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
Jan 25, 2016
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
2
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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
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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]