MC Tools and NLO Monte Carlos or The Good, the Bad & the Ugly - - PowerPoint PPT Presentation

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MC Tools and NLO Monte Carlos or The Good, the Bad & the Ugly - - PowerPoint PPT Presentation

Dec 04, 2023 131 likes •547 views

Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly MC Tools and NLO Monte Carlos or The Good, the Bad & the Ugly Frank Krauss Institute for Particle Physics Phenomenology Durham University Higgs Hunting

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

MC Tools and NLO Monte Carlos

  • r

The Good, the Bad & the Ugly

Frank Krauss

Institute for Particle Physics Phenomenology Durham University

“Higgs Hunting 2016”, Paris, 1.9.2016

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

what the talk is about the cutting edge in theory inputs matching & merging with parton showers where we are and where we (should/could/would) go

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

motivation & introduction

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

reminder: fixed-order and its limits

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

the aftermath of the NLO (QCD) revolution

establishing a wide variety of automated tools for NLO calculations

BLACKHAT, GOSAM, MADGRAPH, NJET, OPENLOOPS, RECOLA + automated IR subtraction methods (MADGRAPH, SHERPA)

first full NLO (EW) results with such 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 MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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)

t¯ t (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) V γ (1504.01330) γγ (1110.2375; 1603.02663) Vj (1507.02850; 1512.01291; 1602.06965) Hj (1408.5325; 1504.07922; 1505.03893; 1508.02684) jj (1310.3993)

WBF at NNLO and N3LO (1506.02660 and N3LO 1606.00840) different IR subtraction schemes: N-jettiness slicing, antenna subtraction, sector decomposition,

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

challenging the revolution

some technical issues at NNLO (and beyond)

(stability of automated NLO, robustness under integration, subtraction vs. slicing)

more scales (internal or external) complicated – need integrals going to higher power of N often driven by need to include larger FS multiplicity – maybe not the most efficient method structural questions concerning convergence/importance limitations of perturbative expansion:

breakdown of factorisation at HO (Seymour et al.) higher-twist: compare (αS/π)n with ΛQCD/MZ

(see Melnikov’s talk last week)

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

matching @ (N)NLO merging @ (N)LO

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

some parton shower fun with DY

(example of accuracy in description of standard precision observable)

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 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 b b b b b b b b b b

Sherpa MC 0 ≤ |yZ| ≤ 1 1 ≤ |yZ| ≤ 2 (×0.1) 2 < |yZ| ≤ 2.4 (×0.01)

b

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

1 2

10−9 10−8 10−7 10−6 10−5 10−4 10−3 10−2 10−1 pT spectrum, Z→ ee (dressed)

1 σfid dσfid dpT [GeV−1]

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 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 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

Sherpa MC |yZ| < 0.8 0.8 ≤ |yZ| ≤ 1.6 (×0.1) 1.6 < |yZ| (×0.01)

b

ATLAS data Phys.Lett. B720 (2013) 32 ME+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) 1 σfid. dσfid. 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

0.8 0.9 1.0 1.1 1.2 | | MC/Data

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

0.8 0.9 1.0 1.1 1.2 → | | MC/Data

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

1 ≤ |yZ| ≤ 2

1 2

0.8 0.9 1.0 1.1 1.2 | | MC/Data

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

0.8 ≤ |yZ| ≤ 1.6

3 2 1

0.8 0.9 1.0 1.1 1.2 → ≤ | | MC/Data

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 0.8 0.9 1.0 1.1 1.2 | | pT,ll [GeV] MC/Data

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

1.6 < |yZ| 10−3 10−2 10−1 1 0.8 0.9 1.0 1.1 1.2 → | | ≥ φ∗

η

MC/Data

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 region MC@NLO: − → visible structures, especially in gg → H POWHEG: − →: high tails, cured by h dampening factor well-established and well-known methods

(no need to discuss them any further)

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

different basic ideas MINLO: S + j at NLO with p(S)

T

→ 0 and capture divergences by reweighting internal line with analytic Sudakov, NNLO accuracy ensured by reweighting with full NNLO calculation for S production UN2LOPS identifies and subtracts and adds parton shower terms at FO from S + j contributions, maintaining unitarity available for two simple processes only: DY and gg → H

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

NNLOPS for H production: MINLO

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

also available for Z production

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

NNLOPS for Z production: UN2LOPS

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

Sherpa+BlackHat

NNLO NLO' FEWZ = 7 TeV s <120 GeV

ll

60 GeV<m NLO'

ll

<2m

R/F

µ /2<

ll

m NNLO

ll

<2m

R/F

µ /2<

ll

m

[pb]

  • e

+

e

/dy σ d 20 40 60 80 100 120 140 160 180 200 Ratio to NLO 0.96 0.98 1 1.02 1.04

  • e

+

e

y

  • 4
  • 3
  • 2
  • 1

1 2 3 4

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

ATLAS PLB705(2011)415

b

UN2LOPS mll/2 < µR/F < 2 mll mll/2 < µQ < 2 mll 10−6 10−5 10−4 10−3 10−2 10−1 Z pT reconstructed from dressed electrons 1/σ dσ/dpT,Z [1/GeV]

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/Data

also available for H production

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 low scales 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 MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

merging example: p⊥,γγ in MEPS@LO vs. NNLO

(arXiv:1211.1913 [hep-ex])

[pb/GeV]

γ γ T,

/dp σ d

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 10

  • 1

Ldt = 4.9 fb

∫

Data 2011, 1.3 (MRST2007) × PYTHIA MC11c 1.3 (CTEQ6L1) × SHERPA MC11c

ATLAS

= 7 TeV s

data/SHERPA 0.5 1 1.5 2 2.5 3 [GeV]

γ γ T,

p 50 100 150 200 250 300 350 400 450 500 data/PYTHIA 0.5 1 1.5 2 2.5 3 [pb/GeV]

γ γ T,

/dp σ d

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 10

  • 1

Ldt = 4.9 fb

∫

Data 2011, DIPHOX+GAMMA2MC (CT10) NNLO (MSTW2008) γ 2

ATLAS

= 7 TeV s

data/DIPHOX 0.5 1 1.5 2 2.5 3 [GeV]

γ γ T,

p 50 100 150 200 250 300 350 400 450 500 NNLO γ data/2 0.5 1 1.5 2 2.5 3

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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 and can be further supplemented with LO simulations for even higher final state multiplicities different implementations, parametric accuracy not always clear

(MEPS@NLO, FxFx, UNLOPS)

starts being used, still lacks careful cross-validation

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets Sherpa S-MC@NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO pp → h + 2j @ NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut MC@NLO pp → h + 2jets for Qn+2 > Qcut

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO pp → h + 2j @ NLO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut MC@NLO pp → h + 2jets for Qn+2 > Qcut iterate

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO pp → h + 2j @ NLO pp → h + 3j @ LO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut MC@NLO pp → h + 2jets for Qn+2 > Qcut iterate

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO pp → h + 2j @ NLO pp → h + 3j @ LO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut MC@NLO pp → h + 2jets for Qn+2 > Qcut iterate sum all contributions

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

illustration: pH

⊥ in MEPS@NLO

pp → h + jets pp → h + 0j @ NLO pp → h + 1j @ NLO pp → h + 2j @ NLO pp → h + 3j @ LO 50 100 150 200 250 300 10−4 10−3 10−2 10−1 Transverse momentum of the Higgs boson p⊥(h) [GeV] dσ/dp⊥ [pb/GeV]

first emission by MC@NLO , restrict to Qn+1 < Qcut MC@NLO pp → h + jet for Qn+1 > Qcut restrict emission off pp → h + jet to Qn+2 < Qcut MC@NLO pp → h + 2jets for Qn+2 > Qcut iterate sum all contributions

  • eg. p⊥(h)>200 GeV

has contributions fr. multiple topologies

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

results from various schemes in H+jets through ggF

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

aside: quark mass effects

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 MC Tools and NLO Monte Carlos

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b–mass effects: playtime

use LO multijet merging for tb-interference vary around µQ = mb vary around µQ = mh vary µF,R with Qcut = mb fixed

HEFT mt effects only mt and mb effects, µb

s =

√ 2mb mt and mb effects, µb

s = mb

0.1 0.2 0.3 0.4 0.5 0.6 Higgs boson transverse momentum dσ/dp⊥ [pb/GeV] 10 20 30 40 50 0.2 0.4 0.6 0.8 1 p⊥(H) [GeV] Ratio to HEFT HEFT mt effects only mt and mb effects, µb

s = mh/

√ 2 mt and mb effects, µb

s = mh

√ 2 10−5 10−4 10−3 10−2 10−1 1 Higgs boson transverse momentum dσ/dp⊥ [pb/GeV] 1 10 1 10 2 10 3 0.9 0.95 1.0 1.05 p⊥(H) [GeV] Ratio to HEFT HEFT µ f ∈ [0.5, 2]mh µr ∈ [0.5, 2]mh mt and mb effects, Qcut = mb 10−5 10−4 10−3 10−2 10−1 1 Higgs boson transverse momentum dσ/dp⊥ [pb/GeV] 1 10 1 10 2 10 3 0.7 0.8 0.9 1.0 1.1 1.2 1.3 p⊥(H) [GeV] Ratio to HEFT

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

limitations of full simulations

lots of routinely used tools for large FS multis (4 and more) at NLO accuracy, but − → not many detailled comparisons

(critical appraisals and learning curve in their phenomenological use still in infancy)

− → no standard way of estimating uncertainties (yet?) to improve: description of loop–induced processes − → potentially important for new physics searches users of codes: higher orders tricky → training needed

(MC = black box attitude problematic - a new brand of pheno/experimenters needed?)

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

a systematic uncertainty

showering a source of uncertainty − → (N)LL only, scale variations?

(quite often just used as black box)

maybe include higher orders? example right: µR uncertainty in p(emit)

⊥

in ggF

Dire PS pp → [h → τ+τ−] @ 8TeV Γ1 ⊕ γ1 Γ1 ⊕ γ1 ⊕ Γ2 1/2 t < µR < 2 t Γ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2 Γ1 ⊕ γ1 ⊕ Γ2 ⊕ γ2 ⊕ Γ3 1/2 t < µR < 2 t 10−3 10−2 10−1 Differential 0 → 1 jet resolution dσ/d log10(d01/GeV) [pb] 0.5 1 1.5 2 2.5 0.8 0.9 1.0 1.1 1.2 log10(d01/GeV) Ratio

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

limitations

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

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) will NNLO (or beyond) become as automated as NLO? − → or more precisely: when and how? we also need unglamorous improvements on existing tools:

systematically check advanced scale-setting schemes (MINLO) automatic (re-)weighting for PDFs & scales scale 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 still bugs me)

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

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

plan

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

achievable goals in fixed order calculations: a roadmap (?)

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 stable enough for NNLO at higher multiplicity? matching for generic processes at NNLO?

(MINLO or UN2LOPS or something new?)

NLO for loop-induced processes:

fixed-order starting, MC@NLO tedious but straightforward

EW NLO corrections with tricky/time-consuming calculational setup

but important at large scales: effect often ∼ QCD, but opposite sign need maybe faster approximation for high-scales (EW Sudakovs)

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

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Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

“curable” bottleneck: colours/spins in parton showers

parton shower usually is spin-averaged, leading colour, (next-to) leading log start including next-to leading colour

(first attempts by Platzer & Sjodahl; Nagy & Soper)

no big effects in e−e+ → hadrons seen maybe more exclusive observables? aside: can also include spin-correlations important for EW emissions

(maybe relevant for ultra-high energies)

HO being implemented at nthe moment

0.001 0.01 0.1 1 average transverse momentum w.r.t. n3 DipoleShower + ColorFull 0.8 0.9 1 1.1 1.2 1 10 p⊥/GeV full shower strict large-Nc GeV N −1 dN/dp⊥ x/full

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos

slide-41
SLIDE 41

Introduction The Good: Fixed Order The Bad: Matching & Merging The Ugly

  • utlook

will need precision for ballistics of smoking guns

  • F. Krauss

IPPP MC Tools and NLO Monte Carlos