Underlying Event Studies at RHIC - Yale Universitycaines/Presentations/...Helen Caines – DPF 2009...

Underlying Event Studies at RHIC

Helen Caines - Yale University - for the STAR Collaboration

DPF 2009 Detroit, MI

July 29th 2009

Outline• Jets and our data set• z and ξ distributions• The underlying event • Summary and outlook

Helen Caines – DPF 2009 2

STAR : PRL 97 (2006) 252001

Jets at RHIC – a calibrated probe?

• Jet cross-section in p+p is well described by NLO pQCD calculations over 7 orders of magnitude.• Excellent description when included in world data

T. Kluge, K.Rabbertz, M.Wobish


STAR : PRL 97 (2006) 252001

• Minimum bias particle production in p+p also well modeled.


STAR : PLB 637 (2006) 161S. Albino et al, NPB 725 (2005) 181• Jet cross-section in p+p is well described by NLO pQCD

calculations over 7 orders of magnitude.• Excellent description when included in world data



STAR : PRL 97 (2006) 252001

• Minimum bias particle production in p+p also well modeled.


STAR : PLB 637 (2006) 161S. Albino et al, NPB 725 (2005) 181• Jet cross-section in p+p is well described by NLO pQCD

calculations over 7 orders of magnitude.• Excellent description when included in world data

What about fragmentation?


Helen Caines – DPF 2009

Fragmentation functions (FF)

CDF

Sapeta&Wiedemann, hep-ph/0707.3494

• No previous comparisons at RHIC energies available.

• Measurements at higher √s agree well with theory.

MLLA Theory

Test energy scaling of fragmentation functions.

3


Fragmentation functions (FF)

CDF

Sapeta&Wiedemann, hep-ph/0707.3494

• No previous comparisons at RHIC energies available.

• Measurements at higher √s agree well with theory.

MLLA Theory

• FF are particle species dependent.

Need to study composition of jets and complete event.

Test energy scaling of fragmentation functions.

3


Jets at RHIC: √s=200 GeV p+p

4

•Unpolarized measurements are a crucial part of the RHIC program

De Florian,Vogelsang, hep-ph 0704.1677

√s=200 GeV•Inclusive hadron and jet cross section measurements at RHIC add new results to existing data from other accelerators at different energies

•Constrain fragmentation functions:• Fits currently dominated by e+e-

data• Still large uncertainties, especially

in the gluon fragmentation functions

Significant contribution from gluons in the RHIC regime


There is also the Underlying Event

5

• p-p events are complicated. More than just hard scattering.

• Underlying Event: soft or semi-hard multiple parton interactions (MPI), initial & final state radiation, beam-beam remnants

The Underlying Event is everything BUT the hard scattering

Figure from Rick Field


Energy Scaling of the Underlying event

6

• An important scaling factor is the hard scattering cut-off for the MPI in UE:

PT0(Ecm) = PT0(Ecm/Eo)ε

From Rick Field

• PYTHIA is tuned to 1.8 TeV - does the tune scale to another collision energy.

• ε = 0.16 - initial estimate

= 0.25 (suggested by 630 GeV Tevatron)

• Pivots around the tuning energy

Correct scaling could improve LHC predictions prior to turn-on


Effect of hard scattering cut-off scaling

From Rick Field

• Increasing ε creates smaller energy dependence for UE

➝ 35% more RHIC➝ 26% less LHC

• ε = 0.16 (DWT) ➝ 0.25 (DW)

Measurable effect at RHIC

RHIC 200 GeV

LHC 14 TeV


The p+p data set - √s = 200 GeV

• TPC tracks to identify charged particles contribution.

• Barrel EMCal for neutral energy contribution.

2006 RunSampled luminosity for Jet-Patch triggers: ~8.7 pb-1

(~8 M events)

Jet-Patch Trigger: BBC coincidence + EMCal Jet-Patch

Jet-Patch: ET > 8 GeV in Δη x Δφ = 1x1

8


The p+p data set - √s = 200 GeV

• TPC tracks to identify charged particles contribution.

• Barrel EMCal for neutral energy contribution.

2006 RunSampled luminosity for Jet-Patch triggers: ~8.7 pb-1

(~8 M events)

Jet-Patch Trigger: BBC coincidence + EMCal Jet-Patch

Jet-Patch: ET > 8 GeV in Δη x Δφ = 1x1

Jet-Patch - NEF FF bias - use non-triggered jet for studies.

8


Jet reconstruction - algorithms

• Rcone=√(Δφ2+Δη2)

• all particles used.• Splitting/Merging destroys cone shape.

Fastjet package - [Cacciari, Soyez, arXiv:0704.0292]Seedless Cone - SISCone

9





Fastjet package - [Cacciari, Soyez, arXiv:0704.0292]

[Cacciari, Salam, Soyez, arXiv:0802.1189]

Seedless Cone - SISCone

Recombination kT

Anti-kT

• starts from lowest pT. • merges weighted by 1/pT i.e. high pT is dis-favored.

• starts from high pT.• merges weighted by pT i.e. low pT is dis-favored.

9





Fastjet package - [Cacciari, Soyez, arXiv:0704.0292]

[Cacciari, Salam, Soyez, arXiv:0802.1189]

Seedless Cone - SISCone

Recombination kT

Anti-kT

• starts from lowest pT. • merges weighted by 1/pT i.e. high pT is dis-favored.

• starts from high pT.• merges weighted by pT i.e. low pT is dis-favored.

Compare results to explore effects in data

9

Radius of Jet Cone

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fra

cti

on

of

En

erg

y w

ith

in J

et

Co

ne R

ad

ius

0

0.2

0.4

0.6

0.8

1

< 30 GeV/cT

20 < Jet p

< 40 GeV/cT

30 < Jet p

< 50 GeV/cT

40 < Jet p


Jet reconstruction - the resolution parameter

10

PreliminarypT

(GeV/c)R

0.4R

0.720-30 77% 94%

30-40 83% 96%

40-50 89% 98%

% Energy within resolution parameter R

• Larger energy → more focussed jet.

• CDF > 80% R=0.3. (Jet pT~ 50 GeV)Compare FF using different radii.

|ηjet|<1-R

(GeV/c)TJet

Input p0 5 10 15 20 25 30 35 40 45 50

T)/

pT

(p!

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

SISCone

Tk

TAnti-k

Calculated in two way:

• Simulation• MC input compared to reconstructed output.

(GeV)TJet

Input p0 10 20 30 40 50 60

(G

eV

)T

Jet

Reco

nstr

ucte

d p

0

10

20

30

40

50

60

-310

-210

-110


Energy resolution - the jet energy scale

Anti-kT R<0.7

Ejet Pythia vs Ejet Reco

• Offset due to missing energy:• Detector efficiencies.• Undetected particles (n, K0L).

11

Preliminary

|ηjet|<1-R

|ηjet|<1-R

• Resolution ~15-20% for pTJet>15GeV/c.

(GeV/c)TJet

Input p0 5 10 15 20 25 30 35 40 45 50

T)/

pT

(p!

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

SISCone

Tk

TAnti-k

Calculated in two way:

• Simulation• MC input compared to reconstructed output.

(GeV)TJet

Input p0 10 20 30 40 50 60

(G

eV

)T

Jet

Reco

nstr

ucte

d p

0

10

20

30

40

50

60

-310

-210

-110


Energy resolution - the jet energy scale

Anti-kT R<0.7

Ejet Pythia vs Ejet Reco

• Offset due to missing energy:• Detector efficiencies.• Undetected particles (n, K0L).

(GeV/c)TJet

Input p0 5 10 15 20 25 30 35 40 45 50

T)/

pT

(p!

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

SISCone

Tk

TAnti-k

Simulation

Read data di-jets

• Real data• Energy balance of di-jets.

11

Preliminary

|ηjet|<1-R

|ηjet|<1-R

• Resolution ~15-20% for pTJet>15GeV/c.

Z

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dN

/dZ

Jet

1/N

-310

-210

-110

1

10

SISCone

TK

TAnti-K

Simulation

p-p data

Z

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dN

/dZ

Jet

1/N

-310

-210

-110

1

10

SISCone

TK

TAnti-K

Simulation

p-p data

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2SISCone

TK

TAnti-K

Pythia 6.4

p-p data

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8 SISCone

TK

TAnti-K

Pythia 6.4

p-p data


ξ and z distributions for charged hadrons

R=0.420 <Jet pTreco< 30 GeV/c

•20-30 GeV•20-30 GeV

30 <Jet pTreco< 40 GeV/c

Reasonable agreement between data and PYTHIA+GEANT. 12

Preliminary

|ηjet|<1-R

Preliminary

|ηjet|<1-R

Preliminary

|ηjet|<1-R

Preliminary

|ηjet|<1-R

Data not corrected to particle

level.

“PYTHIA” =

PYTHIA +GEANT

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2SISCone

TK

TAnti-K

Pythia 6.4

p-p data

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2SISCone

TK

TAnti-K

Pythia 6.4

p-p data

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8 SISCone

TK

TAnti-K

Pythia 6.4

p-p data

)TPart

/pTjet

(=ln(p!0 1 2 3 4 5

!d

N/d

jet

1/N

0.5

1

1.5

2

2.5 SISCone

TK

TAnti-K

Pythia 6.4

p-p data


Charged hadrons ξ for different R and jet pT

20<pTreco<30 GeV/c 30<pTreco<40 GeV/c

R=0.4

R=0.7

Agreement similar between PYTHIA and data for both radii. 13

Preliminary

PreliminaryPreliminary

Preliminary

|ηjet|<1-RpTtrack > 0.2

Data not corrected to particle level.

“PYTHIA” = PYTHIA+GEANT


Measuring the Underlying Event

14

Define:• |Δφ| − Angle relative to leading jet

• “Toward” |Δφ| < 60o

• “Away” |Δφ| > 120o.

• “Transverse” 60o < |Δφ| < 120o

• TransMax - Trans. region with highest ΣpT or ΣNtrack

• TransMin Trans. region with least ΣpT or ΣNtrack

Underlying Event is the data in the Transverse regions.


Sensitivities of the variablesleading : Most basic jet cut, one jet in our acceptance.

back-to-back : Sub-set of leading jet collection. Require |Δφ| > 150, pTAway/pTLead > 0.7 Suppresses hard initial and final state radiation.

TransMin : Sensitive to beam-beam remnants and soft multiple parton interactions.

TransMax : Enhanced probability of containing hard initial and/or final state radiation component.

15


Sensitivities of the variablesleading : Most basic jet cut, one jet in our acceptance.

back-to-back : Sub-set of leading jet collection. Require |Δφ| > 150, pTAway/pTLead > 0.7 Suppresses hard initial and final state radiation.

TransMin : Sensitive to beam-beam remnants and soft multiple parton interactions.

TransMax : Enhanced probability of containing hard initial and/or final state radiation component.

15

Compare TransMin and TransMax data from leading and back-to-back jet samples →

Information about large angle initial/final state radiation.

(GeV/c)T

Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SISCone

TK

TAnti-K

Jet

Max UE

Min UE

(G eV /c )T

Lead J et p0 5 1 0 1 5 2 0 2 5 30 35 40

of

ch

arg

ed

tra

ck

s (

Ge

V/c

)T

Me

an

p0

0 .5

1

1 .5

2

2 .5

3

3.5

4

4.5

SISC one

K t

Anti-K t

J et

M ax UE

M in UE


Underlying event vs jets properties

16


Preliminary

Back-to-Back, R=0.7, |ηjet| < 1-R, pTtrack > 0.2 GeV/c

UE largely independent of jet pT

Preliminary

• Jet charged track density and <pT> rise with jet pT as expected

(GeV/c)T

Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SISCone

Kt

Anti-Kt

Pythia

Jet

Max UE

Min UE

(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

of

ch

arg

ed

tra

ck

s (

Ge

V/c

)T

Me

an

p

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SISCone

Kt

Anti-Kt

Pythia

Jet

Max UE

Min UE

Preliminary


Checking energy scaling at RHIC

17

RHIC data support ε = 0.25

Preliminary

Back-to-Back, R=0.7, |ηjet| < 1-R, pTtrack > 0.2 GeV/c

• Many standard PYTHIA tunes (including those labeled “ATLAS” in PYTHIA) tunes have ε = 0.16 this is INCORRECT activity in min-bias events wrong

MPI scaling factor ε = 0.25 Data not corrected to particle level, “PYTHIA” = PYTHIA +GEANT


TransMin vs TransMax regions of UE

18

CDF √s=1.96 TeV• leading TransMax > back-to-back TransMax Significant initial/final state radiation at large angles.


(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lead Jet Data

Back-to-Back Jet data

Max UE

Min UE



18


STAR √s=200 GeV• leading TransMax ~ back-to-back TransMax

SISCone,R=0.7, |ηjet| < 1-R, pTtrack > 0.2 GeV/c

Preliminary

Small initial/final state radiation at large angles.


(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lead Jet Data


Max UE

Min UE



18




Preliminary

• TransMax > TransMin



(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lead Jet Data


Max UE

Min UE



18



(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

!d"

/dc

hd

N

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lead Jet Data


Max UE

Min UE


Preliminary

• TransMax > TransMin


Poisson distribution with average dNch/dηdφ = 0.36

• UE ~independent of jet pT.


(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

of

ch

arg

ed

tra

cks (

GeV

/c)

TM

ean

p

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SISCone

Kt

Anti-Kt

Pythia

Jet

Max UE

Min UE


Mean pT charged tracks

19

• Agreement between PYTHIA and data OK

, Back-to-Back, R=0.7, |ηjet| < 1-R, pTtrack > 0.2 GeV/c Data not corrected to particle level. “PYTHIA” = PYTHIA +GEANT

Preliminary

(GeV/c)T

Lead Jet p0 5 10 15 20 25 30 35 40

of

ch

arg

ed

tra

cks (

GeV

/c)

TM

ean

p

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SISCone

Kt

Anti-Kt

Pythia

Jet

Max UE

Min UE


Mean pT charged tracks

19

CDF higher than STAR merely due to lower pT cut?

• Agreement between PYTHIA and data OK

, Back-to-Back, R=0.7, |ηjet| < 1-R, pTtrack > 0.2 GeV/c Data not corrected to particle level. “PYTHIA” = PYTHIA +GEANT

Preliminary

Mean Charged

Track pT

!"#$%&

'#$(

!%$)*+,%*,

-.

/$0$1!

UE <Data> <Pythia>

CDF 1.1 1.0

STAR 0.55 0.55

“Back-to-Back”


!"#"$%&'()$$*+,-./$0#12034$5&6.,78&,8$&8/-9.,$59$,5':;$5<8$'.:8&=;-./

878.5$-.$&'.$>65$5<8$?8765&9.@$+03ABCADDEFC)$>EEG"

“Back-to-Back”!"#$%&

'#$(

!%$)*+,%*,

-.

/$0$1!


Max Charged Track pT

UE <Data> <Pythia>

CDF 1.2 1.0

STAR 0.65 0.6

!"#$%&

'#$(

!%$)*+,%*,

-.

/$0$1!

“Back-to-Back”

!"#"$%&'()$$*+,-./$0#12034

5&6.,78&,8$&8/-9.,$59$,5':;$5<8

'.:8&=;-./$878.5$-.$&'.$>65$5<8

?8765&9.@$+03ABCADDEFC)$>EEG"



Max pT charged track

20

RHIC UE is a little softer

Data not corrected to particle level“PYTHIA” = PYTHIA +GEANT


Summary & outlook• Different jet algorithms produce consistent results • Charged hadron ξ and z distributions at √s=200 GeV similar to PYTHIA 6.4.• Underlying Event largely decoupled from hard scattering.• The energy scaling suggested by PYTHIA for the MPI more accurate in the newer tunes - better predictions for LHC • Large angle initial/final state radiation is small. • Particle pT spectra are significantly softer out of the jet cone compared to in the jet.

Outlook

• Compare more jet-variables (kT, jT, etc) to pQCD models.• Look at particle composition in and out of jets • Repeat measurements at √s=500 GeV.• Measure PID FF in heavy ion collisions.

21

(GeV/c)T

Particle p0 2 4 6 8 10

Td

p!

d"

dN

/dT

1/N

1/p

-810

-710

-610

-510

-410

-310

-210

-110

1

10Jets

Transverse Max

Transverse Min

Min-Bias


pT spectra in jet, UE, Min-Bias event

22

Minbias close to but not equal to UE

Preliminary ⟨pT⟩GeV/c

Jet 1.2

Max UE 0.5

Min UE 0.4

Min-Bias 0.6

Uncorrected Spectra

•15<pTjet<20 GeV/c, |ηjet|<1-R, R=0.7

(GeV/c)T

Particle p0 2 4 6 8 10

Td

p!

d"

dN

/dT

1/N

1/p

-810

-710

-610

-510

-410

-310

-210

-110

1

10Jets

Transverse Max

Transverse Min

Min-Bias

(GeV/c)T

Particle p0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Td

p!

d"

dN

/dT

1/N

1/p

-310

-210

-110

1

10

210

Jets

Transverse Max

Transverse Min

Min-Bias


pT spectra in jet, UE, Min-Bias event

22

Minbias close to but not equal to UE

Preliminary ⟨pT⟩GeV/c

Jet 1.2

Max UE 0.5

Min UE 0.4

Min-Bias 0.6

Uncorrected Spectra

•15<pTjet<20 GeV/c, |ηjet|<1-R, R=0.7

Underlying Event Studies at RHIC - Yale Universitycaines/Presentations/...Helen Caines – DPF 2009...

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Transcript of Underlying Event Studies at RHIC - Yale Universitycaines/Presentations/...Helen Caines – DPF 2009...