Briefphysicssummaryofthe MonteCarlogeneratorworkshop · Briefphysicssummaryofthe...

Brief physics summary of the

Monte Carlo generator workshop[ Hadron Collider Physi s Summer S hool 2010 ℄----------------------------------------------Jan Winter� Fermilab �Monte Carlo event generationVe tor boson produ tion and parton showersV + n-jets using tree-level ME+PS mergingV + n-jets � next-to-leading order

Jan Winter FNAL, August 16, 2010 – p.1

Monte Carlo modelling of a (high-pT ) eventFa torization approa h: divide jet simulation into di�erent phasesPerturbative Phases: [parton jets]Hard pro ess/intera tion (hard jet produ tion)exa t matrix elements |M|2QCD bremsstrahlung (soft/ oll multiple emissions)initial- and �nal-state parton showeringMultiple/Se ondary intera tionsmodelling the underlying eventNon-perturbative Phases: [jet confinement – particle jets]Hadronizationphenomenologi al models to onvert partons into primary hadronsHadron de aysphase-spa e or e�e tive models to de ay unstable intostable hadrons as observed in dete tors ��

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predi tions at hadron level � omparable to experimental data if orre ted for dete tor e�e ts

Pre ise and ompatible jet de�nition is ne essary on theoreti al and experimental level.

Type: onemaximize energy within a oneradius (pseudo-)rapidity azimuthal angle

Type: lusteridenti� ation and ombination of nearest neighbouring parti lesinfra-red safe -measure: �nd minimal value of



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Type: onemaximize energy within a oneradius (pseudo-)rapidity azimuthal angle

Type: lusteridenti� ation and ombination of nearest neighbouring parti lesinfra-red safe -measure: �nd minimal value of



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Type: onemaximize energy within a one

⇒ radius × (pseudo-)rapidity × azimuthal angle

Type: lusteridenti� ation and ombination of nearest neighbouring parti lesinfra-red safe kT -measure: �nd minimal value ofQ2

ij = min{k2T,i, k

2T,j} · (∆R2

ij/D2) and

Q2iB = k2

T,i

where e.g. ∆R2ij = 2 [cosh(ηi − ηj) − cos(φi − φj)]


Vector boson production al ulation of the hadroni ross se tion relying on fa torization theorem... ... expe ted to hold for A + B → V + X [COLLINS, SOPER, STERMAN, 2004 REVIEW]

σhadr =X

ij

Z

dx1dx2 fi(x1, µF) fj(x2, µF) σpart(ij → V → . . .)

σpart ... al ulable in pQCD; fi = parton density fun tions (PDFs) ... extra ted from data;separation of perturbative and non-perturbative regimes pQCD used to predi t rossse tions in ompli ated hadron ollider environmentV produ tion � LO: two initial-state partons fuseto make either W± → ℓν or Z/γ∗ → ℓ+ℓ−ve tor boson has no transverse momentumV + n-jet produ tion � LO: ve tor bosonre oils against one or more jets (parton-level jets)highly automated ME generators � tree levelAlpgen, MadGraph, Hela , Amegi , Comix,Whizard, LO MCFMJan Winter FNAL, August 16, 2010 – p.3

Parton shower concept

t = p2a

Traditional approa h: des ribe additional jet a tivity by parton showers.QCD emissions preferably populate ollinear and soft phase-spa e regions.[ Pythia, Herwig, Ariadne ] QCD amplitudes fa torize in the oll/soft limit.Re ursive de�nition of multiple emissions:

dσn+1 = dσn

αs(t)

2 π

dt

tdz Pa→bc(z) (e.g. oll limit)

oll/soft parton emissions iteratively added to the initial/�nal states [ LL resummation ]good des ription of bulk of radiation and parti le multipli ity growthpartoni ensemble evolved down to hadronization s ale [ ordering variable Q, ϑ, pT ]provides suitable input for universal hadronization models [ O(1 GeV) ]ve tor boson produ tion: in lusive V + n-jets predi tion � LO+LL


Example: Sherpa’s Catani–Seymour showeruniversal dipole terms des ribe 1 → 2 parton splittings, exponentiated in Sudakov form fa tor orre t soft & oll. limits; 2 → 3 kinemati s: spe tator used to onserve E, ~p lo allyDrell�Yan produ tion:

CS Shower [SCHUMANN, KRAUSS, JHEP 03 (2008),038]

�

γ∗/Z0 1st emission

=⇒ . . .

�

γ∗/Z0

�γ∗/Z0

hard s ale �xed by M2ee ⇒ k2

⊥,maxtransverse momentum of lepton-pair determined by multiple QCD emissions

0 25 50 75 100 125 150 175 200p

T [GeV]

10-4

10-3

10-2

10-1

100

101

dσ/d

p T [

pb/G

eV]

CDF 2000CS show. + Py 6.2 had.CS show. + Py 6.2 had. (enhanced start scale)

0 5 10 15 20p

T [GeV]

5

10

15

20

25

30

dσ/d

p T [

pb/G

eV]

omparison with Tevatron CDF datarate normalised to datadominant ontribution for peeTSudakov damping for pee

T → 0hardest emission below k⊥,max

→ peeT > k⊥,max matrix-element regimeME orre tions an be implemement to im-prove 1st emission


V + n-jet predictions @ LO+LL and beyondExamples for shower Monte CarlosPythia � virtuality ordering, 1 → 2 (old) and pT ordering, 2 → 3 [SJOSTRAND, SKANDS, MRENNA]Herwig � angular ordering, 1 → 2 [WEBBER, MARCHESINI, SEYMOUR, RICHARDSON]Ariadne � Lund olour-dipole model, pT ordering, full 2 → 3 [LONNBLAD, GUSTAFSON, ANDERSSON]Sherpa's CS shower � based on CS subtra tion terms, pT ordering, 2 → 3 [SCHUMANN, KRAUSS]

Limitationsshower seeds are LO (QCD) pro esses onlyla k of high-energeti large-angle emissionssemi- lassi al pi ture; quantum interferen es and orrelations only approximateshower evolution pro eeds in the limit of large (number of olours)Possible improvements�rst few hardest emissions given by tree-level MEs improved LO+LL predi tions

use NLO QCD ore pro esses and mat h to parton showers NLO+LL predi tions


V + n-jet predictions @ LO+LL and beyondExamples for shower Monte CarlosPythia � virtuality ordering, 1 → 2 (old) and pT ordering, 2 → 3 [SJOSTRAND, SKANDS, MRENNA]Herwig � angular ordering, 1 → 2 [WEBBER, MARCHESINI, SEYMOUR, RICHARDSON]Ariadne � Lund olour-dipole model, pT ordering, full 2 → 3 [LONNBLAD, GUSTAFSON, ANDERSSON]Sherpa's CS shower � based on CS subtra tion terms, pT ordering, 2 → 3 [SCHUMANN, KRAUSS]Limitationsshower seeds are LO (QCD) pro esses onlyla k of high-energeti large-angle emissionssemi- lassi al pi ture; quantum interferen es and orrelations only approximateshower evolution pro eeds in the limit of large NC (number of olours)

Possible improvements�rst few hardest emissions given by tree-level MEs improved LO+LL predi tions

use NLO QCD ore pro esses and mat h to parton showers NLO+LL predi tions


V + n-jet predictions @ LO+LL and beyondExamples for shower Monte CarlosPythia � virtuality ordering, 1 → 2 (old) and pT ordering, 2 → 3 [SJOSTRAND, SKANDS, MRENNA]Herwig � angular ordering, 1 → 2 [WEBBER, MARCHESINI, SEYMOUR, RICHARDSON]Ariadne � Lund olour-dipole model, pT ordering, full 2 → 3 [LONNBLAD, GUSTAFSON, ANDERSSON]Sherpa's CS shower � based on CS subtra tion terms, pT ordering, 2 → 3 [SCHUMANN, KRAUSS]Limitationsshower seeds are LO (QCD) pro esses onlyla k of high-energeti large-angle emissionssemi- lassi al pi ture; quantum interferen es and orrelations only approximateshower evolution pro eeds in the limit of large NC (number of olours)Possible improvements�rst few hardest emissions given by tree-level MEs improved LO+LL predi tions

[ called (tree-level/LO) ME+PS merging – CKKW, L-CKKW, MLM, ME&TS – No NLO xsecs! ]use NLO QCD ore pro esses and mat h to parton showers NLO+LL predi tions

[ called NLO+PS matching – MC@NLO, POWHEG – Full NLO xsecs! ]


matrix element:

˛

˛

˛

˛

˛

˛

˛

˛

˛

+

˛

˛

˛

˛

˛

˛

˛

˛

˛

2

|AR|2 + |BR|

2 + 2 Re(ARB∗R)

Combine ME&PS advantages,remove ME&PS weaknesses.

Beware of double ounting,preserve universality ofhadronization.

parton shower:

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

2

+

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

˛

2

|AR|2 + |BR|

2

αS vs. Log

resummed in PS

exact ME

LO 5jet, but also

NLO 4jet

L

α n

m

NLLexact ME

LO 4jet

4

4

4

4

4

5

5

5

5

5

5

5


Tree-level ME+PS merging ombine parton-shower pros (soft emissions) +ME pros (hard emissions, quantum interferen es, orrelations)avoid double ounting and missing phase-spa e regionsDivide multi-jet phase spa e into two regimes:tree-level MEs: jet seed (hard parton) produ tion Q > Qjetparton showers: (intra-)jet evolution Qjet > Q > Qcut−offExamples for ME+PS merging Monte Carlos:Alpgen � MLM; interfa ed to Pythia or Herwig [MANGANO ET AL.]MadGraph � MLM, one or kT jets; interfa ed to Pythia [MALTONI ET AL.]Sherpa � CKKW, ME&TS from vs1.2; truly inter onne ted with PSs [KRAUSS ET AL.]Methods mainly di�er in:the jet de�nition used to de�ne/regularize the MEs,the way of a epting/reje ting jet on�gurations stemming from the MEs,the details on erning the starting onditions of the parton showering andthe jet vetoing inside the shower algorithm.

AMEGIC + APACIC onstant K-fa torintrinsi -smearingof order 1 GeV


Tree-level ME+PS merging ombine parton-shower pros (soft emissions) +ME pros (hard emissions, quantum interferen es, orrelations)avoid double ounting and missing phase-spa e regionsDivide multi-jet phase spa e into two regimes:tree-level MEs: jet seed (hard parton) produ tion Q > Qjetparton showers: (intra-)jet evolution Qjet > Q > Qcut−offExamples for ME+PS merging Monte Carlos:Alpgen � MLM; interfa ed to Pythia or Herwig [MANGANO ET AL.]MadGraph � MLM, one or kT jets; interfa ed to Pythia [MALTONI ET AL.]Sherpa � CKKW, ME&TS from vs1.2; truly inter onne ted with PSs [KRAUSS ET AL.]Methods mainly di�er in:the jet de�nition used to de�ne/regularize the MEs,the way of a epting/reje ting jet on�gurations stemming from the MEs,the details on erning the starting onditions of the parton showering andthe jet vetoing inside the shower algorithm.

CKKW [CATANI, KRAUSS, KUHN, WEBBER, JHEP 11 (2001) 063]

[KRAUSS, JHEP 08 (2002) 015]

/ GeV Z P0 20 40 60 80 100 120 140 160 180 200

10-3

10-2

10-1

1

10pt Z

Z + 0 jet

Z + 1 jetZ + 2 jet

Z + 3 jetCDF

GeVpb

/

dPσ

d

/ GeV Z P0 5 10 15 20 25 30 35 40 45 50

G

eVpb

/

dPσ

d

1

10

pt Z

Z + 0 jet

Z + 1 jetZ + 2 jet

Z + 3 jetCDF

⇒ IN SHERPA VS1.0 AND VS1.1

E.G. Z + JETS @ 1.8 TEV

KRAUSS ET AL. PRD 70 (2004) 114009

• AMEGIC + APACIC

• onstant K-fa tor

• intrinsi kT -smearingof order 1 GeVJan Winter FNAL, August 16, 2010 – p.8

Comparison between merging approachesTree-level ME+PS merging predi tions of Alpgen, Ariadne, Hela , MadEvent and Sherpa.W + + X

dσ/d

E⊥

1 (p

b/G

eV)

(a)Alpgen

AriadneHelac

MadEventSherpa

10-2

10-1

100

101

102

E⊥ 1 (GeV)

-1-0.5

0 0.5

1

0 50 100 150 200 250 300 350 400 450 500

dσ/d

E⊥

2 (p

b/G

eV)

(b)

10-2

10-1

100

101

102

E⊥ 2 (GeV)

-1-0.5

0 0.5

1

0 50 100 150 200 250 300 350 400

dσ/d

E⊥

3 (p

b/G

eV)

(c)

10-3

10-2

10-1

100

101

E⊥ 3 (GeV)

-1-0.5

0 0.5

1

0 50 100 150 200 250 300

dσ/d

E⊥

4 (p

b/G

eV)

(d)

10-3

10-2

10-1

100

101

E⊥ 4 (GeV)

-1-0.5

0 0.5

1

0 50 100 150 200

jet ET spe tra atthe LHC

jet spe tra atthe LHC

similar patternwrt Tevatronextrapolation toLHC energiesmakes di�eren esmore pronoun edResults in arXiv:0706.2569(EPJC 53 (2008) 473)Jan Winter FNAL, August 16, 2010 – p.9

Comparison between merging approachesTree-level ME+PS merging predi tions of Alpgen, Ariadne, Hela , MadEvent and Sherpa.W + + X

(1/σ

)dσ/

dη1

(a)

AlpgenAriadne

HelacMadEvent

Sherpa

0.1

0.2

η1

-0.4-0.2

0 0.2 0.4

-4 -3 -2 -1 0 1 2 3 4

(1/σ

)dσ/

dη2

(b)

0.1

0.2

η2

-0.4-0.2

0 0.2 0.4

-4 -3 -2 -1 0 1 2 3 4

(1/σ

)dσ/

dη3

(c)

0.1

0.2

η3

-0.4-0.2

0 0.2 0.4

-4 -3 -2 -1 0 1 2 3 4

(1/σ

)dσ/

dη4

(d)

0.1

0.2

η4

-0.4-0.2

0 0.2 0.4

-4 -3 -2 -1 0 1 2 3 4

jet spe tra atthe LHC

jet η spe tra atthe LHCsimilar patternwrt Tevatronextrapolation toLHC energiesmakes di�eren esmore pronoun edResults in arXiv:0706.2569(EPJC 53 (2008) 473)


Comparison with CDF data: W+jets production[T. AALTONEN ET AL., PRD 77 (2008) 011108]Monte Carlos need to be validated and tuned against most re ent Tevatron data.Sherpa vs1.1.3 predi tions normalized to total in lusive ross se tion. Two hoi es of PDFs.Tree-level ME+PS an reprodu e W+>=n-jet xse s to 20% after applying overall K-fa tor.

SHERPASHERPASHERPA

W + jets−1320 pb

Tevatron Run 2

CDF Run 2 data (2008)

W + jets−1320 pb

Tevatron Run 2

Sherpa 112 CKKW 3jet (cteq6m)

W + jets−1320 pb

Tevatron Run 2

Sherpa 112 CKKW 3jet (cteq6l)

+ in

cl n

jets

) [p

b]ν

e→

(Wσ

−110

1

10

210

310

(MC

−dat

a) /

data

−0.4−0.2

00.20.4

Jet multiplicity (incl n jets)0 1 2 3 4

SHERPASHERPASHERPA

W + jets 1st jet

Tp

Tevatron Run 2


W + jets 1st jet

Tp

Tevatron Run 2


W + jets 1st jet

Tp

Tevatron Run 2

Sherpa 112 CKKW 3jet (cteq6l) [pb

/GeV

]T

,1/d

pσd

−310

−210

−110

1

10

(MC

−dat

a) /

data

−1

−0.5

0

0.5

1

[GeV]T,1

p100 200 300



SHERPASHERPASHERPA

W + jets−1320 pb

Tevatron Run 2


W + jets−1320 pb

Tevatron Run 2


W + jets−1320 pb

Tevatron Run 2


+ in

cl n

jets

) [p

b]ν

e→

(Wσ

−110

1

10

210

310

(MC

−dat

a) /

data

−0.4−0.2

00.20.4


SHERPASHERPASHERPA

W + jets 2nd jet

Tp

Tevatron Run 2


W + jets 2nd jet

Tp

Tevatron Run 2


W + jets 2nd jet

Tp

Tevatron Run 2


/GeV

]T

,2/d

pσd

−310

−210

−110

1

(MC

−dat

a) /

data

−1

−0.5

0

0.5

1

[GeV]T,2

p50 100 150



SHERPASHERPASHERPA

W + jets−1320 pb

Tevatron Run 2


W + jets−1320 pb

Tevatron Run 2


W + jets−1320 pb

Tevatron Run 2


+ in

cl n

jets

) [p

b]ν

e→

(Wσ

−110

1

10

210

310

(MC

−dat

a) /

data

−0.4−0.2

00.20.4


SHERPASHERPASHERPA

W + jets 3rd jet

Tp

Tevatron Run 2


W + jets 3rd jet

Tp

Tevatron Run 2


W + jets 3rd jet

Tp

Tevatron Run 2


/GeV

]T

,3/d

pσd

−310

−210

−110

(MC

−dat

a) /

data

−1

−0.5

0

0.5

1

[GeV]T,3

p20 40 60 80


V + n-jets ....

at & beyond NLO ... ..

In lusion of all diagrams ontributing at a given order in leads to LO, NLO, NNLO, ... results.


V + n-jets ....

at & beyond NLO ... ..

LO

∣

∣

∣A

(0)qq′

∣

∣

∣

2

NLO

∣

∣

∣A

(0)qq′

∣

∣

∣

2

, ℜ(A(0)qq′A

(1)⋆

qq′ )

NNLO

∣

∣

∣A

(0)qq′

∣

∣

∣

2

, ℜ(A(0)qq′A

(1)⋆

qq′ ),

ℜ(A(0)qq′A

(2)⋆

qq′ ),∣

∣

∣A

(1)qq′

∣

∣

∣

2

and so forth...

In lusion of all diagrams ontributing at a given order in αs leads to LO, NLO, NNLO, ... results.


A well known example

O(αs) orre tions to virtual-photon de ay to quark�antiquark pair, γ∗

→ qq: K = 1 +αs(µ2

R)

π

∣

∣

∣

∣

∣

∣

∣

∣

+

∣

∣

∣

∣

∣

∣

∣

∣

2 real gluon emission, real → O(gs)Born + one additional leginfrared divergen ies|AR|

2 + |BR|2 + 2Re(ARB∗

R)

∣

∣

∣

∣

∣

∣

∣

∣

+ + +

∣

∣

∣

∣

∣

∣

∣

∣

2

virtual gluon orre tions, virtual → O(g2s), Born + one additional loopinfrared (soft & ollinear) and ultraviolet divergen ies (removed by renormalization)

|A0|2 + 2Re(A0A

∗V + A0B

∗V + A0C

∗V ) + |AV + BV + CV |2× ×


Need for NLO calculationsLessons learned from LEP, HERA, Tevatron:LO predi tions are �ne, yet often only give rough estimates� NLO: 1st real predi tion of normalization of many observablesless sensitivity to unphysi al input s ales (fa torization and renormalization s ales, µF & µR)more physi s (parton merging, jet substru ture, ISR, more IS parton spe ies)0.25 0.5 1 2 4 8

0

10

20

30

40

50

60

σ [

pb

]

LONLO

0.25 0.5 1 2 4 8µ / µ0

0.5

1

1.5

K-f

acto

r

µ0 = 2 M

W = 160.838 GeV

W- + 3 jets + X

BlackHat+Sherpa

√s = 14 TeV

ET

jet > 30 GeV, | ηjet

| < 3

ET

e > 20 GeV, | ηe

| < 2.5

ET/ > 30 GeV, M

T

W > 20 GeV

R = 0.4 [siscone]

Components of NLO al ulationstree-level amplitudes(LO & real radiation)one-loop orre tion to Born levelsubtra tion terms to handle and ombine singularitiesphase-spa e generatorfor example,



0

10

20

30

40

50

60

σ [

pb

]

LONLO

0.25 0.5 1 2 4 8µ / µ0

0.5

1

1.5

K-f

acto

r

µ0 = 2 M

W = 160.838 GeV

W- + 3 jets + X

BlackHat+Sherpa

√s = 14 TeV

ET


| < 3

ET

e > 20 GeV, | ηe

| < 2.5

ET/ > 30 GeV, M

T

W > 20 GeV

R = 0.4 [siscone]

Components of NLO al ulationstree-level amplitudes(LO & real radiation)

+ one-loop orre tion to Born level+ subtra tion terms to handle and ombine singularities

+ phase-spa e generator

for example,



0

10

20

30

40

50

60

σ [

pb

]

LONLO

0.25 0.5 1 2 4 8µ / µ0

0.5

1

1.5

K-f

acto

r

µ0 = 2 M

W = 160.838 GeV

W- + 3 jets + X

BlackHat+Sherpa

√s = 14 TeV

ET


| < 3

ET

e > 20 GeV, | ηe

| < 2.5

ET/ > 30 GeV, M

T

W > 20 GeV

R = 0.4 [siscone]

Components of NLO al ulationstree-level amplitudes(LO & real radiation)

+ one-loop orre tion to Born level+ subtra tion terms to handle and ombine singularities

+ phase-spa e generatorfor example, BLACKHAT+SHERPA


BlackHat+Sherpa W+2,3-jets predictions[GLEISBERG, KRAUSS, EPJC53 (2008) 501] [BERGER ET AL., PHYS REV D80 (2009) 074036]

20 40 60 80 100 120 140 160 180 200

10-4

10-3

10-2

10-1

100

dσ /

dET

[ p

b / G

eV ]

LONLOCDF data

20 40 60 80 100 120 140 160 180 200Second Jet E

T [ GeV ]

0.5

1

1.5

2 LO / NLOCDF / NLO

NLO scale dependence

W + 2 jets + X

BlackHat+Sherpa

LO scale dependence

ET


| < 2

ET

e > 20 GeV, | ηe

| < 1.1

ET/ > 30 GeV, M

T

W > 20 GeV

R = 0.4 [siscone]

√s = 1.96 TeV

µR = µ

F = E

T

W

20 30 40 50 60 70 80 90

10-3

10-2

10-1

dσ /

dET

[ p

b / G

eV ]

LONLOCDF data

20 30 40 50 60 70 80 90Third Jet E

T [ GeV ]

0.5

1

1.5

2 LO / NLOCDF / NLO

NLO scale dependence

W + 3 jets + X

BlackHat+Sherpa

LO scale dependence

ET


| < 2

ET

e > 20 GeV, | ηe

| < 1.1

ET/ > 30 GeV, M

T

W > 20 GeV

R = 0.4 [siscone]

√s = 1.96 TeV

µR = µ

F = E

T

W

Redu ed s ale un ertainties: grey NLO bands are smaller wrt yellow LO bandsLO rates are too low: �rst few bins ontain . ross se tion (log plot!) where LO undershootsLO shapes are distorted: Data/NLO is onsistent with one.


MCFM[CAMPBELL, ELLIS, HTTP://MCFM.FNAL.GOV/] [T. AALTONEN ET AL., PRL 100 (2008) 102001]

) [fb

]

- e+ e→*γ

BR

(Z/

× je

tsNσ

210

310

410

510(a)

-1 CDF Data L = 1.7 fb Systematic uncertainties NLO MCFM CTEQ6.1M corrected to hadron level

=1.3sep(Z), R2T + p2

Z = M20µ

/20µ = µ ; 0µ = 2µ NLO scale

NLO PDF uncertainties LO MCFM hadron level

jets N≥1 2 3

Rat

io t

o L

O

1

1.2

1.4

1.6

1.8(b)

[fb

/(G

eV/c

)]

jet

T/d

pσd

-110

1

10

210

310

410

510

20)×1 jet inclusive (≥) + -e+e→*(γZ/

2 jets inclusive≥) + -e+e→*(γZ/(a)

-1 CDF Data L = 1.7 fb Systematic uncertainties NLO MCFM CTEQ6.1M Corrected to hadron level

=1.3sep(Z), R2T + p2

Z = M20

µ /2

0µ = µ ;

0µ = 2µ

PDF uncertainties

Dat

a / T

heo

ry

0.40.60.8

11.21.41.61.8

1 jet inclusive≥) + -e+e→*(γZ/

(b)

[GeV/c] jetTp30 100 200

Dat

a / T

heo

ry

0.40.60.8

11.21.41.61.8

2 jets inclusive≥) + -e+e→*(γZ/

(c)

Z+jetsCDF data20071.7/fb

NLO parton-level event generator for a range of pro esses at hadron olliders.Anybody an study V + 0,1,2 jets � NLO (and LO) by running MCFM themselves.Spin orrelations maintained in de ays. Heli ity amplitudes. Slightly modi�ed CS subtra tion.


Comparison between MCFM and ME+PS mergingJ.M. Campbell and R.K. Ellis, Phys. Rev. D 60 (1999) 113006pp → W+W− + X � Tevatron Run II: • pT of the WW system

0 0.5 1 1.5 2 2.5log[p

T(W

+W

-) / GeV]

1e-05

0.0001

0.001

0.01

0.1

1

(1/σ

) d

σ / d

log[

p T(W

+W

- ) / G

eV]

MCFM NLO (µ=MW

)

Sherpa 1jet ME levelSherpa 0jetSherpa 1jet

W+W

- --> e

+µ-νeνµ production @ Tevatron Run II

PDF: cteq6l

Cuts: pT

lep > 20 GeV, |ηlep

| < 1.0,

pT

jet > 15 GeV, |ηjet

| < 2.0,

∆Rll > 0.2, ∆R

lj > 0.4

MCFM � parton level vs.Sherpa � shower level� LO, distributiondes ribed by delta peak at 0� NLO, the pT of the

WW system �diverges�for soft pT s.ME+PS a ounts for multiplesoft-parton emission leadingto a Sudakov suppressionfor pT → 0for large pT , shapes ofNLO and ME+PS agree well,sin e both in ludereal-emission orre tions� same orderJan Winter FNAL, August 16, 2010 – p.16

Beyond NLONLO+PS mat hingmat h PS to NLO preserving good features of PS (Sudakov suppression at small pT ,multiple soft/ oll emissions) and NLO (rate, high-pT shape, redu ed s ale dependen e)among other pro esses, V produ tion + de ays fully orrelatedMC�NLO: http://www.hep.phy. am.a .uk/theory/webber/MCatNLO/[FRIXIONE, WEBBER; ...]POWHEG: http://moby.mib.infn.it/ nason/POWHEG/[NASON; OLEARI, ...] work on Z+1jet under wayNNLO

pp → Z/γ∗ → ℓ+ℓ− al ulated by Petriello and Melnikov,show s ale dependen e is further redu edqT resummation + mat hing to higher orders al ulations taylored to des ribe spe i� observable very a urately, e.g. pT of Vfor example ResBos [C.-P. YUAN ET AL.]


NLO+PS matchingfor example MC�NLO: S. Frixione and B.R. Webber, JHEP 0206 (2002) 029pp → W+W− + X � 14 TeV LHC: • pT of the WW systemrate & shape omparisonMC�NLO vs. Herwig PSand NLO predi tionnaive NLO+PS leads todouble ountingPS has real-emission ontribution due to�nal-state bran hingPS has virtual ontributiondue to no-bran hing probabilitysolution: subtra t PS evolutionterms from 2 → n + 1 andadd ba k to 2 → nNLO results re overed upon expansion of NLO+PS in αs,mat hing is smooth, no phase-spa e separation ut, �nal states an be hadronized


SummaryIn a high-energy hadron ollider environment the produ tion of W/Z bosons is alwaysa�e ted by QCD radiation.

⇒ re oiling against �QCD� generates the ve tor boson's pT distribution.Parton showers an apture the leading e�e ts of soft and ollinear emissions.⇒ for ertain observables, e.g. pT,V , analyti resummations go beyond these limitsVe tor bosons often ome with additional hard jets.⇒ V + n-jets is a major ba kground to all new physi s sear hes.Parton showers an be improved by merging them with real-emission MEs for hard radiation.

⇒ CKKW, MLM, ...Comparison with data: di�eren es are on 20�40% level if an overall K-fa tor is used to orre t for the total in lusive ross se tion as measured in the experiment.V + n-jets � NLO: not only predi ts shapes but also total rate � NLO.Observables that are sensitive to multiple soft parton emission annot be des ribed.Hadronization and jet orre tions are needed to ompare to data.

⇒ NLO+PS mat hing improves on these last two points.

Please do not hesitate to ask your questions now(and later during the s hool).Look forward to hear and learn more inJohn Campbell's le tures on QCD.


SummaryIn a high-energy hadron ollider environment the produ tion of W/Z bosons is alwaysa�e ted by QCD radiation.

⇒ re oiling against �QCD� generates the ve tor boson's pT distribution.Parton showers an apture the leading e�e ts of soft and ollinear emissions.⇒ for ertain observables, e.g. pT,V , analyti resummations go beyond these limitsVe tor bosons often ome with additional hard jets.⇒ V + n-jets is a major ba kground to all new physi s sear hes.Parton showers an be improved by merging them with real-emission MEs for hard radiation.

⇒ CKKW, MLM, ...Comparison with data: di�eren es are on 20�40% level if an overall K-fa tor is used to orre t for the total in lusive ross se tion as measured in the experiment.V + n-jets � NLO: not only predi ts shapes but also total rate � NLO.Observables that are sensitive to multiple soft parton emission annot be des ribed.Hadronization and jet orre tions are needed to ompare to data.

⇒ NLO+PS mat hing improves on these last two points.

Please do not hesitate to ask your questions now(and later during the s hool).Look forward to hear and learn more inJohn Campbell's le tures on QCD.


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