Precision Simulations for LHC physics and beyond

Introduction Matching & Merging EW corrections Parton Showers Soft Physics Future directions

Precision Simulations for LHC physics andbeyond

Frank Krauss

Institute for Particle Physics PhenomenologyDurham University

KEK, 10.3.2017

F. Krauss IPPP

Precision Simulations for LHC physics and beyond


what the talk is about

matching & merging with parton showers

Electroweak corrections

Revisit Parton Showers

Revisit Soft Physics simulation

where we are and where we (should/could/would) go

F. Krauss IPPP



motivation & introduction

F. Krauss IPPP



motivation: the need for (more) accurate tools

- to date no survivors in searches for new physics & phenomena(a pity, but that’s what Nature hands to us)

- push into precision tests of the Standard Model(find it or constrain it!)

- statistical uncertainties approach zero(because of the fantastic work of accelerator, DAQ, etc.)

- systematic experimental uncertainties decrease(because of ingenious experimental work)

- theoretical uncertainties are or become dominant(it would be good to change this to fully exploit LHC’s potential)

=⇒ more accurate tools for more precise physics needed!

F. Krauss IPPP



motivation: aim of the exercise

review the state of the art in precision simulations(celebrate success)

highlight missing or ambiguous theoretical ingredients(acknowledge failure)

suggest some further studies – experiment and theory(. . . )

F. Krauss IPPP



matching @ (N)NLO

and

merging @ (N)LO

F. Krauss IPPP



the aftermath of the NLO (QCD) revolution

establishing a wide variety of automated tools for NLO calculationsBLACKHAT, GOSAM, MADGRAPH, NJET, OPENLOOPS, RECOLA + automated IR subtraction methods (MADGRAPH, SHERPA)

first full NLO (EW) results with automated tools

technical improvements still mandatory(higher multis, higher speed, higher efficiency, easier handling, . . . )

start discussing scale setting prescriptions(simple central scales for complicated multi-scale processes? test smarter prescriptions?)

steep learning curve still ahead: “NLO phenomenology”(example: methods for uncertainty estimates beyond variation around central scale)

F. Krauss IPPP



Fixed Order (higher-order QCD & EW)

automated Catani-Seymour subtraction, in two independent matrixelement generators within SHERPA

(that was a long time ago, though)

used in conjunction with many one-loop tools:BLACKHAT, GOSAM, NJET, OPENLOOPS, RECOLA

for practically all cutting-edge calculations(tt + 3 jets, V + 5 jets, 5 jets, . . . )

extending subtraction to EW corrections

added full UFO functionality for BSM models

F. Krauss IPPP



example: tt + 3 jets at NLO(QCD) with OPENLOOPS

first computation of tt+3 jetsat NLO / MINLO accuracy

SHERPA NLO MC framework usingCOMIX combined with OPENLOOPS

public results in NTuple formata la BLACKHAT for easy analysis &recycling

scale dependence studied usingHT ,m =

∑m⊥ and MINLO

extended to massive partons

F. Krauss IPPP



prequel: parton showers vs. resummation calculations

various schemes for various logs in analytic resummation

concentrate on parton shower instead ←→ compare with QT

resummation(transverse momentum of Higgs boson etc.)

parametric accuracy by comparing Sudakov form factors:

∆ = exp

−∫

dk2⊥

k2⊥

[A log

k2⊥

Q2+ B

],

where A and B can be expanded in αS(k2⊥)

showers usually include terms A1,2 and B1 (NLL)

A2 often realised by pre-factor multiplying scale µR ' k⊥

F. Krauss IPPP



some parton shower fun with DY(example of accuracy in description of standard precision observable)

b

b b b b b b b bb

bbbbb b b b

bbb

b

b

b

b

b

b

b b b b b b b bb

bbbbbb b b b

bb

b

b

b

b

b

b

b b b b b b b b bbb b

bb b b b b

bb

b

b

b

b

b

Sher

paM

C

0 ≤ |yZ| ≤ 1

1 ≤ |yZ| ≤ 2 (×0.1)

2 < |yZ| ≤ 2.4 (×0.01)

b ATLAS dataJHEP 09 (2014) 145ME+PS (1-jet)5 ≤ Qcut ≤ 20 GeV

1 210−9

10−8

10−7

10−6

10−5

10−4

10−3

10−2

10−1pT spectrum, Z→ ee (dressed)

1σ

fidd

σfid

dp T

[GeV

−1 ]

b b b b b b b b b b b b b b b b b b b bbbbbb

b

b

b

b

b

b

b

b

b

b b b b b b b b b b b b b b b b b bb b

bbbbb

b

b

b

b

b

b

b

b

b

b b b b b b b b b b b b b b b b b b bb b

bbbb

b

b

b

b

b

b

b

bb

Sher

paM

C

|yZ| < 0.8

0.8 ≤ |yZ| ≤ 1.6 (×0.1)

1.6 < |yZ| (×0.01)

b ATLAS dataPhys.Lett. B720 (2013) 32ME+PS (1-jet)5 ≤ Qcut ≤ 20 GeV

3 2 1

10−4

10−3

10−2

10−1

1

10 1

φ∗η spectrum, Z→ ee (dressed)

1σ

fid.

dσ

fid.

dφ

∗ η

b b b b b b b b b b b b b b b b b b b b b b b b b b

0 ≤ |yZ| ≤ 1

1 2

0.80.91.01.11.2

| |

MC

/Dat

a

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

|yZ| < 0.8

3 2 1

0.80.91.01.11.2

→ | |

MC

/Dat

ab b b b b b b b b b b b b b b b b b b b b b b b b b

1 ≤ |yZ| ≤ 2

1 2

0.80.91.01.11.2

| |

MC

/Dat

a


0.8 ≤ |yZ| ≤ 1.6

3 2 1

0.80.91.01.11.2

→ ≤ | |

MC

/Dat

a

b b b b b b b b b b b b b b b b b b b b b b b b b b

2 < |yZ| ≤ 2.4

1 10 1 10 2

0.80.91.01.11.2

| |

pT,ll [GeV]

MC

/Dat

a


1.6 < |yZ|

10−3 10−2 10−1 1

0.80.91.01.11.2

→ | | ≥

φ∗η

MC

/Dat

a

F. Krauss IPPP



matching at NLO and NNLO

avoid double-counting of emissions

two schemes at NLO: MC@NLO and POWHEG

mismatches of K factors in transition to hard jet regionMC@NLO: −→ visible structures, especially in gg → HPOWHEG: −→ high tails, cured by h dampening factorwell-established and well-known methods

(no need to discuss them any further)

two schemes at NNLO: MINLO & UN2LOPS (singlets S only)

different basic ideasMINLO: S + j at NLO with p

(S)T → 0 and capture divergences by

reweighting internal line with analytic Sudakov, NNLO accuracyensured by reweighting with full NNLO calculation for S productionUN2

LOPS identifies and subtracts and adds parton shower terms atFO from S + j contributions, maintaining unitarityavailable for two simple processes only: DY and gg → H

F. Krauss IPPP



NNLOPS for H production: MINLO

K. Hamilton, P. Nason, E. Re & G. Zanderighi, JHEP 1310

also available for Z/W /VH production

F. Krauss IPPP



NNLOPS for Z production: UN2LOPS

S. Hoche, Y. Li, & S. Prestel, Phys.Rev.D90 & D91

She

rpa+

Bla

ckH

at

NNLONLO'FEWZ

= 7 TeVs<120 GeV

ll60 GeV<m

NLO'll<2mR/F

µ/2< ll m NNLOll<2m

R/Fµ/2< ll m

[pb]

-e

+e

/dy

σd

20

40

60

80

100

120

140

160

180

200

Rat

io to

NLO

0.960.98

11.021.04

-e+ey

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

b

bb

b bb

bbb b

bbb b

b

b

b

b

b

ATLAS PLB705(2011)415b

UN2LOPSmll/2 < µR/F < 2 mllmll/2 < µQ < 2 mll

10−6

10−5

10−4

10−3

10−2

10−1Z pT reconstructed from dressed electrons

1/σ

dσ

/d

p T,Z

[1/G

eV]

b b b b b b b b b b b b b b b b b b b

1 10 1 10 2

0.6

0.8

1

1.2

1.4

pT,Z [GeV]

MC

/Dat

a

also available for H production

F. Krauss IPPP



NNLOPS: shortcomings/limitations

MINLO relies on knowledge of B2 terms from analytic resummation−→ to date only known for colour singlet production

MINLO relies on reweighting with full NNLO result−→ one parameter for H (yH), more complicated for Z , . . .

UN2LOPS relies on integrating single- and double emission to lowscales and combination of unresolved with virtual emissions−→ potential efficiency issues, need NNLO subtraction

UN2LOPS puts unresolved & virtuals in “zero-emission” bin−→ no parton showering for virtuals (?)

F. Krauss IPPP



merging example: p⊥,γγ in MEPS@LO vs. NNLO(arXiv:1211.1913 [hep-ex])

[p

b/G

eV

]γγ

T,

/dp

σd

510

410

310

210

110

1

10

1Ldt = 4.9 fb ∫ Data 2011,

1.3 (MRST2007)×PYTHIA MC11c

1.3 (CTEQ6L1)×SHERPA MC11c

ATLAS

= 7 TeVs

data

/SH

ER

PA

00.5

11.5

22.5

3

[GeV]γγT,

p

0 50 100 150 200 250 300 350 400 450 500

da

ta/P

YT

HIA

00.5

11.5

22.5

3 [

pb

/Ge

V]

γγT,

/dp

σd

510

410

310

210

110

1

10

1Ldt = 4.9 fb ∫ Data 2011,

DIPHOX+GAMMA2MC (CT10)

NNLO (MSTW2008)γ2

ATLAS

= 7 TeVs

da

ta/D

IPH

OX

00.5

11.5

22.5

3

[GeV]γγT,

p

0 50 100 150 200 250 300 350 400 450 500

NN

LO

γd

ata

/2

00.5

11.5

22.5

3

F. Krauss IPPP



merging for loop-induced processesexample: merging for gg → ZH

(looking for H → inv. in `¯+ E/ final states at 13 TeV LHC)

10−8

10−7

10−6

10−5

10−4

10−3

10−2

dσ

/dE

mis

sT

[pb/

GeV

]

Sherpa+OpenLoops

H(inv)Z(ll)gg

H(inv)Z(ll)qq

ZZ, WWgg

ZZ, WWqq

100 200 300 400 500 600Emiss

T [GeV]

1

2

3

4 MEPS@Loop2/Loop2+PS

10−8

10−7

10−6

10−5

10−4

10−3

10−2

dσ

/d

pj lead⊥

[pb/

GeV

]

Sherpa+OpenLoops

H(inv)Z(ll)gg

H(inv)Z(ll)qq

ZZ, WWgg

ZZ, WWqq

100 200 300 400 500 600pjlead⊥ [GeV]

100

101

102 MEPS@Loop2/Loop2+PS

F. Krauss IPPP



multijet-merging at NLO

sometimes “more legs” wins over more loops

basic idea like at LO: towers of MEs with increasing jet multi(but this time at NLO)

combine them into one sample, remove overlap/double-counting

maintain NLO and LL accuracy of ME and PS

this effectively translates into a merging of MC@NLO simulations andcan be further supplemented with LO simulations for even higherfinal state multiplicities

different implementations, parametric accuracy not always clear(MEPS@NLO, FxFx, UNLOPS)

starts being used, still lacks careful cross-validation

F. Krauss IPPP



illustration: pH⊥ in MEPS@NLO

pp → h + jetsSherpa S-MC@NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1

Transverse momentum of the Higgs boson

p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

] first emission byMC@NLO

MC@NLO pp → h + jetfor Qn+1 > Qcut

restrict emission offpp → h + jet toQn+2 < Qcut

MC@NLO

pp → h + 2jets forQn+2 > Qcut

iterate

sum all contributions

eg. p⊥(h)>200 GeVhas contributions fr.multiple topologies

F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

first emission byMC@NLO , restrict toQn+1 < Qcut



MC@NLO


iterate



F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLOpp → h + 1j @ NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLOpp → h + 1j @ NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLOpp → h + 1j @ NLOpp → h + 2j @ NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLOpp → h + 1j @ NLOpp → h + 2j @ NLO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP




pp → h + jetspp → h + 0j @ NLOpp → h + 1j @ NLOpp → h + 2j @ NLOpp → h + 3j @ LO

0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP





0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP





0 50 100 150 200 250 30010−4

10−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]




MC@NLO


iterate



F. Krauss IPPP



detailed comparison of approaches

in

H+jets

F. Krauss IPPP



inclusive Higgs boson rapidity

LH

15pp

→h

+je

tsco

mpa

riso

n

ResBos 2Sherpa h Nnlo

0 1 2 3 4 50

5

10

15

20

25Higgs boson rapidity

y(h)

dσ

/dy

[pb]

0.6

0.8

1

1.2

1.4

Higgs boson rapidity

Rat

ioto

hN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hN

nlo

0 1 2 3 4 5

0.6

0.8

1

1.2

1.4

y(h)

Rat

ioto

hN

nlo

excellent agreement between NNLO and NNLOPS

multijet merged with NLO normalisation, PDF effects

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

ResBos 2Sherpa h NnloPowheg NnloPsSherpa NnloPs

0 1 2 3 4 50

5

10

15

20


y(h)

dσ

/dy

[pb]

0.6

0.8

1

1.2

1.4


Rat

ioto

hN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hN

nlo

0 1 2 3 4 5

0.6

0.8

1

1.2

1.4

y(h)

Rat

ioto

hN

nlo



F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

ResBos 2Sherpa h NnloPowheg NnloPsSherpa NnloPsMG5_aMC FxFxSherpa MePs@NloHerwig 7.1

0 1 2 3 4 50

5

10

15

20


y(h)

dσ

/dy

[pb]

0.6

0.8

1

1.2

1.4


Rat

ioto

hN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hN

nlo

0 1 2 3 4 5

0.6

0.8

1

1.2

1.4

y(h)

Rat

ioto

hN

nlo



F. Krauss IPPP



Higgs boson transverse momentum (nj ≥ 0)

LH

15pp

→h

+je

tsco

mpa

riso

n

HqTResBos 2

0 50 100 150 20010−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Hq

T

0.6

0.8

1

1.2

1.4

Rat

ioto

Hq

T

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Hq

T

good agreement between HqT and NNLOPS, scale choice at high p⊥multijet merged with NLO normalisation, very different at low p⊥

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

HqTResBos 2

Powheg NnloPsSherpa NnloPs

0 50 100 150 20010−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Hq

T

0.6

0.8

1

1.2

1.4

Rat

ioto

Hq

T

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Hq

T

good agreement between HqT and NNLOPS, scale choice at high p⊥

multijet merged with NLO normalisation, very different at low p⊥

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

HqTResBos 2

Powheg NnloPsSherpa NnloPsMG5_aMC FxFxSherpa MePs@NloHerwig 7.1

0 50 100 150 20010−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Hq

T

0.6

0.8

1

1.2

1.4

Rat

ioto

Hq

T

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Hq

T

good agreement between HqT and NNLOPS, scale choice at high p⊥multijet merged with NLO normalisation, very different at low p⊥

F. Krauss IPPP



Higgs boson transverse momentum (nj = 0)

LH

15pp

→h

+je

tsco

mpa

riso

n

Sherpa h Nnlo

0 20 40 60 80 10010−5

10−4

10−3

10−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

fixed-order has various unphysical features

good agreement between the NNLOPS

multijet merged with NLO normalisation,HERWIG7 has much less soft radiation

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


0 20 40 60 80 10010−5

10−4

10−3

10−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0 20 40 60 80 100

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg




F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


0 20 40 60 80 10010−5

10−4

10−3

10−2

10−1

1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0 20 40 60 80 100

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg




F. Krauss IPPP



exclusive over inclusive rate

LH

15pp

→h

+je

tsco

mpa

riso

n

inclusive

exclusive


0 20 40 60 80 10010−2

10−1

1

Ratio of exclusive over inclusive Higgs production

p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.2

0.4

0.6

0.8

1

1.2

Ratio of exclusive over inclusive Higgs production

excl

/inc

l

0.2

0.4

0.6

0.8

1

1.2

excl

/inc

l

0 20 40 60 80 100

0.2

0.4

0.6

0.8

1

1.2

p⊥(h) [GeV]

excl

/inc

l

≈ 20% of Higgs with p⊥ = 60 GeV are not accompanied by a jet

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

BFGLP hj Nnlo

0 50 100 150 20010−3

10−2

10−1

1Higgs boson transverse momentum (nj ≥ 1)

p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

fixed-order has Sudakov shoulder at ph⊥ = 30 GeV due to jet cuthere: bins left and right set to average

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


0 50 100 150 20010−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0.6

0.8

1

1.2

1.4

Rat

ioto

Powheg

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg

good agreement between NNLOPS, different scales at large p⊥excess of POWHEG as p⊥ → 0 (Higgs strahlung off dijet)

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


0 50 100 150 20010−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0.6

0.8

1

1.2

1.4

Rat

ioto

Powheg

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg

multijet merged different shape at p⊥ . 60 GeV

except aMC@NLO MADGRAPH5 (due to different scales?)

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


GoSam+Sherpa hjNLO

0 50 100 150 20010−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0.6

0.8

1

1.2

1.4

Rat

ioto

Powheg

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg

multijet merged different shape at p⊥ . 60 GeV

same as at fixed-order NLO

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


BFGLP hj Nnlo

GoSam+Sherpa hjNLOMiNLOnNLO

0 50 100 150 20010−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0.6

0.8

1

1.2

1.4

Rat

ioto

Powheg

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg

NNLO impacts on shape at p⊥ . 60 GeV

NLL+NLO resummation gets close to NNLO result

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


ResBos 2BFGLP hj Nnlo

GoSam+Sherpa hjNLO

0 50 100 150 20010−3

10−2

10−1


p⊥(h) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

Powheg

0.6

0.8

1

1.2

1.4

Rat

ioto

Powheg

0 50 100 150 200

0.6

0.8

1

1.2

1.4

p⊥(h) [GeV]

Rat

ioto

Powheg

NNLO impacts on shape at p⊥ . 60 GeV

NLL+NLO resummation gets close to NNLO result

F. Krauss IPPP



leading jet transverse momentum (nj ≥ 1)

LH

15pp

→h

+je

tsco

mpa

riso

n

BFGLP hj Nnlo

40 60 80 100 120 140 160 180 20010−3

10−2

10−1

1Leading jet transverse momentum (nj ≥ 1)

p⊥(j1) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

NNLO and NLO show very good convergence for this scale choice

multijet merged ≈ 20% lower in high-p⊥ (due to showering)

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

STWZResBos 2BFGLP hj Nnlo

40 60 80 100 120 140 160 180 20010−3

10−2

10−1


p⊥(j1) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4

Leading jet transverse momentum (nj ≥ 1)

Rat

ioto

hjN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hjN

nlo

40 60 80 100 120 140 160 180 200

0.6

0.8

1

1.2

1.4

p⊥(j1) [GeV]

Rat

ioto

hjN

nlo



F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n


GoSam+Sherpa hjNLO

40 60 80 100 120 140 160 180 20010−3

10−2

10−1


p⊥(j1) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

hjN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hjN

nlo

40 60 80 100 120 140 160 180 200

0.6

0.8

1

1.2

1.4

p⊥(j1) [GeV]

Rat

ioto

hjN

nlo



F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n



GoSam+Sherpa hjNLO

40 60 80 100 120 140 160 180 20010−3

10−2

10−1


p⊥(j1) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

hjN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hjN

nlo

40 60 80 100 120 140 160 180 200

0.6

0.8

1

1.2

1.4

p⊥(j1) [GeV]

Rat

ioto

hjN

nlo



F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n



GoSam+Sherpa hjNLO

40 60 80 100 120 140 160 180 20010−3

10−2

10−1


p⊥(j1) [GeV]

dσ

/dp ⊥

[pb/

GeV

]

0.6

0.8

1

1.2

1.4


Rat

ioto

hjN

nlo

0.6

0.8

1

1.2

1.4

Rat

ioto

hjN

nlo

40 60 80 100 120 140 160 180 200

0.6

0.8

1

1.2

1.4

p⊥(j1) [GeV]

Rat

ioto

hjN

nlo



F. Krauss IPPP



jet vetoed cross sections – inclusive

LH

15pp

→h

+je

tsco

mpa

riso

n

inclusive

STWZ

10 20 50 100 2000

10

20

30

40

50Jet veto cross section

pveto⊥ [GeV]

σ0(

pveto

⊥)

[pb]

0.6

0.8

1

1.2

1.4

Jet veto cross section

Rat

ioto

STW

Z

10 20 50 100 200

0.6

0.8

1

1.2

1.4

pveto⊥ [GeV]

Rat

ioto

STW

Z

very good agreement between NNLOPS and STWZ

multijet merged with larger spread in shape,but within uncertainties once NLO normalisation accounted for

PS resummation uncertainties nowhere fully assessed

F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

inclusive

STWZPowheg NnloPsSherpa NnloPs

10 20 50 100 2000

10

20

30

40


pveto⊥ [GeV]

σ0(

pveto

⊥)

[pb]

0.6

0.8

1

1.2

1.4


Rat

ioto

STW

Z

10 20 50 100 200

0.6

0.8

1

1.2

1.4

pveto⊥ [GeV]

Rat

ioto

STW

Zvery good agreement between NNLOPS and STWZ



F. Krauss IPPP




LH

15pp

→h

+je

tsco

mpa

riso

n

inclusive

STWZPowheg NnloPsSherpa NnloPs

MG5_aMC FxFxSherpa MePs@NloHerwig 7.1

10 20 50 100 2000

10

20

30

40


pveto⊥ [GeV]

σ0(

pveto

⊥)

[pb]

0.6

0.8

1

1.2

1.4


Rat

ioto

STW

Z

10 20 50 100 200

0.6

0.8

1

1.2

1.4

pveto⊥ [GeV]

Rat

ioto

STW

Zvery good agreement between NNLOPS and STWZ



F. Krauss IPPP



aside: quark mass effects in GGF

include effects of quark masses

reweight NLO HEFT with LO ratio:(reweight virtual with Born ratio, real with real ratio)

dσ(NLO)mass ≈ dσ

(NLO)HEFT ×

dσ(LO)mass

dσ(LO)HEFT

F. Krauss IPPP



example: mass effects in gg → H (LO merging for b contribution)

−0.2

−0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

dσ

/d

pH ⊥[p

b/G

eV]

HEFTSM, mb = 0

SM, µbPS = mb

SM, µbPS = 9 GeV

SM, µbPS = 31 GeV

100 101 102

pH⊥ [GeV]

0.20.40.60.81.0

Rat

ioto

HEF

T

Sherpa+OpenLoopsy2

t -contributions: MC@NLOytyb-contributions: LO+PSpp→ H + X√

s = 13 TeV

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

dσ

/d

pH ⊥[p

b/G

eV]

HEFTSM, mb = 0

SM, Qbcut ∈ [1/

√2mb,

√2mb]

100 101 102 103

pH⊥ [GeV]

0.88

0.96

1.04

1.12

Rat

ioto

HEF

T

Sherpa+OpenLoopsy2

t -contributions: MC@NLOytyb-contributions: CKKW

pp→ H + X√s = 13 TeV

10−6

10−5

10−4

10−3

10−2

10−1

100

dσ

/dpH ⊥

[pb/

GeV

]

0 jet SM1 jet SM2 jet SM3 jet SM

0 jet HEFT1 jet HEFT2 jet HEFT3 jet HEFT

0 200 400 600 800 1000pH⊥ [GeV]

0.20.40.60.81.0

Rat

ioto

HEF

T

Sherpa+OpenLoopsMEPS@NLOpp→ H + X√

s = 13 TeV

F. Krauss IPPP



Z+jets at 13 Tev: comparison with ATLAS datavarious merging codes at LO and NLO

jetsN

0≥ 1≥ 2≥ 3≥ 4≥ 5≥ 6≥ 7≥

) [p

b]je

ts*+

Nγ

(Z/

σ

-210

-110

1

10

210

310

410

510

610ATLAS Preliminary

1−13 TeV, 3.16 fb

jets, R = 0.4tanti-k

< 2.5jet

y > 30 GeV, jet

Tp

) + jets−l+ l→*(γZ/

Data

HERPAS + ATHLACK B

2.1HERPA S

6YP + LPGEN A

8 CKKWLYP + MG5_aMC

8 FxFxYP + MG5_aMC

jetsN

0≥ 1≥ 2≥ 3≥ 4≥ 5≥ 6≥ 7≥

Pre

d./D

ata

0.5

1

1.5

jetsN

0≥ 1≥ 2≥ 3≥ 4≥ 5≥ 6≥ 7≥

Pre

d./D

ata

0.5

1

1.5

jetsN

0≥ 1≥ 2≥ 3≥ 4≥ 5≥ 6≥ 7≥

Pre

d./D

ata

0.5

1

1.5

[GeV]TH

200 400 600 800 1000 1200 1400

[pb/

GeV

]T

/dH

σd

-410

-310

-210

-110

1

10

210

310ATLAS Preliminary

1−13 TeV, 3.16 fb

jets, R = 0.4tanti-k

< 2.5jet

y > 30 GeV, jet

Tp

1 jet≥) + −l+ l→*(γZ/ Data

NNLOjetti

1 jet N≥ Z + HERPAS + ATHLACK B

2.1HERPA S6YP + LPGEN A

8 CKKWLYP + MG5_aMC8 FxFxYP + MG5_aMC

[GeV]T

H

200 400 600 800 1000 1200 1400

Pre

d./D

ata

0.5

1

1.5

[GeV]T

H

200 400 600 800 1000 1200 1400P

red.

/Dat

a 0.5

1

1.5

[GeV]TH

200 400 600 800 1000 1200 1400

Pre

d./D

ata

0.5

1

1.5

F. Krauss IPPP



including EW corrections

F. Krauss IPPP



EW corrections

EW corrections sizeable O(10%) at large scales: must include them!

but: more painful to calculate

need EW showering & possibly corresponding PDFs(somewhat in its infancy: chiral couplings)

example: Zγ vs. pT (right plot)(handle on pZ⊥ in Z → νν)

(Kallweit, Lindert, Pozzorini, Schoenherr for LH’15)

difference due to EW charge of Z

no real correction (real V emission)

improved description of Z → ``

bb

b

bbb

b bb

bb b

bb

b

b

b

b

bb

b

b

bb

bb

bb

bb

bb

b b b b

bb

b

b

bb

bb

bb

bb

b b b b b b

Sher

pa+O

pen

Lo

ops

b NLO QCDb NLO QCD+EW

b CMS dataJHEP10(2015)128

0

0.01

0.02

0.03

0.04

0.05Z/γ ratio for events with njets ≥ 1

dσ

/dpZ T

/d

σ/d

pγ T

b

b

b b

b

b

b

b b

b bb

b

b

b

b

b

b

b

b

b b

b

b

b

b b

b bb

b

b

b

b

b

b

b b b b b b b b b b b b b b b b b b

100 200 300 400 500 600 700 800

0.8

0.9

1.0

1.1

1.2

pZ/γT [GeV]

MC

/Dat

a

F. Krauss IPPP



inclusion of electroweak corrections in simulation

incorporate approximate electroweak corrections in MEPS@NLO

1 using electroweak Sudakov factors

Bn(Φn) ≈ Bn(Φn) ∆EW(Φn)

2 using virtual corrections and approx. integrated real corrections

Bn(Φn) ≈ Bn(Φn) + Vn,EW(Φn) + In,EW(Φn) + Bn,mix(Φn)

real QED radiation can be recovered through standard tools(parton shower, YFS resummation)

simple stand-in for proper QCD⊕EW matching and merging→ validated at fixed order, found to be reliable,→ difference . 5% for observables not driven by real radiation

F. Krauss IPPP



results: pp → `−ν + jets(Kallweit, Lindert, Maierhofer, Pozzorini, Schoenherr JHEP04(2016)021)

Sher

pa+O

pen

Lo

ops

Qcut = 20 GeV

MEPS@LOMEPS@NLO QCDMEPS@NLO QCD+EWvirtMEPS@NLO QCD+EWvirt w.o. LO mix

100

10–3

10–6

10–9

pp → ℓ−ν + 0,1,2 j @ 13 TeV

dσ

/dp T

,V[p

b/G

eV]

50 100 200 500 1000 20000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

pT,V [GeV]

dσ

/dσ

NL

OQ

CD

Sher

pa+O

pen

Lo

ops

Qcut = 20 GeV

MEPS@LOMEPS@NLO QCDMEPS@NLO QCD+EWvirtMEPS@NLO QCD+EWvirt w.o. LO mix

100

10–3

10–6

10–9

pp → ℓ−ν + 0,1,2 j @ 13 TeV

dσ

/dp T

,j 1[p

b/G

eV]

50 100 200 500 1000 20000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

pT,j1 [GeV]

dσ

/dσ

NL

OQ

CD

⇒ particle level events including dominant EW corrections

F. Krauss IPPP



NLO EW predictions for ∆R(µ, j1)

Sher

pa+O

pen

Lo

ops

LHC 8 TeVpp → µν + 1j inclusive

LO

0 1 2 3 4 50

0.05

0.1

0.15

0.2

0.25Angular separtion of leading jet and muon

∆R(µ, j)

dσ

/d∆

R[p

b] measure collinear W emission?LHC@8TeV, pj1⊥ > 500GeV, central µ and jet

LO pp →Wj with ∆φ(µ, j) ≈ π

NLO corrections neg. in peaklarge pp →Wjj component opening PS

sub-leading Born (γPDF) at large ∆R

restrict to exactly 1j , no pj2⊥ > 100 GeV

describe pp →Wjj @ NLO, pj2⊥ > 100 GeV

pos. NLO QCD,

neg. NLO EW,

∼ flat

sub-leading Born contribs positive

sub2leading Born (diboson etc) conts. pos.→ possible double counting with BG

merge using exclusive sums

F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCD

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D

measure collinear W emission?LHC@8TeV, pj1⊥ > 500GeV, central µ and jet

LO pp →Wj with ∆φ(µ, j) ≈ πNLO corrections neg. in peaklarge pp →Wjj component opening PS




pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCDNLO QCD+EW

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D






pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCDNLO QCD+EWNLO QCD+EW+subLO

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D






pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops

LHC 8 TeVpp → µν + 1j exclusive


0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D






pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LO

0 1 2 3 4 50

0.05

0.1

0.15

0.2


∆R(µ, j)

dσ

/d∆

R[p

b] measure collinear W emission?LHC@8TeV, pj1⊥ > 500GeV, central µ and jet





pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCD

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D






pos. NLO QCD,

neg. NLO EW,

∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCDNLO QCD+EW

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D






pos. NLO QCD, neg. NLO EW, ∼ flat




F. Krauss IPPP




Sher

pa+O

pen

Lo

ops



0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D










F. Krauss IPPP




Sher

pa+O

pen

Lo

ops


LONLO QCDNLO QCD+EWNLO QCD+EW+subLONLO QCD+EW+subLO+sub2LO

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D










F. Krauss IPPP




Sher

pa+O

pen

Lo

ops

LHC 8 TeVpp → µν + 1j exclusive+pp → µν + 2j inclusive

LONLO QCDNLO QCD+EWNLO QCD+EW+subLONLO QCD+EW+subLO+sub2LO

0

0.05

0.1

0.15

0.2


dσ

/d∆

R[p

b]

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0.5

1

1.5

Rat

iow

rt.N

LO

QC

D

0 1 2 3 4 5

0.5

1

1.5

∆R(µ, j)

Rat

iow

rt.N

LO

QC

D










F. Krauss IPPP



NLO EW predictions for ∆R(µ, j1)R

) [fb]

∆/d

(σ

d

20

40

60

80

100

120

140

160

180 1 = 8 TeV, 20.3 fbs

Data

ALPGEN+PYTHIA6 W+jets

PYTHIA8 W+j & jj+weak shower

SHERPA+OpenLoops W+j & W+jj

NNLOjetti

1 jet N≥W +

> 500 GeVT

Leading Jet p

ATLAS

0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2

, closest jet)µR(∆

0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2

Data comparison(M. Wu ICHEP’16, ATLAS arXiv:1609.07045)

ALPGEN+PYTHIA

pp →W + jets MLM merged(Mangano et.al., JHEP07(2003)001)

PYTHIA 8pp →Wj + QCD showerpp → jj + QCD+EW shower

(Christiansen, Prestel, EPJC76(2016)39)

SHERPA+OPENLOOPS

NLO QCD+EW+subLOpp →Wj/Wjj excl. sum

(Kallweit, Lindert, Maierhofer,)

(Pozzorini, Schoenherr, JHEP04(2016)021)

NNLO QCD pp →Wj(Boughezal, Liu, Petriello, arXiv:1602.06965)

F. Krauss IPPP




) [fb]

∆/d

(σ

d

20

40

60

80

100

1201 = 8 TeV, 20.3 fbs

Data




< 600 GeVT

500 GeV < Leading Jet p

ATLAS

0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2


0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2


ALPGEN+PYTHIA




SHERPA+OPENLOOPS





F. Krauss IPPP




) [fb]

∆/d

(σ

d

5

10

15

20

25

30

35

40

45

501 = 8 TeV, 20.3 fbs

Data




> 650 GeVT

Leading Jet p

ATLAS

0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2


0 0.5 1 1.5 2 2.5 3 3.5 4

Pre

d./

Da

ta

0.5

1

1.5

2


ALPGEN+PYTHIA




SHERPA+OPENLOOPS





F. Krauss IPPP



improving parton showers

F. Krauss IPPP



another systematic uncertainty

parton showers are approximations, based onleading colour, leading logarithmic accuracy, spin-averaged

parametric accuracy by comparing Sudakov form factors:

∆ = exp

−∫

dk2⊥

k2⊥

[A log

k2⊥

Q2+ B

],

where A and B can be expanded in αS(k2⊥)

showers usually include terms A1,2 and B1 (NLL)

A2 often realised by pre-factor multiplying scale µR ' k⊥(CMW rescaling: Catani, Marchesini, Webber, Nucl Phys B,349 635)

fixed-order precision necessitates to consistently assess uncertaintiesfrom parton showers (quite often just used as black box)

maybe improve by including higher orders?

F. Krauss IPPP



event generation (on-the-fly scale variations)

basic idea: want to vary scales to assess uncertainties

simple reweighting in matrix elements straightforward

reweighting in parton shower more cumbersome

shower is probabilistic, concept of weight somewhat alienintroduce relative weightevaluate (trial-)emission by (trial-)emission

F. Krauss IPPP



implementation in HERWIG7

Dipole (µR/√

2, µF /√

2)

Dipole (√

2µR,√

2µF )

Dipole (µR, µF )10−7

10−6

10−5

10−4

10−3

10−2

1/σ

×dσ/d

p⊥

(H)

[1/G

eV]

1 10 1 10 2

0.6

0.8

1

1.2

1.4

p⊥(H) [GeV]

Rew

eigh

t/D

irec

t

AO (µR/√

2, µF /√

2)

AO (√

2µR,√

2µF )

AO (µR, µF )10−7

10−6

10−5

10−4

10−3

10−2

1/σ

×dσ/d

p⊥

(H)

[1/G

eV]

1 10 1 10 2

0.6

0.8

1

1.2

1.4

p⊥(H) [GeV]

Rew

eigh

t/D

irec

t

F. Krauss IPPP



weight variation for W+jets with MEPS@NLO

uncertainties in pW⊥

pW⊥ [GeV]

10−6

10−3

100

103

106

dσ/dpW T

[pb/GeV

]

NLOPS

W pT uncertainty bands

Sherpa MEPS@NLO

pp → W[eν], √s = 13 TeVnNLOPS = 1, nPS = 2

µF ,RαS

CT14

dedicated

100 101 102 103

pWT [GeV]

0.7

1.0

1.3

ratio

toCV

100 101 102 103

0.7

1.0

1.3

scale uncertainty

100 101 102 103

0.9

1.0

1.1

ratio

toCV

αS uncertainty

100 101 102 103

pWT [GeV]

0.9

1.0

1.1

CT14 uncertainty

CPU budget

0 1 2 3 4 6 8nPS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

t rew/t ded

nPS =∞

Sherpa MEPS@LO

0/0 1/0 1/1 1/2 1/3 1/5 1/7nNLOPS / nPS

nNLOPS = 1, nPS =∞

Sherpa MEPS@NLO

0 1 2 3 4 6 8nPS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

t rew/t ded

nPS =∞

Sherpa LOPS

pp → W[eν], √s = 13 TeV

0/0 1/0 1/1 1/2 1/3 1/5 1/7nNLOPS / nPS

nNLOPS = 1, nPS =∞

Sherpa NLOPS

parton-level only

+ non-perturbative eects

+ unweighting

F. Krauss IPPP



going beyond leading colour

start including next-to leading colour(first attempts by Platzer & Sjodahl; Nagy & Soper)

0.001

0.01

0.1

1

average transverse momentum w.r.t. ~n3

DipoleShower + ColorFull

0.80.9

11.11.2

1 10

〈p⊥〉/GeV

fullshower

strict large-Nc

GeV

N−1

dN/d

〈p⊥

〉x/f

ull

also included in 1st emission in SHERPA’s MC@NLO

F. Krauss IPPP



towards higher logarithmic accuracy(Catani, Hoeche, FK, Prestel, in prep.)

reproduce DGLAP evolution at NLOinclude all NLO splitting kernels

corrections to standard 1→ 2 trivial

2-loop cusp term subtracted &combined with LO soft contributionuse weighting algorithms

(Hoeche, Schumann, Siegert, 0912.3501)

new topology at NLO fromq → q and q → q′ splittings

generic 1→ 3 process in parton shower

first branching treated as soft gluonradiation, second as collinear splitting(to match diagrammatic structure)

implementation complete andcross-checked (PYTHIA vs. SHERPA)

y23 × 2

y34 × 10−1

y45 × 10−2

y56 × 10−3

Dir

ePS

e+e− → qq @ 91.2 GeV

1 → 2 only1 → 2 & 1 → 3

10−2

10−1

1

10 1

10 2

10 3

10 4

Differential jet resolution at parton level (Durham algorithm)

dσ

/d

log 10

y nn+

1[p

b]

y23

0.940.960.98

1.01.021.041.06

Rat

io

y34

0.940.960.98

1.01.021.041.06

Rat

io

y45

0.940.960.98

1.01.021.041.06

Rat

io

y56

-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5

0.940.960.98

1.01.021.041.06

log10 yn n+1

Rat

io

F. Krauss IPPP



some first results:DY (pp → e+e−, left) ggF (pp → H → τ+τ−, right)

added scale uncertainties in parton shower by varying µR

Dir

ePS

pp → e+e− @ 8TeV

Γ1 ⊕ γ1Γ1 ⊕ γ1 ⊕ Γ21/2 t < µR < 2 tΓ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2Γ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2 ⊕ Γ31/2 t < µR < 2 t

1

10 1

10 2

10 3

Differential 0 → 1 jet resolution

dσ

/d

log 10

(d01

/G

eV)

[pb]

0.5 1 1.5 2 2.5

0.8

0.9

1.0

1.1

1.2

log10(d01/GeV)

Rat

io

Dir

ePS

pp → [h → τ+τ−] @ 8TeV

Γ1 ⊕ γ1Γ1 ⊕ γ1 ⊕ Γ21/2 t < µR < 2 tΓ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2Γ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2 ⊕ Γ31/2 t < µR < 2 t

10−3

10−2

10−1

Differential 0 → 1 jet resolution

dσ

/d

log 10

(d01

/G

eV)

[pb]

0.5 1 1.5 2 2.5

0.8

0.9

1.0

1.1

1.2

log10(d01/GeV)

Rat

io

F. Krauss IPPP



some first results - comparison with data:y23 at LEP (left) and φ∗ distribution in Drell-Yan production (right)

F. Krauss IPPP



hadronisation

F. Krauss IPPP



colour reconnections and friends(Fischer, Sjostrand, 1610:09818)

(slide stolen from Torbjorn Sjostrand)

F. Krauss IPPP



strange strangeness

(ALICE collaboration)

universality of hadronisation assumed

parameters tuned to LEP datain particular: strangeness suppression

for strangeness: flat ratiosbut data do not reproduce this

looks like SU(3) restorationnot observed for protons

needs to be investigated

|< 0.5η|⟩η/d

chNd⟨

10 210 310)+ π+− π

Rat

io o

f yie

lds

to (

3−10

2−10

1−10

16)× (+Ω+−Ω

6)× (+Ξ+−Ξ

2)× (Λ+Λ

S02K

ALICE = 7 TeVspp,

= 5.02 TeVNNsp-Pb, = 2.76 TeVNNsPb-Pb,

PYTHIA8DIPSYEPOS LHC

F. Krauss IPPP



other “soft” aspects of event generation: QED FSR

QED FSR in Drell-Yan production

F. Krauss IPPP



limitations

and

future challenges

F. Krauss IPPP



limitation: computing short-distance cross sections – LO(Childers, Uram, LeCompte, Benjamin, Hoeche, CHEP 2016)

challenge of efficiency on tomorrow’s (& today’s) computers

2000’s paradigm: memory free, flops expensive(example: 16-core Xeon, 20MB L2 Cache, 64GB RAM)

2020’s paradigm: flops free, memory expensive & must be managed(example: 68-core Xeon KNL, 34MB L2 Cache, 16GB HBM, 96GB RAM)

may trigger rewrites of code to account for changing paradigm

CHEP San Francisco J. Taylor Childers October 2016

Code improvements enable scaling on KNL

31

9hr run-time

matrix element contributions

New results from two days ago… next test on Mira to see if we see similar improvements (figures stolen from Taylor Childers’ talk at CHEP)

F. Krauss IPPP



theory limitations/questions

we have constructed lots of tools for precision physics at LHC−→ but we did not cross-validate them careful enough (yet)−→ but we did not compare their theoretical foundations (yet)

we also need unglamorous improvements on existing tools:

account for new computer architectures and HPC paradigmssystematically check advanced scale-setting schemes (MINLO)automatic (re-)weighting for PDFs & scalesscale compensation in PS is simple (implement and check)

4 vs. 5 flavour scheme −→ really?

how about αS : range from 0.113 to 0.118(yes, I know, but still - it bugs me)

−→ is there any way to settle this once and for all (measurements?)

F. Krauss IPPP



achievable goals (I believe we know how to do this)

NLO for loop-induced processes:

fixed-order starting, MC@NLO tedious but straightforward

EW NLO corrections with tricky/time-consuming calculation setup

but important at large scales: effect often ∼ QCD, but opposite signneed maybe faster approximation for high-scales (EW Sudakovs)work out full matching/merging instead of approximations

improve parton shower:

beyond (next-to) leading log, leading colour, spin-averagedHO effects in shower and scale uncertaintiesstart including next-to leading colourinclude spin-correlations → important for EW emissions

F. Krauss IPPP



more theory uncertainties/issues?

with NNLOPS approaching 5% accuracy or better:

non-perturbative uncertainties start to matter:−→ PDFs, MPIs, hadronization, etc.question (example): with hadronization tuned to quark jets (LEP)−→ how important is the “chemistry” of jets for JES?−→ can we fix this with measurements?example PDFs: to date based on FO vs. data−→will we have to move to resummed/parton showered?

(reminder: LO∗ was not a big hit, though)

g → qq at accuracy limit of current parton showers:−→ how bad are ∼ 25% uncertainty on g → bb?−→ can we fix this with measurements?

F. Krauss IPPP



the looming revolution: going beyond NLO

H in ggF at N3LO (Anastasiou, Duhr and others)

explosive growth in NNLO (QCD) 2→ 2 results(apologies for any unintended omissions)

tt (1303.6254; 1508.03585;1511.00549)

single-t (1404.7116)

VV (1507.06257; 1605.02716;1604.08576; 1605.02716)

HH (1606.09519)

VH (1407.4747; 1601.00658;1605.08011)

Vγ (1504.01330)

γγ (1110.2375; 1603.02663)

Vj (1507.02850; 1512.01291; 1602.06965; 1605.04295; 1610.07922)

Hj (1408.5325; 1504.07922; 1505.03893; 1508.02684; 1607.08817)

jj (1310.3993; 1611.01460)

NLO corrections to gg → VV (1605.04610)

WBF at NNLO (1506.02660) and N3LO (1606.00840)

different IR subtraction schemes:N-jettiness slicing, antenna subtraction, sector decomposition,

F. Krauss IPPP



living with the revolution

we will include them into full simulations(I am willing to place a bet: 5 years at most!)

practical limitations/questions to be overcome:

dealing with IR divergences at NNLO: slicing vs. subtracting(I’m not sure we have THE solution yet)

how far can we push NNLO? are NLO automated results stableenough for NNLO at higher multiplicity?matching for generic processes at NNLO?

(MINLO or UN2LOPS or something new?)

more scales (internal or external) complicated – need integrals

philosophical questions:

going to higher power of N often driven by need to include larger FSmultiplicity – maybe not the most efficient methodlimitations of perturbative expansion:−→ breakdown of factorisation at HO (Seymour et al.)−→ higher-twist: compare (αS/π)n with ΛQCD/MZ

F. Krauss IPPP



outlookwill need precision for ballistics of smoking guns

F. Krauss IPPP



extra: NLO EW subtraction in SHERPA

(M. Schoenherr in preparation)

adapt QCD subtraction (spl. fns. and colour-/spin-correlated MEs)(Catani, Dittmaier, Seymour, Trocsanyi Nucl.Phys.B627(2002)189-265)

replacements: αs → α, CF → Q2f , CA → 0,

replacements: αs → α, TR → Nc,f Q2f , nf TR →

∑f Nc,f Q

2f ,

replacements:Tij ·Tk

T2ij→ QijQk

Q2ij

bcbcbcbc

bc

bcbcbc

bc

νe νe → e+e− @√

s = 200 GeV

10−4 10−3 10−2 10−1 1

-0.33

-0.328

-0.326

-0.324

-0.322

-0.32

-0.318

-0.316

σIRS in dependence on α-parameter

αFF

σIR

S(α

)/σ

Bor

n[%

]

αFF

=α

FI=

αIF

=α

II=

1

αFF

=0.

001,

αFI

=α

IF=

αII

=1

αFI

=0.

001,

αFF

=α

IF=

αII

=1

αIF

=0.

001,

αFF

=α

FI=

αII

=1

αII

=0.

001,

αFF

=α

FI=

αIF

=1

αFF

=α

FI=

αIF

=α

II=

0.00

1

bc bc bc

bc

bc bc

pp → e+e− @√

s = 13 TeV, mee > 60 GeV, no γPDF

0.16

0.17

0.18

0.19

0.2

0.21

0.22

0.23

0.24


σIR

S(α

)/σ

Bor

n[%

]

F. Krauss IPPP








∑f Nc,f Q

2f ,


T2ij→ QijQk

Q2ij

bcbcbcbc

bc

bcbc

bc

bc

νe νe → tt @√

s =2 TeV

10−4 10−3 10−2 10−1 1

-1.936

-1.934

-1.932

-1.93

-1.928

-1.926

-1.924

-1.922


αFF

σIR

S(α

)/σ

Bor

n[%

]

αFF

=α

FI=

αIF

=α

II=

1

αFF

=0.

001,

αFI

=α

IF=

αII

=1

αFI

=0.

001,

αFF

=α

IF=

αII

=1

αIF

=0.

001,

αFF

=α

FI=

αII

=1

αII

=0.

001,

αFF

=α

FI=

αIF

=1

αFF

=α

FI=

αIF

=α

II=

0.00

1

bcbc

bc bc bc

bc

pp → tt @√

s = 13 TeV, no γPDF

3.52

3.54

3.56

3.58

3.6

3.62

3.64


σIR

S(α

)/σ

Bor

n[%

]

F. Krauss IPPP








∑f Nc,f Q

2f ,


T2ij→ QijQk

Q2ij

bcbc

bcbc

bc bcbc bc bc

ldld

ldld

ld ld ld ld ld

νe νe → W+W− @√

s =2 TeVbc Subtraction as massive Quarkld Subtraction as massive Scalar

10−4 10−3 10−2 10−1 1

-8.245

-8.24

-8.235

-8.23

-8.225

-8.22

-8.215


αFF

σIR

S(α

)/σ

Bor

n[%

]

αFF

=α

FI=

αIF

=α

II=

1

αFF

=0.

001,

αFI

=α

IF=

αII

=1

αFI

=0.

001,

αFF

=α

IF=

αII

=1

αIF

=0.

001,

αFF

=α

FI=

αII

=1

αII

=0.

001,

αFF

=α

FI=

αIF

=1

αFF

=α

FI=

αIF

=α

II=

0.00

1

bcbc

bcbc bc

bcld

ld

ldld ld

ld

pp → W+W− @√

s =13 TeV, no γPDFbc Subtraction as massive Quarkld Subtraction as massive Scalar

0.4

0.5

0.6

0.7

0.8


σIR

S(α

)/σ

Bor

n[%

]

F. Krauss IPPP


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Precision Simulations for LHC physics and beyond

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