ALPGEN + [PYTHIA Parameter Variation Effects]

Ben Cooper, Judith Katzy, Michelangelo Mangano, Andrea Messina, Liza MijovicThanks to Peter Skands for useful discussions.

10. September 2010

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Outline

Expanding on Alpgen + Pythia features reported/discussed during Ben’s talkyesterday.

1) Why is Perugia Soft + ALPGEN harder than Perugia Hard + ALPGEN?Which of the Pythia parameters (don’t) contribute to the observed difference?Sketch the main features of PYTHIA parameter variation and ALPGEN matchinginterplay.

Since it turns out PS-related parameters variation is of most relevance for the point 1):

2) Explore ISR and FSR PYTHIA parameter variation effects;Use Perugia 2010 as a central tune and increase/decrease ISR (P2010 ISR ↑, ISR ↓).Use Perugia 2010 as a central tune and increase/decrease FSR (P2010 FSR ↑, FSR ↓).Show the effects on jet-related distributions, x-sections (matching efficiency).

Hard process: V+jets, Pythia 6.4.23, pt-ordered shower, Alpgen: v2.13.

Will not address ALPGEN parameter variations.

2 / 26

Technical Information

hard process: W+ up to 4 jets @ LHC, 7 TeV

ALPGEN: 2.13ebeam 3500.

ih2 1

ickkw 1.

ptjmin 20.

drjmin 0.4

etajmax 3.0

njets: [0,1,2,3,4(inclusive)]

In addition: ET(CLUS)=ptjmin+5 GeV, R(CLUS)=drjmin, ETACLMAX=etajmax.

Pythia: 6.4.23, using the new PS/MI for all the plots.Pythia tunes (P. Skands, arXiv:1005.3457v2):Perugia soft (Psoft), Perugia hard (Phard), Perugia 0 (P0),Perugia 2010,relevant parameter variations will be explained on the fly.

Analysis:jets are obtained from generator-level stable particles using Fastjet (2.4.2)anti-kt alg. with R=0.4.Jet cuts :pt>20 GeV (or higher), and |η| <2.5.Normalization (when applied):

RL = 100 pb−1.

3 / 26

Psoft vs Phard Tune - 1

P0 = a central tune, incl. LEP Z, Tevatron MinBias and DY data.

Phard: more hard pertrubative, less soft non-perturbative activity than P0.

Psoft: less hard pertrubative, more soft non-perturbative activity than P0.

0 1 2 3 4 5 6

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P0 tune

Phard tune

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P0 tune

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1.5

4 / 26

Psoft vs Phard Tune - 2

P0 = a central tune, incl. LEP Z, Tevatron MinBias and DY data.

Phard: more hard pertrubative, less soft non-pertorbative activity than P0.

Psoft: less hard pertrubative, more soft non-pertorbative activity than P0.

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Phard tune

# of jets with pt>40 GeV

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P0 tune

Phard tune

(inclusive) jet pt

jet pt [GeV]50 100 150 200 250 300

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5 / 26

Psoft vs Phard Tune : hard ISR Parameters - 1

Producing 2 additional samples with P0 + (only) ISR parameters varied on top asin Phard and Psoft:

PARP(67): multiplicative factor of a hard scale that sets the allowed phase space forthe hardest ISR emission.PARP(64): multiplicative factor of the ISR evolution scale (Q2, p2

t ).MSTP(64): ISR λ is λMS (for MSTP(64)=2) or λCMW (for MSTP(64)=2).

P0: PARP(67)=1.0, PARP(64)=1.0, MSTP(64)=3,Phard: PARP(67)=4.0, PARP(64)=0.25, MSTP(64)=3,Psoft: PARP(67)=0.5, PARP(64)=2.0, MSTP(64)=2.

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P0 tune

ISR(Phard) sample

# of jets with pt>20 GeV

# of jets0 1 2 3 4 5 6

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ISR(Phard) sample

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6 / 26

Psoft vs Phard Tune : hard ISR Parameters - 2

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ISR(Phard) sample

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P0 tune

ISR(Phard) sample

(inclusive) jet pt

jet pt [GeV]50 100 150 200 250 300

dist

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1.5

effect on high-pt jet #, pt, observed, but not as large as in Psoft, Phard,

little (wrt. Psoft, Phard diff.) effect on pt>20 GeV jets # distribution; thedifference for this distribution comes from other sources (identified in the nextslides).

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Psoft vs Phard Tune : IFSR Parameters - 1Producing 2 additional samples with P0 + (only) IFSR parameters varied on topas in Phard and Psoft:

PARP(71): multiplicative factor of a hard scale that sets the allowed phase space forthe hardest FSR emission.MSTP(72): switch setting a detail of the FSR off ISR (have pasted the definition to thelast slide).n.b.: none of these is the FSR λQCD .

P0: PARP(71)=2.0, MSTP(72)=1,Phard: PARP(71)=4.0, MSTP(72)=1,Psoft: PARP(71)=1.0, MSTP(72)=0 (less FSR off ISR).

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IFSR(Phard) sample

# of jets with pt>20 GeV

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P0 tune

IFSR(Phard) sample

# of jets with pt>30 GeV

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8 / 26

Psoft vs Phard Tune : IFSR Parameters - 2

0 1 2 3 4

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P0 tune

IFSR(Phard) sample

# of jets with pt>40 GeV

# of jets0 1 2 3 4

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P0 tune

IFSR(Phard) sample

(inclusive) jet pt

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Apart from ISR also the IFSR variations also contribute to jet (pt>30, 40) #differences between Psoft and Phard.

9 / 26

Psoft vs Phard Tune : UE Parameters - 1

Producing 2 additional samples with P0 + (only) IFSR parameters varied on topas in Phard and Psoft:

PARP(82): MI IR pt cutoff,

MSTP(90): energy scaling of the MI IR pt cutoff (as EPARP(90)CM ),

PARP(83): hadronic matter overlap,net effect is more non-perturbative activity in Psoft and less in Phard (wrt. P0).

P0: PARP(82)=2.0, PARP(90)=0.26, PARP(83)=1.7,Phard: PARP(82)=2.3, PARP(90)=0.30, PARP(83)=1.7,Psoft: PARP(82)=1.9, PARP(90)=0.24, PARP(83)=1.5.

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P0 tune

UE(Phard) sample

# of jets with pt>20 GeV

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# of jets with pt>30 GeV

# of jets0 1 2 3 4

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10 / 26

Psoft vs Phard tune : UE Parameters - 2

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UE(Phard) sample

# of jets with pt>40 GeV

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UE(Phard) sample

(inclusive) jet pt

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effect on pt>20 GeV # is as expected and also observed when using Pythia stand.

effect on jet # is negligible (wrt. e.g. ISR effects) for pt>40(30) GeV jets.

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Effects on x-Section (Matching Efficiency)

Psoft and Phard values differ from P0 ones for ∼ 10-20%;

this can be assigned to ISR and IFSR differences between Psoft and Phard,

it is not caused by Underlying Event (UE), Colour Reconnections (CR) or othernon-perturbative differences between the tunes.

tune 0j 1j 2j 3j 4j total [pb]Phard 7287 ± 3.9 728 ± 2.6 141 ± 1.3 27 ± 0.2 6.6 ± 0.2 8190 ± 8P0 7556± 3.6 814 ± 2.7 166 ± 1.3 32 ± 0.3 7.8 ± 0.3 8576 ± 8Psoft 7804 ± 3.4 944 ± 2.8 207 ± 1.5 42 ± 0.3 10.1± 0.3 9007 ± 8P0 ISR (Phard) 7207 ± 6.9 735 ± 2.6 143 ± 1.3 27 ± 0.2 6.9 ± 0.2 8119 ± 11P0 ISR (Psoft) 7831 ± 4.9 881 ± 2.7 186 ± 1.4 36 ± 0.3 8.8 ± 0.3 8943 ± 10P0 FISR (Phard) 7548 ± 6.0 814 ± 2.7 167 ± 1.3 32 ± 0.3 7.8 ± 0.3 8569 ± 10P0 FISR (Psoft) 7505 ± 6.1 878 ± 2.7 188 ± 1.4 37 ± 0.3 9.4 ± 0.3 8617 ± 10P0 UE (Phard) 7513 ± 6.1 826 ± 2.7 171 ± 1.4 33 ± 0.3 7.8 ± 0.3 8551 ± 10P0 UE (Psoft) 7576 ± 5.9 817± 2.7 166 ± 1.3 32 ± 0.3 8.1 ± 0.3 8599 ± 10P0 CR (Phard) 7561 ± 5.9 821 ± 2.7 167 ± 1.3 32 ± 0.3 8.1 ± 0.3 8589 ± 10P0 CR (Psoft) 7556 ± 5.9 815 ± 2.7 165 ± 1.3 32 ± 0.3 8.1 ± 0.3 8576 ± 10

12 / 26

P2010 Tune + ISR Parameters Variation - 1

Producing 2 additional samples with P2010 + (only) ISR parameters varied ontop:

PARP(67): multiplicative factor of a hard scale that sets the allowed phase space forthe hardest ISR emission.PARP(64): multiplicative factor of the ISR evolution scale (Q2, p2

t ).

P2010: PARP(67)=1.0, PARP(64)=1.0,P2010 ISR ↑: PARP(67)=4.0, PARP(64)=0.25,P2010 ISR ↓: PARP(67)=0.5, PARP(64)=4.0.

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P2010 tune

↑P2010 ISR

# of jets with pt>20 GeV

# of jets0 1 2 3 4 5 6

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13 / 26

P2010 Tune + ISR Parameters Variation - 2

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P2010 tune

↑P2010 ISR

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(inclusive) jet pt

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Effect on high-pt jet #, pt ∼ as in the case of Psoft and Phard ISR parametersonly variation.

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P2010 Tune + ISR Parameters Variation - 3

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

N

5000

10000

15000↓P2010 ISR

P2010 tune

↑P2010 ISR

ηleading jet

ηjet -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

dist

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E(r

)/E

(R)

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↓P2010 ISR

P2010 tune

↑P2010 ISR

leading jet E-profile

r (R=0.4)0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

dist

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No larga η shape modulation observed in the central region.

Effect on jet energy flow distribution observed, but smaller than the effects due toFSR, UE.

15 / 26

P2010 Tune + FSR Parameters Variation - 1

Producing 2 additional samples with P2010 + (only) FSR λ (PARP(72)) variedon top:

P2010: PARP(72)=0.26,P2010 FSR ↑: PARP(72)=0.52,P2010 FSR ↓: PARP(72)=0.13.

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16 / 26

P2010 Tune + FSR Parameters Variation - 2

Producing 2 additional samples with P2010 + (only) FSR λ (PARP(72)) variedon top:

P2010: PARP(72)=0.26,P2010 FSR ↑: PARP(72)=0.52,P2010 FSR ↓: PARP(72)=0.13.

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17 / 26

P2010 Tune + FSR Parameters Variation 1

Producing 2 additional samples with P2010 + (only) FSR λ (PARP(72)) variedon top:

P2010: PARP(72)=0.26,P2010 FSR ↑: PARP(72)=0.52,P2010 FSR ↓: PARP(72)=0.13.

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

E(r

)/E

(R)

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↓P2010 FSR

P2010 tune

↑P2010 FSR

leading jet E-profile

r (R=0.4)0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

dist

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E(r

)/E

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↓P2010 FSR

P2010 tune

↑P2010 FSR

2nd hard. jet E-profile

r (R=0.4)0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

dist

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Also in matched samples: more FSR ⇒ broader jets.

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P2010 Tune + FSR Parameters Variation - 3

Producing 2 additional samples with P2010 + (only) FSR λ (PARP(72)) variedon top:

P2010: PARP(72)=0.26,P2010 FSR ↑: PARP(72)=0.52,P2010 FSR ↓: PARP(72)=0.13.

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

E(r

)/E

(R)

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↓P2010 FSR

P2010 tune

↑P2010 FSR

[20-30 GeV] jet E-profile

r (R=0.4)0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

dist

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)/E

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P2010 tune

↑P2010 FSR

[40-60 GeV] jet E-profile

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Also in matched samples: more FSR ⇒ broader jets.

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P2010 Tune + ISR/FSR Parameters Effect on W pt

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(stat.3) W pt

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ISR: effect in the matched events reversed wrt. Pythia standalone.

FSR: no significant effect.

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Effects on x-Section (Matching Efficiency)

due to ISR and FSR variation the x-section values change for ∼ 10-20% wrt. thecentral sample (P2010).

the P2010 ISR ↑, P2010 ISR↓, P2010 FSR↑, P2010 FSR↓ variations are asdefined in previous slides.

more hard ISR activity decreases the x-section and the matching efficiency in allsub-samples,

more FSR increases the x-section for 0j, but decreases it for other sub-samples.As a result the effect of FSR variations on the total x-section is small.

tune 0j 1j 2j 3j 4j total [pb]Phard 7287 ± 3.9 728 ± 2.6 141 ± 1.3 27 ± 0.2 6.6 ± 0.2 8190 ± 8P0 7556± 3.6 814 ± 2.7 166 ± 1.3 32 ± 0.3 7.8 ± 0.3 8576 ± 8Psoft 7804 ± 3.4 944 ± 2.8 207 ± 1.5 42 ± 0.3 10.1± 0.3 9007 ± 8P2010 7596 ± 5.8 774 ± 2.7 153 ± 1.3 29 ± 0.2 6.9 ± 0.2 8559 ± 10P2010 ISR ↑ 7226 ± 6.8 693 ± 2.6 132 ± 1.2 25 ± 0.2 6.1 ± 0.2 8082 ± 11P2010 ISR↓ 7846 ± 4.9 822 ± 2.7 166 ± 1.3 32 ± 0.3 7.3 ± 0.3 8874 ± 10P2010 FSR↑ 7639 ± 5.7 733 ± 2.6 136 ± 1.2 24 ± 0.2 5.5 ± 0.2 8538 ± 10P2010 FSR↓ 7546 ± 6.0 809 ± 2.7 164 ± 1.3 33 ± 0.3 8.2 ± 0.3 8560 ± 11

21 / 26

Events Failing the Matching in P2010 Tune - 1

Look at pt of the hardest ISR, FSR and MI branching in events that pass and inevents that do not pass the matching (using P2010 +2 partons for the plots).pt of the hardest branching corresponds to Pythia parameter VINT(357) (ISR)and VINT(358) (FSR), Pythia Manual has the exact definitions.

ISVETO=0 : EVENT PASSES, red histogramISVETO!=0 : EVENT FAILS 1

pt [GeV] 0 20 40 60 80 100 120 140 160

N/N

∆

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 ISVETO = 0

ISVETO = 1

ISVETO = 2

ISVETO = 3

hardest ISR branching pt

pt [GeV] 0 20 40 60 80 100 120 140 160

N/N

∆

00.10.20.30.40.50.60.70.80.9

ISVETO = 0

ISVETO = 1

ISVETO = 2

ISVETO = 3

hardest FSR branching pt

We see that:hard ISR emission : event will more likely fail the matching,hard FSR emission : the event will more likely fail the matching.

1since:

ISVETO=1 : there are fewer reconstructed jets than partons,ISVETO=2 : not all partons match a jet,ISVETO=3 : there are additional jets in an exclusive sample.ISVETO=4 : the matched jets in inclusive sample are not the hardest ones.

22 / 26

Events Failing the Matching in P2010 Tune - 2

ISVETO=0 : EVENT PASSES

ISVETO!=0 : EVENT FAILS 2

pt of the hardest MI branching corresponds to Pythia parameter VINT(356),Pythia Manual has the exact definition.

pt [GeV] 0 20 40 60 80 100

N/N

(Lo

g S

cale

) ∆

-410

-310

-210

-110

ISVETO = 0

ISVETO = 1

ISVETO = 2

ISVETO = 3

hardest MI branching pt

We see that:whether event fails or passes the matching does not depend on the hard(est) MI.

2since:

ISVETO=1 : there are fewer reconstructed jets than partons,ISVETO=2 : not all partons match a jet,ISVETO=3 : there are additional jets in an exclusive sample.ISVETO=4 : the matched jets in inclusive sample are not the hardest ones.

23 / 26

Summary of the Observations

When Pythia parameters are varied for Alpgen + Pythia event generation:increasing hard ISR or hard FSR activity:

will affect the matching efficiency, x-section,will result in softer hard jet # distributions and pt-spectra, which is vice-versa as the effectsobtained with Pythia standalone,some trends will stay as observed in Pythia stand. (e.g. more FSR will still result in broader jets. . .).

UE parameters variations:have not been observed to have a significant effect on matching efficiency, x-section,will (not) push jets above the (relatively low, e.g. 20 GeV) pt threshold, which is also observedin Pythia standalone.

24 / 26

Thank you

25 / 26

What is MSTP(72), cf. Pythia Manual

26 / 26