Effects of initial-state radiation on jet measurements in

Jet Tools 2019Kaya Tatar 1

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


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.

https://doi.org/10.1016/j.physletb.2016.10.037

https://arxiv.org/abs/hep-ph/9608322


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 ?


https://arxiv.org/abs/hep-ph/9608322


Map to PDF – γ+jet at RHIC vs LHCLHCRHIC

Processes squeezed into high-x Much larger phase space


RHIC vs LHC : spectra and acoplanarityJet p

T

Extended power law ~ pT

- (n + c0 * log [pT/pT_0])

n ~ 3

n ~ 4.3


RHIC vs LHC : jet substructureFragmentation Function Jet shape

high-pT

particles

low-pT

particlesp

T > 1 GeV/c for all particles


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)


ISR widens angular corr : RHIC vs LHCLHCRHIC

Smaller phase space at RHIC == > smaller ISR effect


Effect of ISR on spectraPhoton p

TJet p

T

ISR == > smaller jet pT


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


Effect of ISR on jet shape (JS)LHCRHIC

10-20% effectpT > 1 GeV/c for all particles

ISR widens jet shape

6-8% effect


ISR effect for standard kinematicsLHCRHIC

~20%

~95%

Can reduce ISR effects by playing with objects ?


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


in Heavy Ion Background

γ


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

https://github.com/JetQuenchingTools/JetToyHI/blob/master/include/thermalEvent.hh

https://doi.org/10.1103/PhysRevC.87.014902

https://doi.org/10.1103/PhysRevC.83.024913

https://doi.org/10.1103/PhysRevLett.116.222302


FF and JS in toy UE

No significant change ~20% change at small r== > Particles from UE pull the jet axis


FF and JS in toy UE – high pT

Effect reduced at higher jet pT


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

http://fastjet.fr/repo/fastjet-doc-3.2.2.pdf


FF and JS in toy UE – use WTA scheme

JS much more robust with WTA


Angular correlation in toy UE – 1

RHIC

Standard jet axis (E scheme)

3π/4 π

PLB 773 (2017) 672



Angular correlation in toy UE – 2

RHIC

Standard jet axis (E scheme) WTA jet axis

3π/4 π WTA reduces distortions due to UE


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.


BACKUP


γ+jetProduction


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


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


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


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

https://indico.cern.ch/event/634426/contributions/3090477/attachments/1727783/2791534/20181004_songkyo_HP2018_v3.pdf


Map to PDF – γ+jet at RHIC vs LHC

B/c phase space is squeezed into high x

LHCRHIC


q/g fraction – γ+jet vs Z+jet


Mapping γ+jet to PDF – split q vs gγ+quark γ+gluon

dominantly via QCD Compton scattering dominantly via qqbar annihilation


RHIC vs LHC


RHIC vs LHC : spectraPhoton p

TJet p

T

Extended power law ~ pT

- (n + c0 * log [pT/pT_0])

n ~ 3

n ~ 4.3


RHIC vs LHC : angular / momentum correlations


Unity normalized momentum imbalance


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)

http://home.thep.lu.se/~torbjorn/talks/desy16a.pdf

http://home.thep.lu.se/~torbjorn/talks/desy16a.pdf


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


Start with default kinematic selections - xjgLHCRHIC


Increase pT thresholds - xjg

LHCRHIC


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


Effect of ISR on FF - ξTγ

LHCRHIC

high-pT

particles

pT > 1 GeV/c for all particleslow-p

T

particles


Hard parton radiates to large angles



Radiation up to large angles (ΔR ~ 1.5)

LHCLHC


Radiation to large angles – LHC vs RHIC



Recover 80% of parton energy atΔR~0.4 for LHCΔR~0.25 for RHIC


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


FF and JS in toy UE – high pT


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 ?


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

http://fastjet.fr/repo/fastjet-doc-3.2.2.pdf


JS in toy UE – subtract UE via event mixing

JS much more robust with WTAUE distorts JS if recombination is E-scheme


Angular correlation in toy UE – LHC – low pT

LHC



Angular correlation in toy UE - LHCStandard jet axis (E scheme) WTA jet axis

LHC


Angular correlation in toy UE - RHIC

RHIC

Sharper angular correlation at RHIC – > more sensitive to resolution



Prompt vs isolated γ+jet


q/g fraction – isolated γ+jetPRL 121, 242301 (2018)


High and low pT

Documents

Effects of initial-state radiation on jet measurements in