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Effects of initial-state radiation on jet measurements in heavy-ion collisions
Kaya TatarMassachusetts Institute of Technology
Heavy-ion Jet Substructure Workshop (Jet Tools 2019),University of Bergen, Norway
May 14, 2019
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Transverse Momentum Broadeningp
T-broadening = additional momentum perpendicular to jet direction
Radiative energy loss propotional to pT-width
Widens azimuthal correlation
PLB 763 (2016) 208
Sudakov (vacuum radiation) effects large at the LHC – > pT-broadening invisible
BDMPS frameworkarXiv:hep-ph/9608322
– > LHC observed jet quenching, but no pT-broadening.
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Transverse Momentum Broadening - 2
LHCRHIC PLB 773 (2017) 672
BDMPS frameworkarXiv:hep-ph/9608322
Indication of modification in RHIC data Not so much change in LHC data
How are vacuum processes different between RHIC and LHC ?
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Map to PDF – γ+jet at RHIC vs LHCLHCRHIC
Processes squeezed into high-x Much larger phase space
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RHIC vs LHC : spectra and acoplanarityJet p
T
Extended power law ~ pT
- (n + c0 * log [pT/pT_0])
n ~ 3
n ~ 4.3
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RHIC vs LHC : jet substructureFragmentation Function Jet shape
high-pT
particles
low-pT
particlesp
T > 1 GeV/c for all particles
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What widens azimuthal correlation ?
Turn off these processes one by one.Their absence reveals the impact on the observable.
ISR is the process that widens azimuthal angle correlation.
LHCProcesses in the evolution of hard scattering
Multiparton interactions (MPI)Initial-state radiation (ISR)Final-state radiation (FSR)
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ISR widens angular corr : RHIC vs LHCLHCRHIC
Smaller phase space at RHIC == > smaller ISR effect
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Effect of ISR on spectraPhoton p
TJet p
T
ISR == > smaller jet pT
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Effect of ISR on fragmentation functions (FF)LHCRHIC
high-pT
particlesISR reduces jet p
T – > fewer particles per jet
pT > 1 GeV/c for all particleslow-p
T
particles
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Effect of ISR on jet shape (JS)LHCRHIC
10-20% effectpT > 1 GeV/c for all particles
ISR widens jet shape
6-8% effect
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ISR effect for standard kinematicsLHCRHIC
~20%
~95%
Can reduce ISR effects by playing with objects ?
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Increase pT thresholds
LHCRHIC
~10%~60%
From Photon pT > 40 to p
T > 50 GeV/c
== > Drop stats. by ~20x
From Photon pT > 60 to p
T > 200 GeV/c
== > Drop stats. by ~100x
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in Heavy Ion Background
γ
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Creating a toy Underlying Event (UE)
1. Sample toy particles from thermal distributionf(p
T) ~ p
T * exp(-c * p
T)
Same as in JetTools on github : link
< pT > from PRC 87, 014902 (2013)
2. Number of particles taken from multiplicity measurements
RHIC : PRC 83, 024913 (2011)dN / dη ~ 500-700 for Cent:0-10%
LHC : PRL 116, 222302 (2016)dN / dη ~ 1500-2000 for Cent:0-10%
3. Shoot them into Pythia event
4. Cluster jets using all (Pythia+toy) particlesCorrect jet energy by subtracting energy of
toy particles – > JES/JER factored out
5. Construct observables using Pythia particles
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FF and JS in toy UE
No significant change ~20% change at small r== > Particles from UE pull the jet axis
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FF and JS in toy UE – high pT
Effect reduced at higher jet pT
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WTA recombination scheme - JS
Sharper JS with WTA
● Standard jet axis determined via E-scheme
– sum of 4-vec
● Winner-Take-All recombination scheme
– In particular WTA-pt-scheme
– Recombination pr of pi and pj where
== >New axis coincides with that of the harder component
Ref. FastJet v3.2.2 Doc
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FF and JS in toy UE – use WTA scheme
JS much more robust with WTA
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Angular correlation in toy UE – 1
RHIC
Standard jet axis (E scheme)
3π/4 π
PLB 773 (2017) 672
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Angular correlation in toy UE – 2
RHIC
Standard jet axis (E scheme) WTA jet axis
3π/4 π WTA reduces distortions due to UE
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Summary
● pT broadening expected to widen angular correlation
– Hard to observe if vacuum radiation effects large
● effects large at the LHC, small at RHIC● Initial-state radiation (ISR)
– Large ISR effects if large phase space
● ISR smaller at higher pT
– RHIC : small ISR, steep spectra
● Background effects
– Studied using gen-level Pythia embedded into toy heavy-ion UE
– UE particles pull jet axis – > distort angle measurements
– One solution is to change recombination scheme to WTA.
● Reduces resolution effects for JS and angular correlations
– Hope to find a better solution in this workshop
Acknowledgements : The MIT group's work was supported by US DOE-NP.
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BACKUP
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γ+jetProduction
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γ+jet in Pythia 8γ+quark
γ+gluon
dominantly via QCD Compton scattering
dominantly via qqbar annihilation
subdominantly via
Process here is prompt photon (not isolated photon)Quark dominated across the spectrum
80-90% from pT~ 50 GeV to ~ 1 TeV
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Mapping γ+jet to PDFγ+quark
γ+gluon
dominantly via QCD Compton scattering
dominantly via qqbar annihilation
subdominantly via
PDF vs x explored in photon+jet events
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Mapping γ+jet to PDF – large Q
High momentum transfer – > large x
Gluon PDF falls faster with x==>Gluon initiated processes decrease==>Quark jet fraction decreases
PDF vs x explored in photon+jet events
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q/g fraction – γ+jet at RHIC vs LHC
Almost no heavy quark eventsRapid decrease in quark fraction after p
T ~ 50 GeV
sPHENIX not expected to go beyond that pT
See HP2018 talk
LHCRHIC
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Map to PDF – γ+jet at RHIC vs LHC
B/c phase space is squeezed into high x
LHCRHIC
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q/g fraction – γ+jet vs Z+jet
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Mapping γ+jet to PDF – split q vs gγ+quark γ+gluon
dominantly via QCD Compton scattering dominantly via qqbar annihilation
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RHIC vs LHC
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RHIC vs LHC : spectraPhoton p
TJet p
T
Extended power law ~ pT
- (n + c0 * log [pT/pT_0])
n ~ 3
n ~ 4.3
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RHIC vs LHC : angular / momentum correlations
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Unity normalized momentum imbalance
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Hard scattering evolutionCTEQ/MCnet School, Talk by T. Sjöstrand
A lot of things happen already in ppMPI
Initial-state radiation (ISR)Final-state radiation (FSR)
...and
Beam Remnants (BR)
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Shape of momentum imbalance
Processes in the evolution of hard scatteringMultiparton interactions (MPI)Initial-state radiation (ISR)Final-state radiation (FSR)
Turn off these processes one by one.Their absence reveals the impact on the observable.
Left (Right) tail from FSR (ISR)
LHC
PLB 789 (2019) 167
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Start with default kinematic selections - xjgLHCRHIC
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Increase pT thresholds - xjg
LHCRHIC
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Effect of ISR on hard parton radiation
Hard – > parton produced at hard scatteringFinal – > daughters of “hard” partons right before hadronization
Fraction of energy radiated at angle ΔR Fraction of energy inside ΔR
Radiation up to large angles (ΔR ~ 1.5)
LHCLHC
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Effect of ISR on FF - ξTγ
LHCRHIC
high-pT
particles
pT > 1 GeV/c for all particleslow-p
T
particles
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Hard parton radiates to large angles
Hard – > parton produced at hard scatteringFinal – > daughters of “hard” partons right before hadronization
Fraction of energy radiated at angle ΔR Fraction of energy inside ΔR
Radiation up to large angles (ΔR ~ 1.5)
LHCLHC
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Radiation to large angles – LHC vs RHIC
Hard – > parton produced at hard scatteringFinal – > daughters of “hard” partons right before hadronization
Fraction of energy radiated at angle ΔR Fraction of energy inside ΔR
Recover 80% of parton energy atΔR~0.4 for LHCΔR~0.25 for RHIC
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Fragmentation function and jet shape
high-pT particles low-p
T particles
Fragmentation function (FF)Distribution of longitudinal momentum over particles
Jet shape (JS)Energy fraction radiated to transverse direction
pT > 1 GeV/c for all particles
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FF and JS in toy UE – high pT
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Particles from UE and jet axis
● High pT particles from UE pull the jet axis during clustering
● Different shape than with particles from mixed event
● What is distorted here is the jet axis, a vector
– Direction of change ambiguous
● Correlated with the position of particles
– Not reproduced by random smearing
● Need to redefine jet angle ?
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WTA recombination scheme - FF
● Standard jet axis determined via E-scheme
– sum of 4-vec
● Winner-Take-All recombination scheme
– In particular WTA-pt-scheme
– Recombination pr of pi and pj where
Ref. FastJet v3.2.2 Doc
Soft particles slightly removed
== >New axis coincides with that of the harder component
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JS in toy UE – subtract UE via event mixing
JS much more robust with WTAUE distorts JS if recombination is E-scheme
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Angular correlation in toy UE – LHC – low pT
LHC
Standard jet axis (E scheme) WTA jet axis
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Angular correlation in toy UE - LHCStandard jet axis (E scheme) WTA jet axis
LHC
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Angular correlation in toy UE - RHIC
RHIC
Sharper angular correlation at RHIC – > more sensitive to resolution
Standard jet axis (E scheme) WTA jet axis
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Prompt vs isolated γ+jet
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q/g fraction – isolated γ+jetPRL 121, 242301 (2018)
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High and low pT