PoWHEG + Pythia matchinggroups:tools:powheg-matchi… · + Pythia tune New matching gives too much of a hard core to the jet Bolder variation Turn renormMultFac down to 0.5: Low pT:

PoWHEG + Pythia matching

James MonkKiran Joshi

Lily AsquithFelix Müller

Introduction

17

๏ In extreme cases, the shower may be allowed to run wild, e.g. this old version of PoWHEG that has since been patched

๏ PoWHEG provides a scale (SCALUP) that is an indication of where the shower should take over from the perturbative calculation.

๏ There may be some mis-matching between Pwg and Pythia about what is considered the scale of an emission. e.g. pT isn’t very indicative of the hardness of a very forward emission.

partonT

p0 20 40 60 80 100 120 140 160 180 200

truth

Tp

0

20406080

100120140

160180200

#jet

s

1

10

210

310

410

510 = 2.76 TeVsPythia/AUET2bno weights

=5GeVmintborn k

J4

J1

J2

J3

Alternative matching schemes

18

Leg-emission veto

5

For each proposed shower emission, ps, calculate a scale μs

4

1

2

35

µs = maxi

|ps × pj ||pj |

PoWHEG legs

Start the shower at the kinematic limit (“power shower”), but veto any

emission for which μs is above the (unchanged) PoWHEG veto scale

PoWHEG Scale Recalculation Scheme

Recalculate the veto scale, μ, based on the configuration of the PoWHEG legs

4

4

1

2

35

µ = mini,j

|pi × pj ||pj |

Start the shower at the kinematic limit (“power

shower”), but veto any emission above this calculated μ

Original Scale [GeV]0 50 100 150 200 250 300 350 400 450 500

New

Sca

le [G

eV]

0

50

100

150

200

250

300

350

400

450

500

1

10

210

310

Old Vs new scales

PoWHEG legs

Suggested by PoWHEG authors as a way to explore systematics

of (reducing) scale choice

Suggested by Pythia authors as a way to better measure how hard an emission is

Improved ISR description!..

19

Multi-jets

11

ATLAS datavanilla Py8 AU2-CTEQ6L1

Pwg scale recalculationLeg-emission veto

10!1

Inclusive Jet Multiplicity Ratio (R=0.4)

!N

/!

N!

1

3 4 5 6

0.6

0.8

1

1.2

1.4

Njet

MC

/D

ata

Note in particular the 3:2 ratio, which you’d hope to get right with the PoWHEG emission

(something odd in the 4:3 ratio though)

Improved ISR description!..

20

Azimuthal decorrelations

16



10!2

10!1

1

Dijet azimuthal decorrelations for 160 < pmax"

/GeV < 210

1/!

d!

/d

!"

[#/

rad

]

0.5 0.6 0.7 0.8 0.9 1.00

0.5

1

1.5

2

!" [rad/#]

MC

/D

ata

A clear win for the new matchings over the vanilla!

leg-emission matching may be slightly better, but scale reduction is close

All distributions show similar

…but (fatally) worsened FSR description

21

Jet shapes

17



1

Jet shape ! for p! " 40–60 GeV, y " 1.2–2.1

!(r

)

0 0.1 0.2 0.3 0.4 0.5 0.6

0.6

0.8

1

1.2

1.4

r

MC

/D

ata

The new matchings have broken the jet shapes though!

The fact that the jet shapes is so good in the vanilla is a bit surprising - not a combined PoWHEG + Pythia tune

New matching gives too much of a hard core to the jet

Bolder variationTurn renormMultFac down to 0.5:

Low pT: High pT:

→ ρ is pretty insensitive even to large variation.

Look at other |y | bins...

5/9

Varying e.g. alpha_s in shower suggested it would not be

possible to re-obtain the good jet shape behaviour

Ad Hoc Solution: only apply the modified matching to ISR

22


Pwg scale recalcLeg-emission vetoScale recalc, ISR onlymain31, emt010!1

Inclusive jet multiplicity ratio N/N ! 1 (R = 0.4)

!N

/!

N!

1

3 4 5 6

0.6

0.8

1

1.2

1.4

Njet

MC

/D

ata


Pwg scale recalcLeg-emission vetoScale recalc, ISR onlymain31, emt0

1

Jet shape ! for p! " 40–60 GeV, y " 0.8–1.2

!(r

)

0 0.1 0.2 0.3 0.4 0.5 0.6

0.6

0.8

1

1.2

1.4

r

MC

/D

ata



10!2

10!1

1

Dijet azimuthal decorrelations for 160 < pmax"

/GeV < 210

1/!

d!

/d

!"

[#/

rad

]

0.5 0.6 0.7 0.8 0.9 1.00

0.5

1

1.5

2

!" [rad/#]

MC

/D

ata

Improves many distributions while keeping the jet shapes unchanged.



10!4

10!3

10!2

10!1

1

10 1

10 2

10 3

10 4

10 5

10 6

10 7Incl. jet double-diff. x-section, anti-kt 0.4 (|y| < 0.3)

d2!

/d

p "d

y[p

b/

GeV

]

200 400 600 800 1000 1200 1400

0.6

0.8

1

1.2

1.4

p" [GeV]

MC

/D

ata

Even slightly improves differential cross section

at low pT

“Main31”

23

๏ Example matching provided with Pythia. Initially showed very poor description, but recently fixed after interactions with authors.

๏ Author recommended setup. Applies to both ISR and FSR (although agree that there is more theoretical freedom to vary FSR scale choice)

๏ Performs almost as well as ISR-only scale reduction (and much better than vanilla setups). Since it’s less ad-hoc, our likely recommendation for future QCD production

๏ ISR-only setup can provide a nice systematic variation - they both perform well on these distributions.

๏ Full set of plots here: http://www.nbi.dk/~jmonk/powhegmatching/

http://www.nbi.dk/~jmonk/powhegmatching/

http://www.nbi.dk/~jmonk/powhegmatching/

Top, Z

24

ATLAS dataMC (main31)MC (PowhegPythia6 P2011C)

0.7

0.75

0.8

0.85

0.9

0.95

1.0

Gap fraction vs. Q0 for veto region: |y| < 0.8

f gap

50 100 150 200 250 300

0.96

0.98

1.0

1.02

1.04

Q0 [GeV]

MC/data

ATLAS dataMC (main31)MC (PowhegPythia6 P2011C)

0.5

0.6

0.7

0.8

0.9

1.0

Gap fraction vs. Q0 for veto region: |y| < 2.1

f gap

50 100 150 200 250 300

0.9

0.95

1.0

1.05

1.1

Q0 [GeV]

MC/data

Very preliminary studies of top veto by Kiran Joshi suggest there may be issues, but the picture isn’t clear yet

ATLAS dataVetoed showerWimpy showerDefault shower

10!6

10!5

10!4

10!3

10!2

10!1Z p" reconstructed from dressed muons

1/!

d!

/d

p"

[GeV

!1]

1 10 1 10 2

0.6

0.8

1

1.2

1.4

p"(µµ) [GeV]

MC

/D

ata

Did not affect the Z pT distribution much, but

maybe W/Z + jets (c.f. jet Δϕ) will show a

difference. No FSR ambiguity in this case

Documents

PoWHEG + Pythia matchinggroups:tools:powheg-matchi… · + Pythia tune New matching gives too much of a hard core to the jet Bolder variation Turn renormMultFac down to 0.5: Low pT: