Study of jet fragmentation in p p collisions at 200 GeV in the STAR experiment'

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Study of jet fragmentation in pp collisions at
200 GeV in the STAR experiment.

• Elena Bruna, for the STAR Collaboration
• Yale University

Hot Quarks, August 18-23 2008
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Outline

• Jet reconstruction in STAR
• Jet-pT distributions for different jet-finding
  algorithms
• Fragmentation functions for different jet-finding
  algorithms
• Results for different trigger selections
• Fragmentation functions for identified particles
• Summary and Outlook

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Jets in high-energy collisions

• High-pT quarks produced in hard scatterings

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

hadrons
D
c, xc
Jet cross section
a, xa
b, xb
p
p
sab
d, xd
D
hadrons
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Jet Reconstruction
Cone Algorithms

• Seed Cone
• seed (EgtEthreshold)
• iterative approach
• Optional tools
• Midpoint
• Splitting/merging

• Seedless Cone (SIS cone)
• all the particles used as seeds
• Splitting/Merging applied

tracks or towers
Rcone
Rv(?f2??2)
seed
Cacciari, Soyez, arXiv0704.0292
Recombination Algorithms

• Seedless, not bound to a circular structure
• kT starts from merging low pT particles close
  in the phase-space
• Anti-kT starts from merging high pT particles
  close in the phase-space

Cacciari, Salam, Soyez, arXiv0802.1189
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Recent results and perspectives
Midpoint cone jet finder

• Inclusive differential cross section for pp?jet
  X
• Good agreement with NLO over the pT range
  measured
• Increased L and acceptance (x2) in 2006
• More high-pT jets
• PID of jet fragments
• Jet Physics in STAR

2003-2004 data
STAR, PRL 97 (2006), 252001

• Measure jet fragmentation in pp, also as a
  reference for AuAu
• Study the hadrochemical modifications of jets in
  the medium

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Energy resolution (1 of 2)

• Jet Finder (Mid-point Cone Algorithm) applied to
  PYTHIA pp events
• PYTHIA particles ? PYTHIA Jets (no detector
  effects)
• Reconstructed tracks and calorimeter towers ?
  RECO Jets (detector effects)
• Resolution 25

ERECO vs EPYTHIA
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Energy resolution (2 of 2)
From DATA di-jet events
STAR preliminary
20ltEjetlt30 GeV

• From di-jet events
• Jet1 ? E1
• Jet2 ? E2
• Df1,2gt2 rad
• Resolution RMS/v227
• Data confirm simulation
• Complete model independent result

DATA sample pp 2006 data MidPoint Cone Jet
Finder seed0.5 GeV pT cut0.1 GeV/c R0.7 No
trigger constraint applied to di-jets
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STAR Event Selection

• Dataset 2006 pp
• Triggers
• Minbias Beam-Beam-Counter (BBC) coincidence
• High Tower Trigger (HT) BBC coincidence one
  tower (h x f0.05 x 0.05) above threshold (ETgt5.4
  GeV)
• Jet Patch (JP) BBC coincidence Jet-Patch (h x
  f1 x 1) above threshold (ETgt8 GeV)
• Detectors used for Jet Finding TPC and e.m.
  calorimeter (EMC)
• Corrections
• Electrons rejected EMC towers matching to
  electrons candidates only the track pT is taken
• Charged hadrons 1 MIP subtracted from a tower
  when a track (hadron) hits the tower

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Jet pT spectra

• Only the highest energy jet taken per event

Counts
FastJet Package Cacciari,Salam,Soyez
http//www.lpthe.jussieu.fr/salam/fastjet/
R0.7 ?jetlt0.3
STAR preliminary

• Jet Patch DATA
• First raw measurement, no correction for
• Jet energy resolution
• Trigger bias
• Agreement of FastJet Finders and cone with small
  seed
• Different onset for the High-seed Cone

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Fragmentation functions for charged hadrons

• Definition xln (Ejet/phadr)
• We use xln (pT,jet/pT,hadr) (we do not make
  assumptions on the particle mass)

30lt pT,jetlt40 GeV
(seed2 GeV)
STAR preliminary

• Jet Patch DATA
• All the Jet Finders agree

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Jet-Patch vs High Tower triggers
10lt pT,jetlt15 GeV
20lt pT,jetlt30 GeV
STAR preliminary
STAR preliminary
kT algorithm R0.7
30lt pT,jetlt40 GeV
pT,jetgt40 GeV
STAR preliminary
STAR preliminary

• Low Energies Multiplicity of charged particles
  influenced by Neutral Energy Fraction and high z
  fragmenting jets ? stronger trigger BIAS in the
  HT sample
• High Energies JP and HT do not show difference
  due to a smaller bias of different trigger
  selections ? JP and HT fragmentation functions
  similar for higher jet energy

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x for different jet energies
20lt pT,jetlt30 GeV
10lt pT,jetlt15 GeV
JP trigger R0.4
JP trigger R0.7
pT,jetgt40 GeV
30lt pT,jetlt40 GeV

• Different Jet Finders ? Similar performance for
  different cone radii

• Mean x increases with energy
• Average of particles/jet slowly increases with
  jet energy

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Identified particles L, K0s

• L?pp, K0s ? pp-
• Kinematical range of V0 0.5ltpTlt8 -10 GeV/c
• Mid-Point Cone Jet Finder with Eseed0.5 GeV/c
  applied to JP events

pTgt0.5 GeV/c for all particles
10ltErecolt15 GeV
20ltErecolt50 GeV
15ltErecolt20 GeV
M. Heinz, Hard Probes 2008
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Mean of x distributions
mass

• QCD models predict particle mass ordering of mean
  x value
• In pp_at_200 GeV in STAR an inverse ltxgt ordering of
  K0s and L is observed
• Similar observation with kaons and protons shown
  by the BABAR collaboration

ltxgt
STAR Data
Rlt0.4
BABAR Data
K
p
M. Heinz, Hard Probes 2008
F. Anulli, Trento 2008 arXiv0804.2021v1
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Summary and outlook

• Jet-pT spectra measured up to 50 GeV
  (correction for energy resolution to be done)
• Two trigger selections compared Jet-Patch vs
  High Tower. Agreement of fragmentation functions
  at high jet energies, as expected
• Four jet finder algorithms tested on pp 2006
  STAR data. No difference in jet pT, R and x
• No preferred choice of jet finders in pp ? Good
  baseline for jet reconstruction in AuAu
• Fragmentation functions measured for different
  jet energies, for charged and identified
  particles, L and K0s
• ltxgt for K0s and L does not follow a particle mass
  ordering expected from QCD
• Outlook
• PID at high-pT (relativistic rise)
• PID fragmentation functions also for AuAu

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Extra slides
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More on energy Resolution

• Jet Finder (Mid-point Cone Algorithm) applied to
  PYTHIA pp events
• PYTHIA particles ? PYTHIA Jets (no detector
  effects)
• Reconstructed tracks and calorimeter towers ?
  RECO Jets (detector effects)

EPYTHIA vs ERECO

• Given ERECO ltEPYTHIAgt smaller than ERECO
• Folding in the real jet spectrum
• Detector resolution

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x and charged multiplicity/jet
STAR preliminary
R0.7, JP
STAR preliminary
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x and charged multiplicity/jet
R0.7, HT
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R0.7 vs R0.4
STAR preliminary
STAR preliminary
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Comparison with PYTHIA

• Midpoint Cone Algorithm (seed0.5 GeV)
• R0.7
• Charged particles x distributions

20ltErecolt30 GeV
30ltErecolt40 GeV
40ltErecolt50 GeV
M. Heinz, Hard Probes 2008