Full Jet Reconstruction in Heavy Ion Collisions
description
Transcript of Full Jet Reconstruction in Heavy Ion Collisions
Full Jet Reconstruction in Heavy Ion CollisionsFull Jet Reconstruction in Heavy Ion Collisions
Sevil Salur
Charge for this talk:
1) Update on the latest results on jet-medium interactions as seen in A+A collisions at RHIC.
2) New techniques for full jet reconstruction.
1) Results that help move our understanding forward, (this is not an experiment-specific talk).
2Sevil SalurQuark Matter 2009, Knoxville TN
Why Pursue Full Jet Reconstruction?
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• Enables study of jet quenching at the partonic level.
• Uniquely large kinematic reach
• In A+A much reduced geometric biases, full exploration of quenching.
• Multiple channels for consistency checks: Inclusive, di-jets, h-jets, gamma-jets
• Qualitatively new observables: energy flow, jet substructure, fragmentation function
Quark Matter 2009, Knoxville TN
Jets: a theorist’s view
BeamRemnants
BeamRemnants
p
p
,K,p,n,…}
Jet
Initial State Radiation
Final State Radiation
JETS: Colored partons from the hard scatter (2n)• Fragmentation via gluon radiation • Hadronization: “spray” of colorless hadrons
Parton Level: Calculable with pQCD Underlying Event: Beam remnants
Soft BackgroundS.D Drell, D.J.Levy and T.M. Yan, Phys. Rev. 187, 2159 (1969)N. Cabibbo, G. Parisi and M. Testa, Lett. Nuovo Cimento 4,35 (1970)J.D. Bjorken and S.D. Brodsky, Phys. Rev. D 1, 1416 (1970)Sterman and Weinberg, Phys. Rev. Lett. 39, 1436 (1977) … and many more
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Detector
Quark Matter 2009, Knoxville TN
Jets: an experimentalist’s view
BeamRemnants
BeamRemnants
p
p
,K,p,n,…}
Jet
Initial State Radiation
Final State Radiation
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Detector
JETS: Collection of 4-vectors of calorimeter energy clusters and charged track momentum
Quark Matter 2009, Knoxville TN
Jet Reconstruction Algorithms:
Cone jetAnti-kT jet
KT jet
Jet
outgoing parton
Fragmentation process
Hard scatter
1. Mid Point Cone: Merging & Splitting2. SIS CONE3. Leading Order High Seed Cone (LOHSC)
4. KT (starting point: low pT particles)5. Anti-KT (starting point: high pT particles)
Cone Algorithm:
Sequential recombination:Cluster pairs of objects close in relative pT
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Goal: re-associate hadrons to accurately reconstruct the partonic kinematics.
Kinematic ambiguity e.g., E-scheme vs p-scheme:
6. Gaussian filtering. Y. Lai, B. Cole arXiv:0806.1499 See QM 2009 S2A Talk by Y. Lai
Quark Matter 2009, Knoxville TN
The FastJet Algorithms
Suite of modern Collinear and infrared safe jet algorithms• sequential recombination: kT, Cambridge/Aachen, anti-kT
• cone: SISCone (Seedless Infrared-safe Cone)Motivated by high precision jets in high luminosity p+p at LHC (pileup)
• but directly applicable to heavy ion collisions
Two important algorithmic advances:• Large improvements to processing time vs. event multiplicity• Rigorous definition of jet area for subtraction of diffuse event background
M. Cacciari, G. Salam 0707.1378 [hep-ph]M. Cacciari, G. Salam, G. Soyez 0802.1188 [hep-ph]FastJet – http://www.lpthe.jussieu.fr/~salam/fastjet
pT (Jet Measured) ~ pT(Parton) + × A(Jet) ± A(Jet)
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A, ρ, σ are all measurable quantities!
STAR Preliminary
Au+Au Central
A= Jet Area = Diffuse noise, =noise fluctuations
Quark Matter 2009, Knoxville TN
Heavy Ion Background Discussion
pT (Jet Measured) ~ pT(Parton) + × A(Jet) ± A(Jet)
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A= Jet Area = Diffuse noise, =noise fluctuations
Fundamental Assumption: Two separable components: signal and background.
How might it be violated?1. Biases in background estimation due to presence of a jet.
a) Initial state radiation (Expected to be small compared to jet energy). b) Final state “out-of-cone” radiation.
2. Different Algorithms respond differently to background. (kT and Anti-kT)
Quark Matter 2009, Knoxville TN
Cone jetAnti-kT jet
KT jet
Jets in p+p at the Tevatron
Cone Algorithm kT Algorithm
http://www-cdf.fnal.gov/physics/new/qcd/QCD.html
Cone and kT jet spectra are consistent
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Cone Algorithm
Inclusive jet cross section over many orders of magnitude consistent with the NLO QCDInclusive jet cross section over many orders of magnitude consistent with the NLO QCD
http://www-cdf.fnal.gov/physics/new/qcd/QCD.html
Jets in p+p at the Tevatron
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Cone and kT jet spectra are consistent
Quark Matter 2009, Knoxville TN
Phys. Rev. Lett. 97 (2006) 252001
Jets in p+p at RHIC
Reconstructed by a mid-point jet cone algorithm with R = 0.4
Experimental uncertainty ~50%
STAR jet reconstruction: • neutral energy from Barrel EMC• charged hadrons from TPC
Agrees with NLO p-QCDAgrees with NLO p-QCD
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Towards Jets in A+A at RHIC
Phys. Rev. Lett 96 202301 (2006)
Phys. Rev. Lett. 91 (2003) 072304 Phys. Rev. Lett. 97 (2006) 162301
Phys.Rev.Lett.97:162301,2006
High pT hadron suppression described by pQCD+partonic energy lossMedium seems to be transparent to photons colored medium.
Conclusive evidence for large partonic energy loss in dense matter (final state effect)Conclusive evidence for large partonic energy loss in dense matter (final state effect)
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Jet quenching via inclusive hadrons: Quantitative Understanding?
S. A. Bass, C. Gale, A. Majumder , C. Nonaka, G. Qin, T. Renk, J. Ruppert 0808.0908 [nucl-th]
Good fit of theory to data but limited discrimination of underlying physics.
Theory: Modifications of jets in a 3-D hydrodynamic medium
All calculations have same initial structure, final vacuum fragmentation, nuclear geometry.
Parameters can be adjusted to describe data well: varies between 4-18 GeV/c2
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TECHQM!
Di-Hadrons : Quantitative Understanding?
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Di-hadron suppression not yet well-described by NLO theory
H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, 212301 (2007)J.L. Nagle arXiv:0805.0299 [nucl-ex]J. Adams, et al Phys Rev. Lett. 97, 162301 (2006) See the next talk by J. Nagle
reco
il y
ield
per
trig
ger
Vary energy loss parameterzT=pT
recoil/pTtrig
Quark Matter 2009, Knoxville TN
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New developments are in progress!
H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, 212301 (2007)J.L. Nagle arXiv:0805.0299 [nucl-ex]J. Adams, et al Phys Rev. Lett. 97, 162301 (2006) See the next talk by J. Nagle
See the talks in S3Aby A. Hamed, M. Connors, A. Hanks
ZOWW arXiv:0902.4000
reco
il y
ield
per
trig
ger
reco
il y
ield
per
trig
ger
zT=pTrecoil/pT
trig
Di-Hadrons : Quantitative Understanding?
Quark Matter 2009, Knoxville TN
Vary energy loss parameter 0
Full Jet Reconstruction in Heavy Ion Collisions
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Goal is Unbiased Jet Reconstruction:
Reconstruct partonic kinematics independent of fragmentation details - quenched or unquenched.
trigger
recoil
Why Pursue Full Jet Reconstruction?
• Enables study of jet quenching at the partonic level.
• Uniquely large kinematic reach
• In A+A much reduced geometric biases, full exploration of quenching.
• Multiple channels for consistency checks: Inclusive, di-jets, h-jets, gamma-jets
• Qualitatively new observables: energy flow, jet substructure, fragmentation function
Multi-hadronic Observables:• Geometric Biases: dominated by jets that have not interacted! • Limited kinematic reach.• Jet energy not constrained.
Quark Matter 2009, Knoxville TN
Can we see jets at RHIC?
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STAR Preliminary
p+p
Quark Matter 2009, Knoxville TN
QM 2009 Talks by E. Bruna, H. Caines, M. Ploskon, J. Putschke
Can we see jets at RHIC?
STAR Preliminary
Au+Au Central
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PHENIX Preliminary
Cu+Cu
Quark Matter 2009, Knoxville TN
QM 2009 Talks by E. Bruna,M. Ploskon, J. Putschke, Y. S. Lai
Reconstructed Spectra in p+p and Cu+Cu
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QM 2009 Talk by Y. S. Lai
Large pT range within restricted PHENIX acceptance!
Unfolding of the spectra is last step!Forthcoming soon!
Quark Matter 2009, Knoxville TN
Extracting di-jet angular width
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QM 2009 Talk by Y. S. LaiQuark Matter 2009, Knoxville TN
No centrality dependence on the widths!
ET [GeV]
dNJe
t/dE T (
per e
vent
) Nbin scaled p+p
Reconstructed Jet Spectra & Corrections:
Au+Au 0-10% ST
AR P
relim
inar
y
MB-Trig
R=0.4pT
cut =1 GeVSeed=4.6 GeV
LOHSC
S. Salur [STAR Collaboration], arXiv:0809.1609 [nucl-ex]
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MB-Trig: Minimum Bias Trigger
Agreement with Nbin Scaled p+p (~50%).
p+p: Phys. Rev. Lett. 97 (2006) 252001
Quark Matter 2009, Knoxville TN
ET [GeV]
dNJe
t/dE T (
per e
vent
) Nbin scaled p+p
Reconstructed Jet Spectra & Corrections:
Au+Au 0-10% ST
AR P
relim
inar
y
MB-Trig
R=0.4pT
cut =1 GeVSeed=4.6 GeV
LOHSC
S. Salur [STAR Collaboration], arXiv:0809.1609 [nucl-ex]
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MB-Trig: Minimum Bias Trigger
Suppression of backgrounds in heavy ions: Limit jet resolution parameter R Cut on track/calorimeter pT
Agreement with Nbin Scaled p+p (~50%).
p+p: Phys. Rev. Lett. 97 (2006) 252001
Quark Matter 2009, Knoxville TN
ET [GeV]
dNJe
t/dE T (
per e
vent
)
MB-Trigger: Agreement with Nbin Scaled p+p (~50%).HT-Trigger: Bias towards hard fragmentation:
Bad for quenching Studies!
Nbin scaled p+p
Reconstructed Jet Spectra & Corrections:
Large HT-trigger bias persists at least to 30 GeV. Similar to leading particle bias.
Au+Au 0-10% ST
AR P
relim
inar
y
MB-TrigO HT-Trig
R=0.4pT
cut =1 GeVSeed=4.6 GeV
LOHSC
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MB-Trig: Minimum Bias Trigger
S. Salur [STAR Collaboration], arXiv:0809.1609 [nucl-ex]
What about other algorithms?
p+p: Phys. Rev. Lett. 97 (2006) 252001
Quark Matter 2009, Knoxville TN
Suppression of backgrounds in heavy ions: Limit jet resolution parameter R Cut on track/calorimeter pT
HT-Trig: Satisfied Minimum Bias and requires a pion/photon with pT>7.5 GeV
PT Cut
KT KT KT
STAR
Pre
limin
ary
STAR
Pre
limin
ary
STAR
Pre
limin
ary
Au+Au 0-10% MB-Trig Nbin Scaled p+p
Au+Au 0-10% MB-Trig Nbin Scaled p+p
Au+Au 0-10% MB-Trig Nbin Scaled p+p
PT Cut Dependence Bias
Imprecise subtraction of underlying event?Do we introduce a bias with pT-cuts?Sensitivity to fragmentation model?
S. Salur [STAR Collaboration], arXiv:0810.0500 [nucl-ex]
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p+p: Phys. Rev. Lett. 97 (2006) 252001
STAR Preliminary
Au+Au Central
pTcut
Quark Matter 2009, Knoxville TN
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Un-Biased Jet Measurements
1) Minimize the kinematic cuts, e.g PTCut
2) Data driven corrections :a. Experimental characterization of background fluctuations. b. Detailed unfolding of fluctuations. Correcting for smearing
Cros
s-Se
ction
ET
Unfolding bgd from signal
Correct via “unfolding” for the “min-bias” jet reconstruction.
LOHSC
seed=4.6 GeVR=0.4
PyDetPyEmbedPyTrue
ET [GeV]
dNJe
t/dE T (
a.u.
)
STAR Preliminary
Quark Matter 2009, Knoxville TN
S. Salur [STAR Collaboration], arXiv:0809.1609 [nucl-ex]
See QM 2009 S2A Talk by: M. Ploskon
Inclusive jet spectrum:
p+pp+p
STAR Preliminary
STAR
Pre
limin
aryAu+AuAu+Au
Anti-kT and kT jet spectra are consistent.
BEMC calibration UncertaintyBEMC calibration Uncertainty
BEMC calibration UncertaintyBEMC calibration Uncertainty
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Unfolding Uncertainty
Unfolding uncertainty corresponds to a factor of 2 in jet cross-section.
Quark Matter 2009, Knoxville TN
See QM 2009 S2A Talk by: M. Ploskon
RAA of Jets
R = 0.4R = 0.4A large fraction of jets are reconstructed! (Compare pion Rπ
AA = 0.2)
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STAR Preliminary
BEMC calibration UncertaintyBEMC calibration Uncertainty
Quark Matter 2009, Knoxville TN
See QM 2009 S2A Talk by: M. Ploskon
What happens at high pT?
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W. Vogelsang Private Communication
p+p
Relative contributions of quark and gluon vary.
What about quenching dependence on parton species?
Relative contribution of sub-processes to inclusive jet production
Quark Matter 2009, Knoxville TN
What happens at high pT?
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The EMC Effect: Deviation between structure Functions of Au and deuterium.
W. Vogelsang Private Communication
p+p
Initial state effects at large x ~15%σΑ /
σd
What about other high x effects?
Relative contribution of sub-processes to inclusive jet production
Relative contributions of quark and gluon vary.
J. Gomez et al., SLAC–PUB–5813 June 7, 2001 D.F. Geesaman et al., Ann. Rev. Nucl. Part. Sci. 45, 337 (1995)B. A. Cole. et al, arXiv:hep-ph/0702101
SLAC E139
Quark Matter 2009, Knoxville TN
See QM 2009 S2A Talk by: M. Ploskon
STAR Preliminary
STAR
Pre
limin
ary
Jet Energy Profile:
p+pp+p
Au+AuAu+Au
BEMC calibration UncertaintyBEMC calibration Uncertainty
BEMC calibration UncertaintyBEMC calibration Uncertainty
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Au+Au: Stronger decrease in yield within R=0.2 as compared to R=0.4
Quark Matter 2009, Knoxville TN
Cross-section ratios in p+p and Au+ Au with R=0.2/R=0.4
p+pIncrease in the ratio with increasing pT.more focused cone with increasing jet pT
Au+AuDecrease in the ratio with increasing pT.
See QM 2009 S2A Talk by: M. Ploskon
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STAR
Pre
limin
ary
STAR
Pre
limin
ary
Quark Matter 2009, Knoxville TN
Jet Energy Profile:
Quantitative differences due to jet resolution parameter R.Evidence of broadening of the jet energy profile due to quenching!Is R=0.4 large enough to reconstruct the jets in an unbiased way?
Nicolas Borghini arXiv: 0902.2951
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QM 2009 Talk by K. Zapp
JEWEL (Jet Evolution with Energy Loss):K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv:0805.4759Parton shower with microscopic description of interactions with medium
Q-Pythia: N. Armesto, L. Cunqueiro and C. A. Salgado arXiv:0809.4433[hep-ph]MC implementation in Pythia of medium-induced gluon radiation through an additive term in the vacuum splitting functions. QM 2009 Talks by N. Armesto and Salgado
Analytic Calculations:
Quantitative analysis of data requires model building…
Many more….PYQUEN (Lokhtin, Snigriev), PQM (Dainese, Loizides, Paic), HIJING (Gyulassy, Wang)…See other QM2009 talks.
Quark Matter 2009, Knoxville TN
YaJEM:T. Renk arXiv:0808.1803QM 2009 Talk by T. Renk
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We need to confront the calculations with data!
Nicolas Borghini arXiv: 0902.2951
QM 2009 Talk by K. Zapp
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv:0805.4759
Strong broadening of shower in transverse momentum with respect to jet axis.
Angular distribution becomes wider!
No strong broadening of shower when PT
cut >2 GeV is selected. (limitations of broadening observable)
Analytic Calculations vs New Monte Carlos
Quark Matter 2009, Knoxville TN
Fragmentation Functions from di-jets
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“recoil” jet“recoil” jet
“trigger” jet“trigger” jet
Quark Matter 2009, Knoxville TN
Large HT-trigger bias in FF
No-trigger bias in FF
Fragmentation Functions from di-jets
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“recoil” jet“recoil” jet
“trigger” jet“trigger” jet
Quark Matter 2009, Knoxville TN
Large HT-trigger bias in FF
No-trigger bias in FF
20<pt,rec(AuAu)<25 GeV ⇒ < pt,rec(pp)> ~ 18 GeV
pT(trigger) > 10 GeV & PTcut=2 GeV
20<pT(recoil jet) < 25 GeV & PTcut=0.1 GeV
STAR Preliminary
See QM 2009 S2A: Talk by E.Bruna
Apparent modification in the z of Au+Au with respect to p+p.
But a biased population of jets.
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv:0804.3568
K. Zapp Talk 2009 QM
Significant uncertainties due to the sensitivity to hadronisation: Look for new observables unaffected by the hadronisation.
Parton vs Hadron
Clear increase in multiplicity due to radiative energy lossCollisional energy loss when recoils are counted toward the jet
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QCD JET Observables
pT cut infrared safe insensitive observables! : number of subjets, thurst …
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv:0804.3568
In vacuum (LEP) data well understood in pQCD
QM 2009 Talk by K. ZappMedium Induced Radiation More Coarser Jet Structure
Quark Matter 2009, Knoxville TN
Cone jetAnti-kT jet
KT jet
Another way to do it: Jet quenching at the LHC
P. Jacobs and M. van LeeuwenNucl. Phys A774, 237 (2006)
High pT Jets well above the background at LHC
Pb+Pb at 5.5 TeV:enormous jet energy range qualitatively new probes
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N. Grau for ATLAS
Copious production of hard probes : Jets, charm & bottom…
Quark Matter 2009, Knoxville TN
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LHC
ALICE EMCAL Physics Performance Update, CD-2
ALICE
• Pb+Pb background seems to be under control for the reconstructed jet-energy.
CMS Eur. Phys. J. 50 (2007) 117
+jet (Z+jet) cleaner means to determine FF
QM 2009 Talks by:N. Grau, G. VeresPoster by M. Heinz …
• Detector Upgrades: 2 super modules are installed for ALICEFull azimuthal calorimetric coverage for ATLAS & CMS
ATLASN. Grau for ATLAS
Quark Matter 2009, Knoxville TN
Conclusions: Why Pursue Full Jet Reconstruction?
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Full jet reconstruction gives access to the full spectrum of fragmentation topologies:
• Enables study of jet quenching at the partonic level.
“When you have completed 95 percent of your journey, you are only halfway there.”
Japanese Proverb
– First full jet reconstruction at RHIC (0-10% central heavy ion collisions - reach is up to 50 GeV).
– Nbin scaling (50% Syst Uncert.) observed for the least biased case, R=0.4 and pTcut=0.1 GeV
• Uniquely large kinematic reach
– New theory developments FASTJET and New medium-modified shower MC codes… Q-Pythia, JEWEL,…
– But beware of biases: data taking and selection of particles (pTcut, R)
– Path length and jet radius dependencies.– Session 2A
• In A+A much reduced geometric biases, full exploration of quenching.
• Multiple channels for consistency checks: Inclusive, di-jets, h-jets, γ-jets
• Qualitatively new observables: energy flow, jet substructure, fragmentation function
Quark Matter 2009, Knoxville TN
Thank you!
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Nestor ArmestoElena Bruna
Jana BielcikovaMatteo Cacciari
Helen CainesBrian Cole
David d'EnterriaNathan GrauJohn Harris
Wolf G. HolzmanPeter JacobsJan Kapitan
Yue LaiLeticia Mendez
Mateusz PloskonJoern PutschkeThorsten RenkGavin Salam
Carlos SalgadoGregory SoyezGabor Veres
Urs Achim WiedemannKorinna Zapp
…
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RAA of Jets
R = 0.4R = 0.4
R = 0.2R = 0.2
Jets are reconstructed in an unbiased way for the R=0.4
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STAR Preliminary
STAR Preliminary
See QM 2009 S2A Talk by: M. Ploskon
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The FastJet Measurement of the Jet Area
1. Add randomly distributed ghost particles of known density d to the event
2. Run the jet algorithm3. Count the number n of ghost particles
assigned to the jet4. Jet area A= n/d
Accounts for event-wise fluctuations in shape and area of jet
Jets are irregular objects!Jet area is non-trivial (≠ πR2)
Nuclear Modification Factors of Di-jets
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See QM 2009 S2A: Talk by E.Bruna
Maximum path length for the recoil! pT(recoil jet) large suppression
STAR Preliminary
“recoil” jet“recoil” jet
“trigger” jet“trigger” jet
Quark Matter 2009, Knoxville TN
Large HT-trigger bias in FF
No-trigger bias in FF
Significant suppression in di-jet coincidence
The FastJet Algorithms: Background Subtraction in Pb+Pb
Pb+Pb jet cross-section is recovered after the subtraction
M. Cacciari, G. Salam 0707.1378 [hep-ph]
M. Cacciari, G. Salam, G. Soyez 0802.1188 [hep-ph]FastJet – http://www.lpthe.jussieu.fr/~salam/fastjet
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Background subtraction works in the Heavy Ion environment.
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Anti-kT is resistant to absorption by background of jet energy at large radius
M. Cacciari, G. Salam, G. Soyez 0802.1189 [hep-ph]
The FastJet Algorithms: Background Subtraction in p+p
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Modified Fragmentation Function
Modified Leading Logarithmic Approximation:- good description of vacuum fragmentation (basis of PYTHIA)- introduce medium effects at parton splitting
Fragmentation is strongly modified at pThadron~1-5 GeV
Borghini and Wiedemann, hep-ph/0506218
=ln(EJet/phadron)
Jet quenching: Low pT enhancement
pT hadron~ 2 GeV
Fragmentation-function in Au+Au 0-20% and p+p
No apparent modification in the ξ of Au+Au with respect to p+p.
STAR Preliminary
STAR Preliminary
Where is the jet quenching? Biases: Online triggering, PT cut
J. Putschke [STAR Collaboration], arXiv:0809. 1419[nucl-ex]
See QM 2009 S2A: Talks by E.Bruna, H.Caines
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LOCone
FastJet kT
Good agreement between thealgorithms!
Quantitative Understanding: Di-Hadrons?
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vary model parameter 0
Di-hadron suppression not yet well-described by NLO theory
H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, 212301 (2007)J.L. Nagle arXiv:0805.0299 [nucl-ex]J. Adams, et al Phys Rev. Lett. 97, 162301 (2006)
zT=pTrecoil/pT
trig
p+p @ s = 200 GeV Au+Au @ sNN = 200 GeV Pb+Pb @ sNN = 5.5 TeV
A. Pulvirenti
q
q
hadrons
leadingparticle
jet production in quark matter
Multiple interaction inside the collision region Lose energy through medium induced gluon radiation
q
q
hadronsleadingparticle
leading particle
schematic view of jet production
hadrons
Jets at RHIC & LHC:
Early production from parton-parton scatterings Direct probes of partonic phase
Early production from parton-parton scatterings Direct probes of partonic phase
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Jet
Jet
Jet fragmentation getting more diffuse as jet propagate in the medium?
STAR Preliminary
STAR
Pre
limin
ary
Inclusive jet spectrum:
p+pp+p
BEMC calibration UncertaintyBEMC calibration Uncertainty
BEMC calibration UncertaintyBEMC calibration Uncertainty
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New analysis (kT & anti-kT) agrees with published STAR data (Mid-point cone)
Au+AuAu+Au
See QM 2009 S2A Talk by: M. Ploskon
Quark Matter 2009, Knoxville TN
Di-hadrons?
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Di-hadrons not yet well-constrained by NLO theory
zT=pTrecoil/pT
trig
Zhang et al. (ZOWW)PRL 98, 212301
See the next talk by Jamie Nagle
What’s happening!
55
Effect A:Effect A: Biased sample of jets due to the High-Tower Trigger: the HT trigger favors “surface” jets that are not modified by the medium Ejet (AuAu) = Ejet (pp) FF unmodified If this is true HT jets should not binary scale even without Ptcut!
Effect BEffect B: Biased sample of jets due to energy loss and pTcut The jet softens in the medium Its energy is not recovered with pTcut AND
assuming PYTHIA fragmentation Its energy is UNDERESTIMATED ξ=ln(pt
jet/pt) should be larger If this is true Quenching models could address this issue
dN/d
ξ
ξ
Au+Aup+p
dN/d
ξ
ξ
Au+Aup+p
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Fragmentation Functions
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See QM 2009 S2A: Talk by E.Bruna
pT(trigger) > 10 GeV & PTcut=2 GeV
pT(recoil jet) > 25 GeV & PTcut=0.1 GeV
No apparent modification in the z of Au+Au with respect to p+p.
Recoil jet
Trigger jet
STAR Preliminary
Fragmentation Functions
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See QM 2009 S2A: Talk by E.Bruna
pT(trigger) > 10 GeV & PTcut=2 GeV
20<pT(recoil jet) < 25 GeV & PTcut=0.1 GeV
Apparent modification in the z of Au+Au with respect to p+p.
Recoil jet
Trigger jet
STAR Preliminary
Jets look more softer and more diffuser!
H – recoil jet coincidences
STAR Preliminary
58Mateusz Ploskon, LBNL, STAR, QM'09
Incr
easi
ng P
ath
Leng
th
q-PYTHIA(http://igfae.usc.es/QatMC)
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Carlos Salgado Talk 2009 QMSevil Salur
Q-Pythia
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QCD Observables: (Analytic Calculations)
Nicolas Borghini arXiv: 0902.2951
Increase in the subjet distributions
Gluon Jet
Number of subjets vs Opening Angle
Multiplicity is sensitive to hadronisation. observe a strong broadening of the shower in kT
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Need theoretical understanding of jet energy loss
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QCD JET Observables
pT cut infrared safe insensitive observables! : number of subjets, broadening, energy flow…
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv:0804.3568
In vacuum (LEP) data well understood in pQCD
Need theoretical understanding of jet energy loss
100 GeV Jet
Theory: Jet Quenching – Energy LossElastic energy loss: Bjorken ’82Bremsstrahlung: Gyulassy, Wang, Plumer ’92
jet quenching measures color charge density, plasma transport coefficients
But quantitative analysis of data requires model buildingCurrent status: large discrepancies (factor~10) in extracted medium parameters (transport coefficients) → ongoing efforts to resolve this
Renk: medium increases virtuality of partons during evolution
PYQUEN (Lokhtin, Snigriev): PYTHIA afterburner reduces energy of final state partons and adds radiated gluons according to BDMPS expectations.
PQM (Dainese, Loizides, Paic): MC implementation of BDMPS quenching weights
HIJING (Gyulassy, Wang): jet and mini-jet production with induced splitting
JEWEL (Zapp, Ingelman, Rathsman, Stachel, Wiedemann): parton shower with microscopic description of interactions with medium
q-PYTHIA (Armesto, Cunquiero, Salgado, Xiang): includes BDMPS-like radiation in modified splitting function See many QM2009 talks…
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Jet Reconstruction: connect theory and experiment
Goal: re-associate hadrons to accurately reconstruct the partonic kinematics(infrared and collinear safe)
• pQCD theory calculates partons
• experiment measures fragments of partons: hadrons and calorimeter towers (clusters of hadrons)
Apply “same” jet clustering algorithm to data and theory.
hadrons
schematic view of jet production
hadrons
q
q
θ≈0
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Underlying event (UE) & pile-up are distributed uniformly in y and pT(Jet Measured) ~ pT(Parton) + <pT(UE)> X A(Jet)
22 pile-up in p+p +Pythia 6.325 dijet
AjAj
= Diffuse noise (pT added)
/<Aj>
Area Definition: Estimate the active area of each jet by filling event with many very soft particles then count how many are clustered into given jet
M. Cacciari, G. Salam 0707.1378 [hep-ph]
Study of PT/Aj determine the noise density ρon an event-by-event basis
The FastJet Algorithms
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Background in kT
Use the same approach for HIStudy the pT/Aj and remove the contribution Aj
The scaled pp cross-section is recovered after the subtraction M. Cacciari, G. Salam 0707.1378 [hep-ph]
Looks promising:Has to be studied further for RHIC & LHC with experimentalist tools
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M. Cacciari, G. Salam hep-ph/0512210
kT Algorithms are not so slow after all
Geometric nearest neighbor search
Voronoi Diagram
• Divide the plane into cells (one per vertex), • N points can be constructed with O (N ln N )
N3 Nlog(N)
Computational Geometry Algorithms Library
Orders of magnitude fasterLarge N region is feasible.
Geometrical and minimum-finding of the kt jet-finder require O (N ln N )
Event Characteristics: Jet Area & Fluctuations
Jet Area Background Fluctuations
Heavy-ion: Reduction in Jet Area & Increase in fluctuationsPythia Jets embedded in real Au+Au background events have the same area and fluctuations with that of Jets in real Au+Au data.
MB-trigPyEmbedPyTrue
R=0.4 R=0.4KT KT
pT cut = 0.1 GeV
pT cut = 0.1 GeV
Jet Area
MB-trigPyEmbedPyTrue
Counts
Counts
Sigma
STAR Preliminary
STAR Preliminary
Jet ET> 20 GeV
Au+Au 0-10% Au+Au 0-10%
M. Cacciari, G. Salam, G. Soyez 0802.1188 [hep-ph]
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Energy Resolution
LOHSC KT CAMBpTcut =1 GeV
R=0.4
Seed=4.6 GeV
Counts
∆E ∆E ∆E
Event by event comparison of PyTrue vs PyDet vs PyEmbed.
pTcut =1 GeV
R=0.4pT
cut =1 GeV R=0.4
E = EPyDet - EPyTrue
E = EPyEmbed – EPyDet
E = EPyEmbed - EPyTrue
Smearing due to background subtraction in Au+Au.
STA
R
Pre
limin
ar
y STA
R
Pre
limin
ar
y STA
R
Pre
limin
ar
y
Shift of median due to un-measured particles (n, K0L)
and the pT cut.
Tail at positive ∆E causes a kick in the spectrum.
ET=35±5 GeV ET=35±5 GeV ET=35±5 GeV
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Effect of Resolution on Spectrum
pTcut
=0.1 GeV
-Increase pT threshold: Reduce the effect of background fluctuations (jet reconstruction in 0-10% Au+Au is similar in p+p) - The pT cut is expected to produce biases.
Similar effects also observed for KT & Cambridge/Aachen
dN
Jet/d
ET (
a.u
.)
LOHSC
seed=4.6 GeVR=0.4
PyDetPyEmbedPyTrue
PyDetPyEmbedPyTrue
PyDetPyEmbedPyTrue
pTcut
=1.0 GeV
LOHSC
seed=4.6 GeVR=0.4
pTcut
= 2.0 GeV
LOHSC
seed=4.6 GeVR=0.4
ET [GeV]
STAR Preliminary
STAR Preliminary
STAR Preliminary
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Resolution and Efficiency & Acceptance Corrections
Resolution effect corrected assuming Pythia Fragmentation.Embed Pythia Jets in 0-10% Central Events with MBtrig.
ET [GeV]
dN
Jet/d
ET (
a.u
.)
LOHSC
ET [GeV]PyEm
bed /
PyTru
e
pT cut =0.1 GeV
R=0.4Seed=4.6 GeV
pol5
Use the fit functions from the ratio of PyEmbed to PyTrue to correct for energy resolution, efficiency & acceptance.
PyTruePyEmbed
PTcut LOHSC KT CAMB
0.1 GeV 0.2-10 1-4 2-6
1 GeV 0.2-1 0.7-1 1-2
2 GeV 0.2-0.3 0.5-1 0.5-1
ET-dependent correction factors
STAR Preliminary
STAR Preliminary
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ATLAS
• Efficiency and ET resolution as a function of jet ET and eta.
• Gamma-jet measurements possible !• Di-jet correlation expectations, • D(z) determination • Effective RAA of jets (reco/input)
Talk by N. Grau QM 2009
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CMS
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Talk by G. Veres QM 2009 CMS Eur. Phys. J. 50 (2007) 117
+jet (Z+jet) cleaner means to determine fragmentation function
Fake jet contamination
“Fake” jet rate estimation: • Central Au+Au dataset (real data)• Randomize azimuth of each
charged particle and calorimeter tower
• Run jet finder• Remove leading particle from
each found jet• Re-run jet finder
“Fake” jets: signal in excess of background model from random association of uncorrelated soft particles (i.e. not due to hard scattering)
STAR Preliminary
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