MC for t H and SM backgrounds - Durham University

MC for ttH and SM backgrounds

Stefano Pozzorini

Zurich University

Higgs Couplings 2015Lumley Castle, 13 October 2015

Large backgrounds in ttH searches

Dominant ttH(bb) channel suffers from heavy tt+jets contamination

∼ 3′000 ttH events but only 3–4×σSM exclusion at Run1

main bottleneck is the background theory uncertainty

1 / 30

Theory priorities in ttH searches

Key priority is precision for backgrounds

various multi-particle processes: tt+ jets,ttV+ jets, ttγγ, V V+ jets

NLO automation crucial but 2→ 4 CPUintensive

ttH Analyses at the LHC

H

b

b

t

W q

q

b

t

W l

ν

b• direct probe of ttH Yukawa coupling

• ttH(bb) originally considered best discovery channel for

light Higgs

• complicated bbbbℓνjj final state hampers H → bb peak

reconstruction ⇒ very large QCD backgrounds

Status of ttH analyses in various channels

95% exclusion in σSMttH units H → bb H → V V ∗ H → γγ

ATLAS 4.1 (2.6) 4.7 (5.4)

CMS 5.2 (4.1) 6.6 (2.4) 5.3 (5.4)

• sensitivity already ∼ 100%λSMt and ttH searches still quite active

• expected sensitivity ∼ 10%λSMt with 300 fb−1

NLO matching & merging crucial

various new methods (FxFx, MEPS@NLO, MINLO, UNLOPS,. . . )

various automated tools support NLO precision for signal and most backgrounds:MG5 aMC@NLO, Sherpa+OpenLoops/GoSam, Powheg/Powhel

Theory uncertainty estimates nontrivial

still limited experience in NLO matching+merging framework

sophisticated analyses (profile likelihood, MEM, background reweighting, . . . )

2 / 30

Recent progress in theory precision [slide by M. Zaro]

Marco Zaro, 17-09-2015

2015!

(4FS and 5FS) Demartin et al. arXiv:1504.00611 tHW see poster by F. Demartin

2015!

2015!

2015!

2015!

• t tbb • NLO QCD corrections !!

• Matching to PS • t tV • NLO QCD corrections !!!

• Matching to PS !

• Electro-Weak corrections !• t tVV

• NLO QCD corrections + PS

Higher order predictions for signal and backgrounds

4

Beenakker et al. hep-ph/0107081 & hep-ph/0211352Dawson et al. hep-ph/0211438 & hep-ph/0305087

aMC@NLO: Frederix et al. arXiv:1104.5613 Powhel: Garzelli et al. arXiv:1108.0387

Powheg Box: Hartanto et al. arXiv:1501.04498

Frixione et al. arXiv:1407.0823 & arXiv:1504.03446 Zhang et al. arXiv:1407.1110

Kulesza et al. arXiv:1509.02780

Bredenstein et al. arXiv:0905.0110 & arXiv:1001.4006 Bevilacqua et al. arXiv:0907.4723

Denner et al. arXiv:1506.07448

Kardos et al.1303.6201 Cascioli et al. 1309.5912

t t�� Melnikov et al. arXiv:1102.1967

t tW,t t��*/Z, t t�� Hirschi et al. arXiv:1103.0621 t tZ Lazopoulos et al. arXiv:0804.2220

t tZ Kardos et al. arXiv:1111.0610 t tW Campbell et al. arXiv:1204.5678

t tZ Garzelli et al. arXiv:1111.1444 t tW, t tZ Garzelli et al. arXiv:1208.2665

t t��𝛄 Kardos et al. arXiv:1408.0278 all t tVV Maltoni et al. arXiv:1507.05640

t t��𝛄 van Deurzen et al. arXiv:1509.02077

t tW, t tZ (and t tH) Frixione et al. arXiv:1504.03446(5FS) Farina et al. arXiv:1211.3737

(5FS) Campbell et al. arXiv:1302.3856

2015!

2015!

2015!

• t tH • NLO QCD corrections (30% @ RunII)

!• Matching to PS

!!

• NLO QCD corrections to bbℓ+ℓ-ννH !• Weak and Electro-Weak corrections

(1.5% @ RunII) !

• Soft gluon resummation (2-6% @ RunII) !• tH • NLO QCD corrections

!• Matching to PS

3 / 30

Outline

1 ttH Signal

2 Backgrounds for H → bb

3 Backgrounds for H →WW,ZZ, ττ →multi-leptons

4 Backgrounds for H → γγ

NLO tools for ttH

σ(pp → tt_ H + X) [fb]

√s = 14 TeV

MH

= 120 GeV

µ0 = m

t + M

H/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5

0

200

400

600

800

1000

1200

1400 σttH at NLO [Beenakker et al ’01; Reina, Dawson ’01]

moderate uncertainty (9% scale, 8% PDF+αS)

sufficient for mid-term future

Publicly available NLO+PS tools

MG5 aMC@NLO [Frederix et al. ’15]

Powhel [Garzelli at al. ’11] andPowheg Box [Jaeger et al. ’15]

Sherpa+OpenLoops or GoSam

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.001

0 50 100 150 200 250 300 350dm

/dp T

(H) [

pb/G

eV]

POWHEG+PYTHIA 6POWHEG+PYTHIA 8POWHEG+HERWIG

NLO

0.8

1

1.2

0 50 100 150 200 250 300 350

R

pT(H) [GeV]

<∼ 10% uncertainty for inclusive observables (more if sensitive to jet radiation)4 / 30

Spin-correlated Top decays

Marco Zaro, 17-09-2015

Including spin correlations at NLO Frixione, Leanen, Motylinski, Webber, arXiv:hep-ph/0702198

• Generate events (to be showered) for the production process MP

• Before showering, produce a decayed event file starting from the undecayed events

• Exploit the fact that |M2P+D|/|M2P| is bounded from above

• The generation of unweighted decayed events is possible: generate many kinematics configurations until

• In NLO computations use only tree-level matrix elements (with n or n+1 particles)

• Loop effects on spin correlation assumed to be negligible

• Automated in MadSpin (MadGraph5_aMC@NLO) and Decayer (PowHel)

21

qL

qR

t

t

b

b

W+

W�

l+

⌫l

l�

⌫l

|MP+D|2 / |MP |2 > Rand() max

⇣|MP+D|2 / |MP|2

⌘

Artoisenet, Frederix, Mattelaer, Rietkerk, arXiv:1212.3460 Garzelli, Kardos, Trocsanyi, arXiv:1405.5859

Mandatory for signal and all backgrounds

ttγγ : uncorttγγ : corr

tth(γγ) : uncortth(γγ) : corr

Frame 1

cos θℓℓ

1 NdN

dcosθℓℓ

10.60.2-0.2-0.6-1

0.65

0.6

0.55

0.5

0.45

0.4

[Biswas et al ’14]

NLO+PS level automation in Powheg, MG5 aMC@NLO and Sherpa

via full production×decay LO MEs for ttH(+j) assuming same 1-loop correlations

only (NLO production)×(LO decay) accuracy

5 / 30

Soft-gluon NLL resummation for σttH [Kulesza et al. ’15]

Resummation of threshold logarithms of β =√

1− (2mt +mH)2/s

optimal for mH + 2mt →√s

logs are β4-suppressed by ttH phase space ⇒ no realistic estimate of full NNLO

Matched prediction: σttHNLO+NLL = 650+7.9%−5.7% (σttHNLO = 613+6.2%

−9.4%)

moderate +7% correction and modest reduction of scale uncertainty

dominant effects from O (αS) matching coefficient C(1) (formally NNLL)

0.00

200.00

400.00

600.00

800.00

1000.00

0.2 0.5 1 2 5µR/µ0

σ(pp→ Htt+X)[fb]√S = 14 TeV

µ0 = mt +mH/2mH = 125 GeV

LO

NLO

NLO+NLL

NLO+NLL(w C-coef);

0.00

200.00

400.00

600.00

800.00

1000.00

0.2 0.5 1 2 5µF /µ0

σ(pp→ Htt+X)[fb]√S = 14 TeV

µ0 = mt +mH/2mH = 125 GeV

LO

NLO

NLO+NLL

NLO+NLL(w C-coef);

6 / 30

Soft-gluon NNLL resummation for dσ/dΦttH [Broggio et al. ’15]

SCET PIM resummation formalism

resums threshold-type logarithms of 1− z = 1−M2ttH/s

valid for MttH > 2mt +mH and optimal for M2ttH → s

⇒ applicable to differential distributions

nNLO approx. from truncated NLO+NNLL µ0 = mt +mH/2 = 235 GeV

µ = 2.64 µ0 ⇒ +8% nNLO correction & 5% uncertainty (scale+subleading logs)

standard scale µ = µ0 ⇒ smaller nNLO correction

0 1 2 3 4 5

350

400

450

500

550

μ

235GeV

σ[fb

] approx NLO

nNLO

NLO MG5

NLO MG5 no qg

MSTW 2008 PDFs

μ0 = 620 GeV

0 50 100 150 200 250 300 3500

5

10

15

20

25

30

35

PTH [GeV]

crosssectionperbin[fb

]

7 / 30

NLO QCD corrections to ttH + 1 jet

Parton-level NLO with Sherpa+GoSam[Deurzen et al, ’13]

more reliable prediction and uncertainty forextra jet activity

room for significant effects in the hardregion (depending on µ)

NNLO contributions of hard type

Applicable to ttH analysis through ttH + 0, 1 jets NLO merging (automated)

MEPS@NLO merging [Hoche et al. ’12] in Sherpa+OpenLoops/GoSam

FxFx merging [Frederix, Frixione ’12] in MG5 aMC@NLO

⇒ impact on σttH and ttH searches?

8 / 30

ttH (and ttV ) at NLO QCD+EW [Frixione et al. ’14–15; Zhang et al. ’14]

Emerging NLO QCD+EW automation

Recola [1411.0916], OpenLoops [1412.5157],MG5 aMC [1504.03446], GoSam [1507.08579]

applicable to ttH signal and backgrounds(matching/merging automation ongoing)

large NLO EW effects at pT �MW

NLO EW corrections to ttH and ttV

NLO correction to mt = λtv/√

2 relation

impact below QCD scale+PDF uncertainties andmore significant for backgrounds

ttH: −1% for σtot (-8% at pT > 200 GeV)

ttW : −8% for σtot (-12% at pT > 200 GeV)σ

pe

r b

in [

pb

]

tt-H production at the 13 TeV LHC

LO QCD

LO+NLO QCD

LO+NLO QCD+EW

LO+NLO QCD+EW, no γ

10-4

10-3

10-2

10-1

MadGraph5_aMC@NLO

1

1.2

1.4ratio over LO QCD; scale unc.

1

1.2

1.4ratio over LO QCD; PDF unc.

pT(H) [GeV]

relative contributions

NLO QCD

LO+NLO EW

LO+NLO EW, no γ

HBR

-0.2

0

0.2

0.4

0 200 400 600

9 / 30

pp→ ttH → e+νeµ−νµbbH at NLO QCD [Denner and Feger ’15]

Challenging 2→ 5 NLO calculation

heptagons, complex mass scheme for top resonances

Recola+Collier [Denner, Dittmaier, Hofer, Uccirati]

g

g

t

t

H

b

t

W+

e+

νe

t

b

W−

νµ

µ−

g

σ[f

b]

Γt [GeV]

LONLO

2.2

2.3

2.4

2.5

2.6

2.7

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Results for total cross section with tt-type cuts

excellent agreement with NWA (Γt → 0) as for tt

bigger off-shell effects from pp→ tWH if one bunresolved (would require mb > 0)

Distributions with µdyn = (ET,tET,tET,H)1/3

nontrivial NLO shape corrections

due to NLO corrections to production and decay?

⇒ cf. (NLO production)×(LO+PS decay) in MCdσ

dφe+

µ−

[ fb ◦]

Kfa

ctor

φe+µ− [◦]

LONLO

0.005

0.01

0.015

0.02

0.025

0.03

0.81

1.21.4

0 20 40 60 80 100 120 140 160 180

10 / 30

Outline

1 ttH Signal




2→ 8 simulation of ttH(bb) signal and irred. backg. I

g

g

bt

W−u

dt

t

tb

W+

ℓ+

νℓ

Hb

b

g

g

bt

W−u

dt

t

W+

tb

W+

ℓ+

νℓ

b

b

g

g

bt

W−u

dt

t

tb

W+

ℓ+

νℓ

g

b

b

g

g

b

b

g

g

u

g

b

b

b

u

ud

W+

ℓ+

νℓ

O(α1S

)O

(α1S

)O

(α2S

)O

(α3S

)pp→ `ν + 2j + 4b at LO [Denner, Feger, Scharf, 1412.5290]

48 partonic channels and up to 78’000 diagrams/channel at LO

MEs of order α0S , α1

S , α2S , α3

S with interferences and non-resonant effects

11 / 30

2→ 8 simulation of ttH(bb) signal and irred. backg. II

g

g

bt

W−u

dt

t

tb

W+

ℓ+

νℓ

Hb

b

g

g

bt

W−u

dt

t

W+

tb

W+

ℓ+

νℓ

b

b

g

g

bt

W−u

dt

t

tb

W+

ℓ+

νℓ

g

b

b

g

g

b

b

g

g

u

g

b

b

b

u

ud

W+

ℓ+

νℓ

O(α1S

)O

(α1S

)O

(α2S

)O

(α3S

)O (αS) EW contributions to ttbb

ttH (70%), ttZ (∼ 10%), ttγ∗ (∼ 10%), and t-channel W -exchange (∼ 10%)

Interference between O(α2S

)QCD and O (αS) EW contributions to ttbb

−8% wrt ttbb QCD, mostly from t-channel W -exchange (tiny ttH interference)

Off-shell contributions with ≤ 1 top resonance (new qg and qq′ channels)

+12% wrt ttbb QCD (might be attributed to Wbb+jets?)

Non-standard background contributions of order 10–50% σttH

12 / 30

Irreducible ttbb QCD background at NLO

NLO ttbb [Bredenstein et al ’09/’10; Bevilacqua et al ’09];

ttbb dominates ttH(bb) systematics

NLO reduces uncertainty from 80% to 20–30%

NLO+PS ttbb 5F scheme (mb = 0) with Powhel [Garzelli et al ’13/’14]

ttbb NLO MEs cannot describe collinear g → bb splittings

⇒ inclusive tt+b-jets simulation requires parton shower in collinear bb region

⇒ NLO merging tt+ 0, 1, 2 jets (see later)

NLO+PS ttbb 4F scheme (mb > 0) with Sherpa+OpenLoops [Cascioli et al ’13]

ttbb NLO MEs cover full b-quark phase space

⇒ inclusive NLO accurate tt+b-jets simulation possible

13 / 30

S–MC@NLO ttbb 4F scheme [Cascioli et al ’13]

Good perturbative stability but unexpected MC@NLO enhancementttb ttbb ttbb (mbb > 100)

σLO[fb] 2644+71%−38%

+14%−11% 463.3+66%

−36%+15%−12% 123.4+63%

−35%+17%−13%

σNLO[fb] 3296+34%−25%

+5.6%−4.2% 560+29%

−24%+5.4%−4.8% 141.8+26%

−22%+6.5%−4.6%

σNLO/σLO 1.25 1.21 1.15

σMC@NLO[fb] 3313+32%−25%

+3.9%−2.9% 600+24%

−22%+2.0%−2.1% 181+20%

−20%+8.1%−6.0%

σMC@NLO/σNLO 1.01 1.07 1.28

Large enhancement (∼30%) in Higgs region from double g → bb splittings

matching, shower and 4F/5F systematicsremain to be understood!

14 / 30

tt+ jets background and merging at NLO

NLO tt+ 2 jets [Bevilacqua, Czakon, Papadopoulos, Worek ’10/’11]

reduces uncertainty from 80% to 15%

NLO merging (FxFx, MEPS@NLO, UNLOPS,. . . )

0-jet NLO+PS tt1-jet NLO+PS tt+ 1 j. . . . . .≥ n jets NLO+PS tt+ n j

NLO and log accuracy for 0, 1, . . . n jets

separated via kT-algo at merging scale Qcut

smooth PS–MEs transition ↔ MEs with PS-likescale and Sudakov FFs

NLO merging for tt+ 0, 1 jets

FxFx with Madgraph5/aMC@NLO[Frederix, Frixione ’12]

MEPS@NLO with Sherpa+GoSam[Hoche et al ’13]

15 / 30

MEPS@NLO for tt+ 0, 1, 2 jets (Sherpa+OpenLoops)[Hoche, Krauss, Maierhofer, S. P. , Schonherr, Siegert ’14] I

Sherpa+OpenLoops

pl-jet⊥ > 40 GeV

pl-jet⊥ > 60 GeV

×0.1

pl-jet⊥ > 80 GeV

×0.01

[email protected] × MEPS@LOS-MC@NLO10−5

10−4

10−3

10−2

10−1

1Inclusive light jet multiplicity

σ(≥

Nl-

jet)

[pb]

pl-jet⊥ > 40 GeV

1

1.5

2

Rat

ioto

ME

PS@

NL

O

pl-jet⊥ > 60 GeV

1

1.5

2

Rat

ioto

ME

PS@

NL

O

pl-jet⊥ > 80 GeV

0 1 2 3

1

1.5

2

Nl-jet

Rat

ioto

ME

PS@

NL

O

Consistency with LO merging and NLO+PS

decent (10–20%) mutual agreement

Reduction of µR, µF , µQ variationsNlight−jet ≥ 0 1 2

LO 48% 65% 80%NLO 17% 18% 19%

16 / 30

MEPS@NLO for tt+ 0, 1, 2 jets (Sherpa+OpenLoops)[Hoche, Krauss, Maierhofer, S. P. , Schonherr, Siegert ’14] II

Sherpa+OpenLoops

1st jet

2nd jet

3rd jet

[email protected] × MEPS@LOS-MC@NLO10−7

10−6

10−5

10−4

10−3

Light jet transverse momenta

dσ

/d

p T[p

b/G

eV]

1st jet0.5

1

1.5

Rat

ioto

ME

PS@

NL

O

2nd jet0.5

1

1.5

Rat

ioto

ME

PS@

NL

O

3rd jet

40 50 100 200 500

0.5

1

1.5

pT (light jet) [GeV]

Rat

ioto

ME

PS@

NL

O

Consistency with LO merging and NLO+PS

decent (10–20%) mutual agreement

Reduction of µR, µF , µQ variationsNlight−jet ≥ 0 1 2

LO 48% 65% 80%NLO 17% 18% 19%

Differential distributions

similarly mild scale dependence

small shape corrections

Next steps

improve CPU performance

consistent combination with ttbb

17 / 30

MEPS@NLO for tt+ 0, 1, 2 jets (Sherpa+OpenLoops)[Hoche, Krauss, Maierhofer, S. P. , Schonherr, Siegert ’14] III

Sherpa+OpenLoops

MEPS@NLOMEPS@NLO Qcut = 20 GeVMEPS@NLO Qcut = 30 GeVMEPS@NLO Qcut = 40 GeV1.65×MEPS@LOS-MC@NLO

1

10 1

10 2

10 31 → 2 k⊥ jet resolution

dσ

/d

d 12

[pb/

GeV

]

1

1.5

→ ⊥

Rat

ioto

Qcu

t=

30G

eV

0.8

1

1.2

→ ⊥

Rat

ioto

Qcu

t=

30G

eV

3 10 100 3000

0.2

0.4

0.6

0.8tt+ 0j excl.tt+ 1j excl.tt+ 2j incl.

→ ⊥

d12 [GeV]

Rat

ioto

Qcu

t=

30G

eV

Small merging scale choice

Qcut = 30 GeV such that exp. resolvedjets are described by MEs

Merging scale uncertainty

Qcut = 30± 10 GeV

⇒ � 10% dependence

does not spoil tt+ 0, 1, 2 jets NLO precision

18 / 30

Scale dependencies in NLO matching and merging

Scale variations

basis for definition of intrinsic theory precision of (N)NLO Monte Carlo

quantitative and realistic uncertainty estimate is a key advantage of (N)NLO MCwrt (tuned) LO MC

Four scale variations

standard prescription for renormalisation (µR) and factorisation (µF ) scales

no widely accepted prescription for resummation (µQ) and merging (Qcut) scales

19 / 30

Benefits of NLO matching and merging (top-pair pT in tt+ 0, 1 jets)LO

−→NLO

Sherpa+OpenLoops

SMCNLO R1F1Q1SMCNLO R0.5F1Q1SMCNLO R2F1Q1SMCNLO R1F0.5Q0.5SMCNLO R1F2Q2

10−2

10−1

1

pT of top-antitop pair

dσ/dpT[pb/GeV

]

0 50 100 150 200 250 300 350 400

0.6

0.8

1

1.2

1.4

pT [GeV]

MC/Data

Sherpa+OpenLoops

LOPS R1F1Q1LOPS R0.5F1Q1LOPS R2F1Q1LOPS R1F0.5Q0.5LOPS R1F2Q2

10−6

10−5

10−4

10−3

10−2

10−1

1


dσ/dpT[pb/GeV

]

0 50 100 150 200 250 300 350 400

0.6

0.8

1

1.2

1.4

pT [GeV]

MC/Data

Sherpa+OpenLoops

MEPSNLO30 R1F1Q1MEPSNLO30 R0.5F1Q1MEPSNLO30 R2F1Q1MEPSNLO30 R1F0.5Q0.5MEPSNLO30 R1F2Q210−2

10−1

1


dσ/dpT[pb/GeV

]

0 50 100 150 200 250 300 350 400

0.6

0.8

1

1.2

1.4

pT [GeV]

MC/Data

Sherpa+OpenLoops

MEPSLO30 R1F1Q1MEPSLO30 R0.5F1Q1MEPSLO30 R2F1Q1MEPSLO30 R1F0.5Q0.5MEPSLO30 R1F2Q2

10−2

10−1

1


dσ/dpT[pb/GeV

]

0 50 100 150 200 250 300 350 400

0.6

0.8

1

1.2

1.4

pT [GeV]

MC/Data

µR = ξ mt

renorm. scale

µQ = µF = ξ mt

factorisation scale

resummation scale

NLO merging

10% accuracy overwhole spectrum!

matching −→ merging

20 / 30

Choice and variation of merging scale Qcut?!

Virtue of small Qcut

NLO accuracy over whole pT spectrum of experimentally resolved jets

Formal problem at (very) small Qcut [Hamilton, Nason, Oleari, Zanderighi ’12]

when Qcut approaches Sudakov peak, i.e. αS log2(Qcut/Qhard) ∼ 1

⇒ uncontrolled O(α1.5S

)subleading logs can spoil NLO precision

⇒ Optimal Qcut choice? Uncertainty?

Quantitative estimate (conservative)

consider jet-kT dist ⇒ max Qcut sensitivity

push Qcut down to Sudakov region

take local Qcut dependence as uncertainty estimate

stop before you exceed NLO scale dependence

Sherpa+OpenLoops

MEPSNLO30 R1F1Q1MEPSNLO5 R1F1Q1MEPSNLO10 R1F1Q1MEPSNLO20 R1F1Q1MEPSNLO40 R1F1Q1

10−4

10−3

10−2

10−1

1

10 1

10 2

log10(k⊥ jet resolution 0 → 1 [GeV])

dσ/dlog10(d

01/GeV

)[pb]

0.5 1 1.5 2 2.5 3 3.5

0.6

0.8

1

1.2

1.4

log10(d01/GeV)MC/Data

Example: MEPS@LO for tt+ 0, 1 jets

Qcut = 40 . . . 5 GeV ⇒ less than 10% uncertainty for Qcut ≥ 10 GeV

21 / 30

Outline

1 ttH Signal




Most relevant Backgrounds to ttH(H → V V )

tt+W/Z/γ∗+extra jets

MC systematics dominated by ttW + 0, 1, 2 jets

same signature as ttH(H →WW → `νjj)!

tt+ Z/γ(→ `+`−) requires off-shell effects down to small m``

V V+jets

mainly WZ → 3` but also ZZ → 4`

requires inclusive sample with NLO precision up to 2 jets

HF jets crucial (genuine V V bb component and light-jet mistags)

22 / 30

NLO+PS tt+W±/Z with Powheg/Powhel [Garzelli et al ’12]

2

510

-22

510

-12

512

0.90.951.0

1.051.1

0.80.91.01.11.2

0 50 100 150 200 250 300 350 400 450 500

dσ

dp ⊥

,t[fb/G

eV]

√s = 7TeV

mt = 172.9GeVmZ = 91.1876GeVµ0 = mt +mZ/2CTEQ6.6M

(a) PowHel-NLOPowHel-LHEµ0/2 ≤ µ ≤ 2µ0

LHE/N

LO

K-fact

p⊥,t [GeV]

LHE/NLO

K-fact

Moderate NLO+PS corrections

K = 1.1–1.35 and mild uncertainties (10% scale, 8% PDFs)

top decays in NWA (uncorrelated)

ttV + 2 jets from PS

23 / 30

NLO+PS predictions for ttV and ttV V [Maltoni et al. ’15]

Thorough study with scale+PDF variations for all V = W,Z, γ

moderate K-factors and shape corrections at 13–14 TeV

small irreducible ttH(V V ) backgrounds: 2 fb ≤ σttV V ≤ 12 fb

[pb]

NLO

σ

110

1

10

210

H production at pp colliders at NLO in QCDtV, ttt

Ztt

±Wtt

γtt

Htt

, MSTW2008 NLO PDFs (68% cl)g

µ = r

µ = f

µcentral

MadGraph5_aMC@NLO

| | | | |

fa

cto

rsK 1

1.2

1.4

| | | | |

[TeV]s

facto

rsK

1

1.5

2

8 13|

14|

25|

33|

50|

100

[fb

]N

LO

σ

1

10

210

310

410 production at pp colliders at NLO in QCDtttVV, ttt

ZZtt

[4f]

W+

Wtt

γγtt

Z±Wtt

γZtt

γ±Wtt

, MSTW2008 NLO PDFs (68% cl)g

µ = r

µ = f

µcentral

tttt

MadGraph5_aMC@NLO

| | | | |

fa

cto

rsK 1

1.2

1.4

| | | | |

[TeV]s

facto

rsK

1

1.5

2

8 13|

14|

25|

33|

50|

100

24 / 30

Backgrounds to ttH →multileptons [Maltoni et al. ’15]

Simulation of 2-, 3- and 4-lepton CMS searches at 13 TeV

σNLO+PS [fb] 2 same-sign leptons 3 leptons 4 leptons

ttH(H → WW∗) 1.54+5.1%−9.0%

+2.2%−2.6%

1.47+5.2%−9.0%

+2.0%−2.4%

0.095+7.4%−9.7%

+2.0%−2.4%

ttH(H → ZZ∗) 0.0437+5.5%−9.2%

+2.3%−2.8%

0.119+6.3%−9.6%

+2.1%−2.5%

0.0170+5.0%−8.5%

+2.0%−2.4%

ttH(H → τ+τ−) 0.563+4.6%−8.8%

+2.2%−2.7%

0.669+6.0%−9.4%

+2.1%−2.6%

0.0494+7.1%−9.9%

+2.1%−2.5%

ttW± 5.77+15.1%−12.7%

+1.6%−1.2%

2.44+13.1%−11.6%

+1.7%−1.4%

-

ttZ/γ∗ (→ `+`−) 1.61+7.7%−10.5%

+2.0%−2.5%

2.70+9.0%−11.2%

+2.0%−2.5%

0.280+9.8%−11.0%

+1.9%−2.3%

ttW+W− 0.288+8.0%−11.1%

+2.3%−2.6%

0.201+7.4%−10.7%

+2.1%−2.3%

0.0116+6.9%−10.2%

+2.2%−2.3%

tttt 0.340+27.5%−25.8%

+5.5%−6.4%

0.211+27.4%−25.6%

+5.2%−6.1%

0.0110+27.0%−25.5%

+5.0%−5.9%

ttZZ 0.0096+3.5%−8.4%

+1.8%−1.8%

0.0050+3.7%−8.3%

+1.8%−1.7%

0.00025+7.2%−9.6%

+1.9%−1.8%

ttW±Z 0.0620+9.0%−10.2%

+2.2%−1.6%

0.0279+9.2%−10.3%

+2.3%−1.7%

0.00091+7.2%−9.2%

+2.4%−1.7%

B/S up to 350% for ttV and only 15–25% for ttWW + tttt

NLO precision (10–15%) seems sufficient apart from ttW (and ttW+jets!)

25 / 30

Outline

1 ttH Signal




Backgrounds to ttH(γγ) and MC predictions

σttH(γγ) ∼ 1.5 fb contaminated by mild backgrounds

tt+ 1, 2γ, single-top+1, 2γ, jets+1, 2γ

H+jets with H → γγ and HF jets

NLO+PS for ttγ and ttγγ with Powhel [Kardos, Trocsanyi ’14]

mild NLO uncertainty (14%) and MC effects for inclusive observables

realistic IR-safe hard-photon isolation (without q → qγ fragmentation): looseFrixione isolation (generation cuts) + tighter cone isolation (analysis cuts)

2

5

10−3

2

5

10−2

dσ/dmγ1,γ

2[fb/G

eV]

PowHel LHEPowHel NLO

8 TeV, µ = mt

δ0 = 0.4

0.91.01.1

ratio

0 50 100 150 200 250 300 350 400 450 500

mγ1, γ2 [GeV]

5

10−3

2

5

10−2

2

dσ/dmγ1,γ

2[fb/G

eV]

SMC Rγ, q = 0.1SMC δ0 = 0.1

13 TeV, µ = H⊥/2

0.91.01.1

ratio

0 50 100 150 200 250 300 350 400 450 500

mγ1, γ2 [GeV]26 / 30

ttγγ background in ttH(γγ) searches [Maltoni et al. ’15]

MC@NLO simulations of ttH(γγ) and ttγγ see also [van Deurzen at al. ’15]

CMS-like cuts for ttH(γγ) search with one lepton

Higgs peak fully reconstructed within |mγγ −mH | < 2 GeV mass window

dσ

/dm

[p

b/b

in]

NLO+PS (µg), LHC13 ttγγ

ttH(γγ)

10−6

10−5

10−4

MadGraph5_aMC@NLO

ΚP

Stt H

LO unc. NLO unc.

0.5

1

1.5

ΚP

Stt γγ

m(γ1γ2) [GeV]

0.5

1

1.5

100 110 120 130 140 150 160 170 180

20 signal events/100 fb−1

S/Birred. ∼ 10

NLO+PS for S and Birred. well undercontrol: <∼ 10% uncertainty

⇒ MC better than side-band approach forbackground?

What about reducible backgrounds (fake photons, leptons,. . . )?

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Subtleties in tt+multi-γ production

Radiative top decays (not included in recent NLO MC studies)

two photons can arise from tt+ 0, 1, 2 γ production plus 2, 1, 0 γs from top decays

photons with Mb`νγ < mt mainly from top-decay products (light leptons/quarks)

NLO study of tt+ 0, 1 γ with t→ b`+ν + 0, 1 γ [Melnikov, Schulze ’11]

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140 160 180 200

dσX NLO

dpT(γ)

/dσNLO

dpT(γ)

pT(γ) [GeV]

γ in production

γ in decay

1 2 30

0.2

0.4

0.6

0.8

1

dσXNLO

dR

/dσNLO

dR

R(γ, bjet)

γ in production

γ in decay

photons from top decays dominate up to pT(γ) <∼ 60 GeV (smooth isolation, Rγi > 0.4)

ttH(γγ) searches require tt+X background with nγ = 1, 2

radiative top decays can be generated with QED parton shower

tt+ 0, 1, 2 γ MEs need to be merged (to avoid double counting)

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Relevance of radiative top decays for ttH(γγ) searches

�� n�� g

g

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g�� n�� g

g�� Fig. 146: A subsample of the relative Feynman graphs illustrating the three types ofttγγ processes.

Table 27: Cross-sections at leading order (statistical errors in parentheses), number of events generated, numbers ofevents and

statistical weight/generated event for 30 and 100 fb−1 of integrated luminosity for the irreducible backgrounds considered.

Process σ× BR(1 W→ lν) Ngen N 30 fb−1 Wgt 30 fb−1 N 100 fb−1 Wgt 100 fb−1 Generator

ttγγ 1 1.6 fb (≤ 1/mil) 9296 48 .0052 160 .0172 AL,MGttγγ 2 6.1 fb (≤ 1%) 2310 183 .0792 610 .2641 ALttγγ 3 4.9 fb (≤ 1%) 914 147 .1608 490 .5361 ALbbγγ 318.1 fb 159829 9543 .0597 31810 0.1990 MG

Wγγ 4j 11.5 fb (1.2%) 4587 345 0.0752 1150 .2507 ALZγγ 29.0 fb 50005 870 0.0174 2900 0.0580 MGWγγ 23.6 fb 112000 708 0.0063 2360 0.0211 MG

identified due to incomplete detector coverage or other instrumental effects. This could arise if one ormore electrons or jets are misidentified as photons, or a jet as an electron or a muon. Therefore possiblebackground processes can be grouped into the following signature categories:llγ, llj, ljj, lγj, γγj, γjj,jjj, wherel is a lepton andj is a jet. Table 28 lists the reducible background processes to be considereedfor each category. It should be noted that several processescould contribute to more than one signaturecategory.

During the time horizon of the workshop, due to the implementation of the many new generatorprocesses, it has been possible to study only the irreducible backgrounds with acceptable statistics, soonly these will be presented in this report. Low-statisticstests on most of the processes in Table 28 havebeen performed, and as many of these processes as possible will be included with high statistics in adefinitive study now in progress with events fully simulatedand reconstructed in the CMS detector.

All generated signal and background events were fragmentedand hadronized with PYTHIA [27,284] version 6.227.

25.4 Description of preselections

No generator-level preselections were made on signal events. For the irreducible background events, thefollowing preselection was made:

• mγγ ≥80 GeV + where applicable:

• pTγ ≥ 20 GeV,|ηγ | ≤ 2.5 or pTγ ≥ 15 GeV,|ηγ | ≤ 2.7

• pTj,l,b ≥ 15 GeV,|ηj,l,b| ≤ 2.7,∆R(l,j or j,j or b,b orγ,j or γ, γ) ≥ 0.3

where pT refers to the transverse momentum of the particle,η its rapidity and ∆R =√(∆η2 + ∆φ2) whereφ is the azimuthal angle.

146

�� n�� g

g

�� n��g

g�� n�� g
















146

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g

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146

LO Monte Carlo study [LH2005, hep-ph/0604120]

inclusive σ(`νjjbbγγ) dominated by Type 2+3

# of events/100 fb−1 for ttH(γγ) selection with stringentγ-hardness+isolation cuts and |mγγ −mH | < 1.5 GeV

mH [GeV] 120 130 140

ttH(γγ) 13.5 13.1 9.5

ttγγ Type 1 0.38 0.48 0.53

ttγγ Type 2 0.5 0.3 0.5

ttγγ Type 3 0.5 <0.5 <0.5

⇒ radiative top decays still dominant

Implications on shapes and acceptance efficiencies deserve detailed NLO studies

29 / 30

Summary and Outlook

Theory precision for ttH and backgrounds

crucial for measurements of ttH rate and coupling

main priority: multi-particle backgrounds (tt+jets, tt+b-jets, ttV+jets,. . . )

Recent theory progress

powerful automated NLO MC tools

lot of results for ttH signal (soft resummation, NLO EW, off-shell effects)

all backgrounds available at NLO+PS level but not yet optimally understood

Sophisticated MC simulations and experimental analyses

solid understanding of matching/merging/shower uncertainties non-trivial

precision MC simulations in ATLAS/CMS require theory guidance

To follow/contribute to ongoing ttH HXSWG studies

⇒ https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCHXSWGTTH

⇒ subscribe e-group mailing list lhc-higgs-xsbr-tth

30 / 30

MC for t H and SM backgrounds - Durham University

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