Jet fragmentation in p+p collisions at 200 GeV in the

STAR experiment.

Elena Bruna, for the STAR Collaboration

Yale University

DNP Meeting, Oct 23-26 2008

Jet Physics in STARMeasure jet fragmentation in p+p:

test for QCD models at RHIC energies reference for Au+Au Study the hadrochemical

modifications of jets in the medium

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• Jet reconstruction in STAR

• Jet-pT distributions for different jet algorithms

• Systematic study of jet finding algorithms

• Fragmentation functions for charged and identified particles for different jet algorithms

Out

line

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Jets in high-energy collisionsHigh-pT partons produced in hard scatterings

p pa, xa b, xbσab

c, x c

d, x d

D

D

Jet cross section:

Full jet (spray of collimated hadrons) after parton fragmentation access to partonic kinematics

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Cone AlgorithmsCone Algorithms

• ‘seed’ (E>Ethreshold)

• iterative approach• Primarily used at hadron colliders

• Cone• SISCone :all particles as seeds

seed

tracks or towers

Rcone

[Cacciari, Soyez, arXiv:0704.0292]

Jet Reconstruction

• Seedless • Not bound to a circular structure• Start from merging particles close in phase-space

• kT : pairs of high-pT particles clustered first

• Anti-kT :pairs of low-pT particles clustered first

Recombination AlgorithmsRecombination Algorithms

R=√(Δφ2+Δη2)

[Cacciari, Salam, Soyez, arXiv:0802.1189]

outgoing parton

fragmentation

particles

seed

STAR Detectors: TPC + e.m. calorimeter

Jet pT spectraEle

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R=0.7 |ηjet|<0.3

• Raw measurement, no correction for:•Jet energy resolution•Trigger bias• Track reconstruction efficiency

• Agreement of FastJet Finders and cone with low-pT seed

• Different onset for the high-pT seed Cone

Only the highest energy jet taken per event

STAR preliminary

FastJet Package [Cacciari,Salam,Soyez http://www.lpthe.jussieu.fr/~salam/fastjet/]C

ou

nts

p+p √s=200 GeVJP trigger

Fragmentation functions for charged hadrons

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Definition: =ln (Ejet/phadr)

We use: =ln (pT,jet/pT,hadr) : we do not make assumptions on the particle mass mpart=0 jet Minv=0

ξξ ppT,hadrT,hadr (GeV/c)(GeV/c)

(p(pTjetTjet=20 GeV)=20 GeV)

11 7.47.4

22 2.7 2.7

33 1 1

44 0.4 0.4

55 0.1 0.1

Charged particlesCharged particles (no e-)

20< pT,jet<30 GeV

Electrons:• identified with dE/dx and p/E• mainly from 0 and •few electrons: ≈10% of events have at least 1 e- •No major difference in fragmentation functions w/ & w/o e-

p+p √s=200 GeVJP trigger

for different jet energiesEle

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• Uncorrected spectra • Different Jet Finders show similar performance for a given R• Mean increases with jet momentum

10< pT,jet<15 GeV

20< pT,jet<30 GeV

30< pT,jet<40 GeV

pT,jet>40 GeVR=0.7R=0.7R=0.7R=0.7

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pT>=1 GeV/c

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

Identified particles: , K0s

p, K0s +-

Kinematical range of V0 in STAR: 0.5<pT<8 -10 GeV/c Cone Jet Finder with Eseed=0.5 GeV/c

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10<Ejet<15 GeV10<Ejet<15 GeV 20<Ejet<50 GeV20<Ejet<50 GeV15<Ejet<20 GeV15<Ejet<20 GeV

[M. Heinz, Hard Probes 08]

pT>0.5 GeV/c for all particles

pT>0.5 GeV/c for all particles

Here: =ln(Ejet/phadr)

Summary and outlook Jet-pT spectra measured up to ~ 50 GeV (correction for energy

resolution to be done)

Systematic study of jet finding algorithms in p+p:

No difference in jet pT and observed for a given R

No preferred choice of jet finders in p+p Good baseline for jet reconstruction in Au+Au

Fragmentation functions measured for different jet energies and particle species (hadrons, and K0

s).

Electron contamination not significant.

Outlook:

PID at high-pT (relativistic rise of dE/dx)

PID fragmentation functions also for Au+Au

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Extra sli

des

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and charged multiplicity/jetEle

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R=0.7, JPR=0.7, JP

STAR preliminary

STAR preliminary

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R=0.7 vs R=0.4Ele

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STAR preliminary

STAR preliminary

Energy resolution Jet Finder (Mid-point Cone Algorithm) applied to PYTHIA p+p

events: PYTHIA particles PYTHIA Jets (no detector effects) Reconstructed tracks and calorimeter towers RECO Jets

(detector effects)

Resolution: ~25%

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ERECO vs EPYTHIA

Comparison with PYTHIA

Midpoint Cone Algorithm (seed=0.5 GeV)

R=0.7

Charged particles distributions

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20<Ereco<30 GeV20<Ereco<30 GeV 30<Ereco<40 GeV30<Ereco<40 GeV 40<Ereco<50 GeV40<Ereco<50 GeV

[M. Heinz, Hard Probes 2008]