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Parameters - R size (= √dϕ2 +dη2) - pT cut of single particle - Jet enrgy threshold

Procedure of Jet FindingCalculate particle distance : dij

Calculate Beam distance : diB=kti2p

Find smallest distance (dij or diB)If dij is smallest combine particlesIf diB is smallest and the cluster momentum larger than threshold call the cluster a Jet.

arXiv:0802.1189v2 [hep-pn] (2008)


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dij = min(kti2p,ktj

2p ) ΔR2

R2

p =1p = 0p = −1

⎧ ⎨ ⎪

⎩ ⎪

kT algorithmCambridge/Aachen algorithmanti-kT algorithm Cone jetKT jet

Event Generation with PYTHIA

p+p 5500GeV

pThard : 20-40 40-60 60-80 100-120 120-140Detector smearing: not included

AliRoot v4-16-Rev-03# of event 10000Process MB & All

QCDon ϕ 0 ~ 2πη -1 ~ 1

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PYTHIA(60GeV)We use all final state particles to find

jet.

Status of PYTHIA (tuned for ALICE)

PYTHIA: event generator in pp collisionshttp://home.thep.lu.se/~torbjorn/Pythia.html

Jet ReconstructionΔϕ resolution Δη resolution measured energy

pThard(pt of scatterd parton) 100GeV/cJet threshold 20GeV/c

kT algorithm

anti-kT algorithm

Cambridge algorithm

Resolution : σ of sharp gaussian Performance of energy reconstruction of Jet with anti-kT algorithm is about 50% better than kT algorithm!

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We analyze for the closest Jet to parton


R R R

Definition of Efficiency for Jet

ΔEt/parton Et

kT

anti-kT

CambEfficiency =# 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

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|η of parton|<1.|η of Jet|<1.


How many jets dose keep on the property of the partons ?

Jet Efficiency with PYTHIA9

pT hard = 100 GeV/cthreshold = 20 GeV/c

R = 0.2threshold = 20 GeV/c

kT algorithmanti-kT algorithmCambridge 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!


HIJING + Jet[PYTHIA]

Pb+Pb 5500GeV

Centrality : 0-20 20-40 40-60 60-80 80-100%

AliRoot v4-16-Rev-03# of event 10000pThard min 2GeV/c

Jet Quenching onShadowing on

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We embed PYTHIA Jets in HIJING event centrality 0-20 20-40HIJING 40-60 + PYTHIA Jets (same

Jets) 60-80 10000 Jets 80-100


HIJING+Jet [PYTHIA(60GeV)]

HIJING: event generator in heavy ion collisionshttp://www-nsdth.lbl.gov/~xnwang/hijing/index.html

We use all final state particles to find

jet.

Status of HIJING (tuned for ALICE)

Energy reconstruction with HIJINGanti-kt algorithmpT hard :100GeV/cR :0.2Jet threshold :20 GeV/c

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Jets has about 40GeV background at central collision at pTcut=0.5 GeV/c


Jet EnergyIf 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.

Jet Efficiency with HIJINGanti-kt algorithmpT hard :100GeV/cR :0.2Jet threshold :20 GeV/c

We should set pTcut 1.0GeV/c at most central,0.5GeV/c for the other centrality.

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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 dϕ &In 3σ of broad gaussian for dη

Summary & OutlookWe 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 willinclude detector smearing.consider effect of Jet energy threshold & cone radius in high multiplicity.optimize algorithm for low energy Jets.

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Thank you very much!


Backup


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ALICE-EMcal

• • •

4 〜 6 SM

J-Cal : EMCal for back-to back jets trigger

J-Cal

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Appendix [J-Cal] 09.10.17APS/JPS HAW09

J-Cal is approved by ALICE!

Area of Jet & background Et in PYTHIA Jet 09.10.17APS/JPS HAW09

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pT hard = 100 GeV/cthreshold = 20 GeV/c

R = 0.2threshold = 20 GeV/c

Jet Area

background Et = (Jet Area)*(EtJET/dϕdη)

EtJET:ΣEt(exclude JetArea)

Measured Energy and Energy Correction 09.10.17APS/JPS HAW09

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ETcorrected = ET

reconstructed - ETbackground/dΦdη*AreaJet

Back ground :Multiple parton interactionBeam remnantthreshold:20GeV/c

pThard 100-120GeV/c

pThard 60 – 80 GeV/c

Black : Cam/AachenBlue : kT

Red : anti-kT

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 algorithmpThard = 100GeV/cJet threshold = 20GeV/c

# of event of N-jets / total events

3-Jets

1-Jet

2-Jets 4-Jets

5-Jets70% of total events has less than 2-jets for setting of all radiation on

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Using particles: StatusCode of PYTHIA =1 (final state particles)


Radiation effect for dϕ resolution


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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


Radiation effect for Energy reconstruction


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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


Efficiency & Purity

3σ

3σEfficiency:# of Jet in 3σ of “broad” gaussian / # of partonS/(S+N)(pulity):# of Jet in 3σ of “sharp” gaussian/ # of Jet in 3σ of “broad” gaussian


anti-ktktCambridge

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Energy reconstruction with HIJINGanti-kT algorithmR :0.2pT cut : 0.5GeV/cJet threshold :20 GeV/c

pT hard :100GeV/cR :0.2Jet threshold :20 GeV/c

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Jets has about 40GeV background at central collision at pTcut=0.5


dE/parton Et distribution 09.10.17APS/JPS HAW09

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pThard = 20GeV/c 60GeV/c 100GeV/c

R =0.2

R =0.6

R =1.0

PYTHIA threshold = 20GeV/c

dE/parton Et

anti-ktCambkt

|dE/parton Et|<0.3

Efficiency for Energy 24


ΔEt/parton Et

kT

anti-kT

Camb


Efficiency for Energy =# of Jets / # of partons

|(parton Et – Jet Et)/(parton Et)|<0.3

Efficiency for direction 25


Efficiency for Direction =# of Jets / # of partons

In 3σ of broad gaussian for dϕ &In 3σ of broad gaussian for dη


Efficiency of HIJINGanti-kT algorithmR :0.2pT cut : 0.5GeV/cJet threshold :20 GeV/c

pT hard :100GeV/cR :0.2Jet threshold :20 GeV/c

We should set pTcut 1.0GeV/c at most central,0.5GeV/c for the other centrality.

Low energy jets are affected by background.

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