Aspects of MC simulations for top quark physics

Stefan Hoche

Fermi National Accelerator Laboratory

Top Quark Physics at the Precision Frontier

Fermilab, 05/15/2019

Fermilab’s special relationship with the top quark

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Observation of the Top Quark

S. Abachi,12 B. Abbott,33 M. Abolins,23 B.S. Acharya,40 I. Adam,10 D.L. Adams,34

M. Adams,15 S. Ahn,12 H. Aihara,20 J. Alitti,36 G. Alvarez,16 G.A. Alves,8 E. Amidi,27

N. Amos,22 E.W. Anderson,17 S.H. Aronson,3 R. Astur,38 R.E. Avery,29 A. Baden,21

V. Balamurali,30 J. Balderston,14 B. Baldin,12 J. Bantly,4 J.F. Bartlett,12 K. Bazizi,7

J. Bendich,20 S.B. Beri,31 I. Bertram,34 V.A. Bezzubov,32 P.C. Bhat,12 V. Bhatnagar,31

M. Bhattacharjee,11 A. Bischoff,7 N. Biswas,30 G. Blazey,12 S. Blessing,13 A. Boehnlein,12

N.I. Bojko,32 F. Borcherding,12 J. Borders,35 C. Boswell,7 A. Brandt,12 R. Brock,23

A. Bross,12 D. Buchholz,29 V.S. Burtovoi,32 J.M. Butler,12 D. Casey,35 H. Castilla-Valdez,9

D. Chakraborty,38 S.-M. Chang,27 S.V. Chekulaev,32 L.-P. Chen,20 W. Chen,38

L. Chevalier,36 S. Chopra,31 B.C. Choudhary,7 J.H. Christenson,12 M. Chung,15 D. Claes,38

A.R. Clark,20 W.G. Cobau,21 J. Cochran,7 W.E. Cooper,12 C. Cretsinger,35

D. Cullen-Vidal,4 M. Cummings,14 D. Cutts,4 O.I. Dahl,20 K. De,41 M. Demarteau,12

R. Demina,27 K. Denisenko,12 N. Denisenko,12 D. Denisov,12 S.P. Denisov,32

W. Dharmaratna,13 H.T. Diehl,12 M. Diesburg,12 G. Diloreto,23 R. Dixon,12 P. Draper,41

J. Drinkard,6 Y. Ducros,36 S.R. Dugad,40 S. Durston-Johnson,35 D. Edmunds,23

A.O. Efimov,32 J. Ellison,7 V.D. Elvira,12,‡ R. Engelmann,38 S. Eno,21 G. Eppley,34

P. Ermolov,24 O.V. Eroshin,32 V.N. Evdokimov,32 S. Fahey,23 T. Fahland,4 M. Fatyga,3

M.K. Fatyga,35 J. Featherly,3 S. Feher,38 D. Fein,2 T. Ferbel,35 G. Finocchiaro,38

H.E. Fisk,12 Yu. Fisyak,24 E. Flattum,23 G.E. Forden,2 M. Fortner,28 K.C. Frame,23

P. Franzini,10 S. Fredriksen,39 S. Fuess,12 A.N. Galjaev,32 E. Gallas,41 C.S. Gao,12,∗

S. Gao,12,∗ T.L. Geld,23 R.J. Genik II,23 K. Genser,12 C.E. Gerber,12,§ B. Gibbard,3

M. Glaubman,27 V. Glebov,35 S. Glenn,5 J.F. Glicenstein,36 B. Gobbi,29 M. Goforth,13

A. Goldschmidt,20 B. Gomez,1 P.I. Goncharov,32 H. Gordon,3 L.T. Goss,42 N. Graf,3

P.D. Grannis,38 D.R. Green,12 J. Green,28 H. Greenlee,12 G. Griffin,6 N. Grossman,12

P. Grudberg,20 S. Grunendahl,35 J.A. Guida,38 J.M. Guida,3 W. Guryn,3 S.N. Gurzhiev,32

Y.E. Gutnikov,32 N.J. Hadley,21 H. Haggerty,12 S. Hagopian,13 V. Hagopian,13

K.S. Hahn,35 R.E. Hall,6 S. Hansen,12 R. Hatcher,23 J.M. Hauptman,17 D. Hedin,28

A.P. Heinson,7 U. Heintz,12 R. Hernandez-Montoya,9 T. Heuring,13 R. Hirosky,13

J.D. Hobbs,12 B. Hoeneisen,1,¶ J.S. Hoftun,4 F. Hsieh,22 Ting Hu,38 Tong Hu,16 T. Huehn,7

S. Igarashi,12 A.S. Ito,12 E. James,2 J. Jaques,30 S.A. Jerger,23 J.Z.-Y. Jiang,38

T. Joffe-Minor,29 H. Johari,27 K. Johns,2 M. Johnson,12 H. Johnstad,39 A. Jonckheere,12

H. Jostlein,12 S.Y. Jun,29 C.K. Jung,38 S. Kahn,3 J.S. Kang,18 R. Kehoe,30 M. Kelly,30

A. Kernan,7 L. Kerth,20 C.L. Kim,18 S.K. Kim,37 A. Klatchko,13 B. Klima,12

B.I. Klochkov,32 C. Klopfenstein,38 V.I. Klyukhin,32 V.I. Kochetkov,32 J.M. Kohli,31

D. Koltick,33 A.V. Kostritskiy,32 J. Kotcher,3 J. Kourlas,26 A.V. Kozelov,32

E.A. Kozlovski,32 M.R. Krishnaswamy,40 S. Krzywdzinski,12 S. Kunori,21 S. Lami,38

G. Landsberg,38 R.E. Lanou,4 J-F. Lebrat,36 J. Lee-Franzini,38 A. Leflat,24 H. Li,38 J. Li,41

Y.K. Li,29 Q.Z. Li-Demarteau,12 J.G.R. Lima,8 D. Lincoln,22 S.L. Linn,13 J. Linnemann,23

R. Lipton,12 Y.C. Liu,29 F. Lobkowicz,35 S.C. Loken,20 S. Lokos,38 L. Lueking,12

A.L. Lyon,21 A.K.A. Maciel,8 R.J. Madaras,20 R. Madden,13 I.V. Mandrichenko,32

Ph. Mangeot,36 S. Mani,5 B. Mansoulie,36 H.S. Mao,12,∗ S. Margulies,15 R. Markeloff,28

L. Markosky,2 T. Marshall,16 M.I. Martin,12 M. Marx,38 B. May,29 A.A. Mayorov,32

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FERMILAB-PUB-95/022-ECDF/PUB/TOP/PUBLIC/3040

Observation of Top Quark Production in pp Collisions

Abstract

We establish the existence of the top quark using a 67 pb−1 data sample

of pp collisions at√s = 1.8 TeV collected with the Collider Detector at Fer-

milab (CDF). Employing techniques similar to those we previously published,

we observe a signal consistent with tt decay to WWbb, but inconsistent with

the background prediction by 4.8σ. Additional evidence for the top quark is

provided by a peak in the reconstructed mass distribution. We measure the

top quark mass to be 176 ± 8(stat.)± 10(sys.) GeV/c2, and the tt production

cross section to be 6.8+3.6−2.4 pb.

The CDF Collaboration

F. Abe,14 H. Akimoto,32 A. Akopian,27 M. G. Albrow,7 S. R. Amendolia,24

D. Amidei,17 J. Antos,29 C. Anway-Wiese,4 S. Aota,32 G. Apollinari,27

T. Asakawa,32 W. Ashmanskas,15 M. Atac,7 P. Auchincloss,26 F. Azfar,22

P. Azzi-Bacchetta,21 N. Bacchetta,21 W. Badgett,17 S. Bagdasarov,27

M. W. Bailey,19 J. Bao,35 P. de Barbaro,26 A. Barbaro-Galtieri,15 V. E. Barnes,25

B. A. Barnett,13 P. Bartalini,24 G. Bauer,16 T. Baumann,9 F. Bedeschi,24

S. Behrends,3 S. Belforte,24 G. Bellettini,24 J. Bellinger,34 D. Benjamin,31

J. Benlloch,16 J. Bensinger,3 D. Benton,22 A. Beretvas,7 J. P. Berge,7 S. Bertolucci,8

A. Bhatti,27 K. Biery,12 M. Binkley,7 D. Bisello,21 R. E. Blair,1 C. Blocker,3

A. Bodek,26 W. Bokhari,16 V. Bolognesi,24 D. Bortoletto,25 J. Boudreau,23

G. Brandenburg,9 L. Breccia,2 C. Bromberg,18 E. Buckley-Geer,7 H. S. Budd,26

K. Burkett,17 G. Busetto,21 A. Byon-Wagner,7 K. L. Byrum,1 J. Cammerata,13

1

Reconstructed Mass (GeV/c2)

Even

ts/(

10 G

eV/c

2)

Top Mass (GeV/c2)

∆ln

(lik

elih

ood)

0

1

2

3

4

5

6

80 100 120 140 160 180 200 220 240 260 280

-1

0

1

2

160 170 180 190

Top as a probe of the Higgs sector

I First direct evidence of Yukawa interactions

I Responsible for stability of EW vacuum?

Httµ1− 0 1 2 3 4 5 6 7

Combined

13 TeV

7+8 TeV

)bH(btt

)-τ+τH(tt

)γγH(tt

H(ZZ*)tt

H(WW*)tt

(13 TeV)-1 (8 TeV) + 35.9 fb-1 (7 TeV) + 19.7 fb-15.1 fb

CMSObserved

syst)⊕ (stat σ1± (syst)σ1±

syst)⊕ (stat σ2±

[CMS] arXiv:1804.02610

SMttH

σ/ttH

σ1− 0 1 2 3 4

Total Stat. Syst. SMATLAS

­1 = 13 TeV, 36.1 ­ 79.8 fbs

Total Stat. Syst.

Combined )0.19

0.21 ± 0.18 , ± ( 0.26

0.28 ± 1.32

H (ZZ)tt < 1.77 at 68% CL

)γγH (tt )0.170.23 ± , 0.38

0.42 ± ( 0.420.48 ± 1.39

H (multilepton)tt )0.270.30 ± , 0.29

0.30 ± ( 0.400.42 ± 1.56

)bH (btt 0.53 )± , 0.280.29 ± ( 0.60

0.61 ± 0.79

[ATLAS] arXiv:1806.00425

Top mass

I Extracted from measurementsof various observablese.g. σtt, mlb, mT2, . . .

I Complex hadronic & leptonicfinal states, often with extra jets

I Subtle theoretical issuesI Top reconstructionI Radiative correctionsI Off-shell effectsI Color reconnectionI MC mass vs. pole massI . . .

[GeV]topm165 170 175 180 185

ATLAS+CMS Preliminary = 7-13 TeVs summary, topmWGtopLHC

November 2018

World comb. (Mar 2014) [2]

stattotal uncertainty

total stat

syst)± total (stat ± topm Ref.s

WGtopLHCLHC comb. (Sep 2013) 7 TeV [1] 0.88)± 0.95 (0.35 ±173.29

World comb. (Mar 2014) 1.96-7 TeV [2] 0.67)± 0.76 (0.36 ±173.34

ATLAS, l+jets 7 TeV [3] 1.02)± 1.27 (0.75 ±172.33

ATLAS, dilepton 7 TeV [3] 1.30)± 1.41 (0.54 ±173.79

ATLAS, all jets 7 TeV [4] 1.2)± 1.8 (1.4 ±175.1

ATLAS, single top 8 TeV [5] 2.0)± 2.1 (0.7 ±172.2

ATLAS, dilepton 8 TeV [6] 0.74)± 0.85 (0.41 ±172.99

ATLAS, all jets 8 TeV [7] 1.01)± 1.15 (0.55 ±173.72

ATLAS, l+jets 8 TeV [8] 0.82)± 0.91 (0.39 ±172.08

ATLAS comb. (Oct 2018) 7+8 TeV [8] 0.41)± 0.48 (0.25 ±172.69

CMS, l+jets 7 TeV [9] 0.97)± 1.06 (0.43 ±173.49

CMS, dilepton 7 TeV [10] 1.46)± 1.52 (0.43 ±172.50

CMS, all jets 7 TeV [11] 1.23)± 1.41 (0.69 ±173.49

CMS, l+jets 8 TeV [12] 0.48)± 0.51 (0.16 ±172.35

CMS, dilepton 8 TeV [12] 1.22)± 1.23 (0.19 ±172.82

CMS, all jets 8 TeV [12] 0.59)± 0.64 (0.25 ±172.32

CMS, single top 8 TeV [13] 0.95)± 1.22 (0.77 ±172.95

CMS comb. (Sep 2015) 7+8 TeV [12] 0.47)± 0.48 (0.13 ±172.44

CMS, l+jets 13 TeV [14] 0.62)± 0.63 (0.08 ±172.25

CMS, dilepton 13 TeV [15] 0.69)± 0.70 (0.14 ±172.33

CMS, all jets 13 TeV [16] 0.76)± 0.79 (0.20 ±172.34 [1] ATLAS-CONF-2013-102[2] arXiv:1403.4427[3] Eur.Phys.J.C75 (2015) 330[4] Eur.Phys.J.C75 (2015) 158[5] ATLAS-CONF-2014-055[6] Phys.Lett.B761 (2016) 350

[7] JHEP 09 (2017) 118[8] arXiv:1810.01772

[9] JHEP 12 (2012) 105[10] Eur.Phys.J.C72 (2012) 2202[11] Eur.Phys.J.C74 (2014) 2758

[12] Phys.Rev.D93 (2016) 072004

[13] EPJC 77 (2017) 354[14] arXiv:1805.01428[15] CMS PAS TOP-17-001[16] CMS PAS TOP-17-008

Top as a constraint on PDF fits

[Czakon,Hartland,Mitov,Nocera,Rojo] arXiv:1611.08609

I Pin down large-x gluon with thehelp of top-quark differentialdistributions

( GeV )XM210 310

Glu

on -

Glu

on L

umin

osity

(R

elE

rr)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Baseline

+ top-quark differential

NNLO, global fits, LHC 13 TeV

( GeV )XM210 310

Glu

on -

Glu

on L

umin

osity

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

Baseline

+ top-quark differential

NNLO, global fits, LHC 13 TeV

( GeV )XM210 310

Qua

rk -

Ant

iqua

rk L

umin

osity

0.85

0.9

0.95

1

1.05

1.1

1.15

Baseline

+ top-quark differential

NNLO, global fits, LHC 13 TeV

Top quark (pair) production

I Studied intensely, both at fixed orderI NNLO QCD [Czakon,Fiedler,Mitov ’13], [Czakon,Heymes,Mitov ’16]

[Catani,Devoto,Grazzini,Kallweit,Mazzitelli,Sargsyan ’19]

I NNLO QCD in production × decay [Gao,Papanastasiou ’17]

[Behring,Czakon,Mitov,Papanastasiou,Poncelet ’19]

I NNLO QCD + NLO EW [Czakon,Heymes,Mitov,Pagani,Tsinikos,Zaro ’17]

I NLO QCD / EW in production × decay [Bernreuther,Brandenburg,Si ’04],[Melnikov,Schulze ’09], [Campbell,Ellis ’15], [Bernreuther,Si ’10]

I NLO QCD / EW WWbb [Bevilacqua,Czakon,vanHameren,Papadopoulos,Worek ’11],[Denner,Dittmaier,Kallweit,Pozzorini ’11+’12], [Heinrich,Maier,Nisius,Schlenk,Winter ’14],[Frederix ’14], [Cascioli,Kallweit,Maierhofer,Pozzorini ’14], [Denner,Pellen ’16]

I NLO QCD tt+(multi-)jet [Dittmaier,Uwer,Weinzierl ’07],[Bevilacqua,Czakon,Papadopoulos,Worek ’10], [Maierhofer,Moretti,Pozzorini,Siegert,SH ’16]

I . . .

I and in the context of particle-level Monte CarloI NLO QCD+PS [Frixione,Nason,Webber ’03], [Frixione,Nason,Ridolfi ’07]

I NLO QCD+PS in production × decay [Campbell,Ellis,Nason,Re ’15]

I NLO QCD+PS WWbb [Garzelli,Kardos,Trocsanyi ’14], [Jezo,Lindert,Nason,Oleari,Pozzorini ’16]

I NLO QCD+PS vs. NLO WWbb [Heinrich,Maier,Nisius,Schlenk,Schulze,Scyboz,Winter ’17]

I NLO QCD+PS tt+(multi-)jet [Kardos,Papadopoulos,Trocsanyi ’11], [Alioli,Moch,Uwer ’11],[Huang,Luisoni,Schonherr,Winter,SH ’13], [Krauss,Maierhofer,Pozzorini,Schonherr,Siegert,SH ’14]

I . . .

Outline of this talk

I Stability of fixed-order& particle-level predictions

I Impact of parton showeron top mass measurement

I ttbb four vs. five flavor& variable flavor number scheme

Scale uncertainties in tt+jets at NLO

I Renormalization/factorization scaletypically used at very high multiplicity:sum of transverse mass H ′T,m =

∑m⊥

I Very different scale in resummation[Hamilton,Nason,Zanderighi] arXiv:1206.3572

I Interpret event in terms of QCDbranchings, like in a parton-shower

I Assign transverse momentum scalesq to splittings, evaluate one αs ateach of these scales

I Multiply with NLL Sudakov factors,subtract first-order expansion

I MINLO scale probes detailed dynamics,typically very small → good candidatefor comparison to H ′T,m

?

cluster oncefind some kT

?

kT

cluster twicefind some k′T

k′T

kT

Scale uncertainties in tt+jets at NLO

[Maierhofer,Moretti,Pozzorini,Siegert,SH] arXiv:1607.06934

Sherpa+OpenLoops

Njet ≥ 1

200 400 600 800 1000

10−4

10−3

10−2

10−1

1

HjetsT [GeV]

dσ

/d

Hje

tsT

[pb/

GeV

]

Njet ≥ 2

200 400 600 800 1000

10−4

10−3

10−2

10−1

1

HjetsT [GeV]

dσ

/d

Hje

tsT

[pb/

GeV

]

Njet ≥ 3

MINLOMILONLO

LO

200 400 600 800 1000

10−4

10−3

10−2

10−1

1

HjetsT [GeV]

dσ

/d

Hje

tsT

[pb/

GeV

]

200 400 600 800 1000

0.60.8

11.21.41.6 NLO

LO

HtTextjets [GeV]

σ/

σN

LO

200 400 600 800 1000

0.60.8

11.21.41.6

HtTextjets [GeV]

σ/

σN

LO

200 400 600 800 1000

0.60.8

11.21.41.6

HtTextjets [GeV]

σ/

σN

LO

200 400 600 800 1000 1200

0.60.8

11.21.41.6 MINLO

MILO

HtTextjets [GeV]

σ/

σM

INL

O

200 400 600 800 1000 1200

0.60.8

11.21.41.6

HtTextjets [GeV]

σ/

σM

INL

O

200 400 600 800 1000 1200

0.60.8

11.21.41.6

HtTextjets [GeV]

σ/

σM

INL

O

NLOLO

200 400 600 800 1000

0.60.8

11.21.41.6

HjetsT [GeV]

σM

I(N)L

O/

σ(N

)LO

200 400 600 800 1000

0.60.8

11.21.41.6

HjetsT [GeV]

σM

I(N)L

O/

σ(N

)LO

200 400 600 800 1000

0.60.8

11.21.41.6

HjetsT [GeV]

σM

I(N)L

O/

σ(N

)LO

Scale uncertainties in tt at NNLO

[Czakon,Heymes,Mitov] arXiv:1606.03350

I Various central scales investigated for perturbative convergenceI Best choices observable dependent and given by

µ0 =

{ mT2

for : pT,t, pT,t and pT,t/t

HT4

for : all other distributions

I Peculiar behavior of H ′T,m based scale at NNLO

-10

-5

-1 0 1

5

10

1/8 1/4 1/2 1 2 4 8

PP → tt-+X (8 TeV)

mt=173.3 GeV

µ0 = HT/2

MSTW2008

σ(

µ)/

σres(mt)-1[%]

µ/µ0

LO

NLO

NNLO

-10

-5

-1 0 1

5

10

1/8 1/4 1/2 1 2 4 8

PP → tt-+X (8 TeV)

mt=173.3 GeV

µ0 = H‘

T/2

MSTW2008

σ(

µ)/

σres(mt)-1[%]

µ/µ0

LO

NLO

NNLO

Multi-jet merging for tt+jets

I NLO-matched & merged simulationsavailable up to tt+2j [Frederix,Frixione ’12],[Krauss,Maierhofer,Pozzorini,Schonherr,Siegert,SH ’14]

I Decays & spin correlations at LO

I Largely reduced µR/F variations,central values agree well withLO merged predictions

Sherpa+OpenLoops

MEPS@NLO1.65 × MEPS@LO

S-MC@NLO

10−5

10−4

10−3

Total transverse energy

dσ

/d

Hto

tT

[pb/

GeV

]

200 400 600 800 1000 1200

0.5

1

1.5

HtotT [GeV]

Rat

ioto

ME

PS@

NL

O

Sherpa+OpenLoops

1st jet

2nd jet

3rd jet

MEPS@NLO1.65 × MEPS@LOS-MC@NLO

10−8

10−7

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

O2nd jet0.5

1

1.5

2

Rat

ioto

ME

PS@

NL

O

3rd jet

40 50 100 200 500

0.51

1.52

2.5

pT (light jet) [GeV]

Rat

ioto

ME

PS@

NL

O

EW corrections in multi-jet merging

[Kallweit,Lindert,Maierhofer,Pozzorini,Schonherr] arXiv:1511.08692

[Gutschow,Lindert,Schonherr] arXiv:1803.00950

I EW virtual corrections & integrated subtraction can be included in merging

I Real corrections recovered to a good accuracy by parton shower or YFS

Sher

pa+O

pen

Lo

ops

+0,1 (NLO QCD) + 2,3,4 (LO)+0,1 (NLO QCD+EWvirt) + 2,3,4 (LO)+0,1 (NLO QCD+EWvirt+LOsub) + 2,3,4 (LO)10−6

10−5

10−4

10−3

10−2

10−1

1

pp → tt + 0, 1(, 2, 3, 4) jets at 13 TeV

dσ

/dp T

,t[p

bG

eV−

1 ]

50 100 300 500 1000

0.7

0.8

0.9

1.0

1.1

1.2

pT,t [GeV]

1/M

EPS

@N

LO

QC

D

b

b

b

b

b

b

b

b

Sher

pa+O

pen

Lo

ops

µ = µCKKW

b ATLAS dataPRD 93 (2016) 3, 032009MEPS@NLO QCDMEPS@NLO QCD+EWvirt

10−2

10−1

1

10 1pp → tt (+ jet) at 8 TeV

dσ

/dp T

[pb

GeV

−1 ]

b b b b b b b b

300 400 500 600 700 800 900 1000 1100 1200

0.6

0.8

1

1.2

1.4

Particle top-jet candidate pT [GeV]

The

ory

/D

ata

Matching – Processes with intermediate resonances

[Jezo,Nason] arXiv:1509.09071

I NLO subtraction methods do not preserve virtuality of possible resonancesIR cancellation takes place highly non-locally → efficiency problem

I Problem worsens in POWHEG, as uncontrollable ratios are exponentiated:

∆(ΦB , pT ) = exp

{−

∑α

∫dΦ1

R(Φ(α)R )

B(ΦB)Θ(pT − kT )

}

I Proposed solution:I Partition phase space such that each region

corresponds to a unique resonance historyI Within each region modify subtraction mappings

such that resonance mass is preserved

I Assignment of resonance histories requires algorithm→ Use kinematic proximity to resonance

Πfb =Pfb∑

f ′b∈res hists Pf ′b

, Pfb =∏i∈ress

M4i

(si −M2i )2 + Γ2

iM2i

Matching – Wt vs tt

[Jezo,Lindert,Oleari,Nason,Pozzorini] arXiv:1607.04538

I Wt production in the 5F scheme:I NLO corrections swamped by LO tt decayI Requires ad-hoc subtraction prescription (DR/DS)

I Wt production in the 4F scheme:I Unified treatment of Wt and tt (identical at LO)I Requires off-shell WWbb calculation

I Sizable differences compared to resonance-unaware matchingand to narrow-width approach [Frixione,Nason,Ridolfi] arXiv:0707.3088

10−2

10−1

dσ/d

mW

jB[pb/G

eV]

8 TeV

dσ/d

σbb4ℓ

dσ/d

mW

jB[pb/G

eV]

8 TeV

dσ/d

σbb4ℓ

bb4ℓres-default

res-off

res-guess

mWjB[GeV]

0.5

1.0

1.5

150 160 170 180 190 200

10−5

10−4

10−3

10−2

10−1

dσ/d

mW

jB[pb/G

eV]

8 TeV

dσ/d

σbb4ℓ

dσ/d

mW

jB[pb/G

eV]

8 TeV

dσ/d

σbb4ℓ

bb4ℓtt

mWjB[GeV]

0.5

1.0

1.5

100 150 200 250 300 350

Top quark mass from kinematical distributions

[Ravasio,Jezo,Nason,Oleari] arXiv:1801.03944

I If top quark mass extracted from kinematical distributionsquality of Monte Carlo modeling limits precision

1. Generators of increasing accuracy →NLO tt, NLO production × decay, NLO W+W−bb

2. Different parton-shower models → Pythia, Herwig

I For a large range of observables good consistency, except PS dependence

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

3.5 4 4.5 5 5.5

dσ/d

log(E

b j)/E

b j[p

b/G

eV]

bb4` Emaxbj

= 71.200 ± 0.081 GeV

ttdec Emaxbj

= 71.361 ± 0.062 GeV

hvq Emaxbj

= 70.744 ± 0.064 GeV

8 TeV

log(Ebj)

bb4`+Py8.2

ttdec+Py8.2

hvq+Py8.2

168

170

172

174

176

178

180

182

184 bb4`, mt = 172.5 GeV

Py8.2: mt = 172.500+0.784−0.766 GeV

Hw7.1: mt = 175.392+1.045−1.138 GeV

Extr

acte

dm

t[G

eV]

1st Mellin moment

2nd Mellin moment

3rd Mellin moment

168

170

172

174

176

178

180

182

184

pT(`+) pT(`

+`−) m(`+`−) E(`+`−) pT(`+)+pT(`

−)

hvq mt = 172.5 GeV

Py8.2: mt = 172.238+0.754−0.748 GeV

Hw7.1: mt = 174.607+0.961−1.097 GeV

Extractedm

t[G

eV]

1st Mellin moment

2nd Mellin moment

3rd Mellin moment

Top quark mass from mlb

[Heinrich,Maier,Nisius,Schlenk,Winter] arXiv:1312.6659

[Heinrich,Maier,Nisius,Schlenk,Schulze,Scyboz,Winter] arXiv:1709.08615

I Determine top mass from template fit to m2lb = (pb−jet + pl)

2

I ATLAS-CONF-2013-077: use lepton b-jet pairing minimizing∑mlb

I Theory uncertainty estimated to 0.8 GeV at√s =7 TeV

I Improve using NLO QCD W+W−bb from GoSam+Sherpa

LHC 7 TeVµR = µF = HT/2MSTW2008(n)lo pdf

NLO, mt = 172.5 GeVNLO, mt = 165.0 GeVNLO, mt = 180.0 GeVLO, mt = 172.5 GeV10−5

10−4

10−3

10−2

(1/

σ)

dσ

/dm

lb[1

/GeV

]

0 50 100 150 200

0.51

1.52

2.53

mlb [GeV]

Rat

io

[GeV]intm

164 166 168 170 172 174 176 178 180

[GeV

]in t

- m

out

tm

1−

0.5−

0

0.5

1

1.5

2

2.5

3

0.07 GeV± calibration, offset -0.01 fullNLOScale varied pseudo-data

0.07 GeV± calibration, offset 0.83 NLOdecNWANLO

Scale varied pseudo-data

fullPseudo-data according to NLO

172.5 GeV, PDF4LHC15 =t = mR/F

µLHC 13 TeV 50/fb,

Top quark mass from mlb

[Heinrich,Maier,Nisius,Schlenk,Winter] arXiv:1312.6659

I m2lb > m2

t −m2W kinematically forbidden in NWA at LO

I NLO corrections flat with symmetric bands in NWA

I Significant changes in shapes & uncertainty bands at NLO in W+W−bbDriven by radiative corrections or non-factorizing contributions?

full calculation NWA factorized

LHC 7 TeVHT/4 < µ < HTMSTW2008(n)lo pdf

W+W−bb (LO)W+W−bb (NLO)10−5

10−4

10−3

10−2

dσ

/dm

lb[p

b/G

eV]

0 50 100 150 2000.60.8

11.21.41.61.8

22.22.4

mlb[GeV]

NLO

/LO

LHC 7 TeVmt/2 < µ < 2mt

MSTW2008(n)lo pdf

tt (LO)tt (NLO)10−5

10−4

10−3

10−2

dσ

/dm

lb[p

b/G

eV]

0 50 100 150 2000.60.8

11.21.41.61.8

22.22.4

mlb[GeV]

NLO

/LO

Top quark mass from mlb

[Heinrich,Maier,Nisius,Schlenk,Schulze,Scyboz,Winter] arXiv:1709.08615

I Accidental agreement between NLO W+W−bb and NLO+PS (Sherpa)Maybe not completely accidental? NLO+PS has BW distribution!

NLOfull

NLONLOdecNWA

NLOLOdecNWA

NLOPS

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

1/

σd

σ/

dm

lb[1

/4

GeV

]

40 60 80 100 120 140 160 180 2000.30.40.50.60.70.80.91.01.11.2

mlb [GeV]

Ra

tio

[GeV]intm

164 166 168 170 172 174 176 178 180

[GeV

]in t

- m

out

tm

1.5−

1−

0.5−

0

0.5

1

1.5

2

2.5

0.07 GeV± calibration, offset -0.01 fullNLOScale varied pseudo-data

0.07 GeV± calibration, offset -0.09 PSNLOScale varied pseudo-data

fullPseudo-data according to NLO

172.5 GeV, PDF4LHC15 =t = mR/F

µLHC 13 TeV 50/fb,

Parton shower cutoff and the top-quark mass

[Hoang,Platzer,Samitz] arXiv:1807.06617

I Investigated jet mass distribution in e+e− → tt

I Established relation between pole mass, mpole, and coherent branchingmass, mCB(Q0), which depends on the parton-shower cutoff Q0

I Comparison between coherent branching formalism and SCET showslimQ0→0mCB(Q0) = mpole, both for massless and massive quarks

I Pole mass and coherent branching mass related for finite Q0 by

mCB(Q0) = mpole −2

3αs(Q0)Q0 +O(α2

s)

Status of ttbb

[F.Siegert SM@LHC ’19]

I ATLAS and CMS ttH(bb) analyses relyon MC modelling for irreducible ttbb BG

I Largest sources of uncertainty on extractedsignal strength related to tt+HF modeling!

I What can be improved?I ATLAS & CMS: relied on NLO+PS tt so

far! More accurate theory with NLO ttbbused only to reweight HF fractions(ATLAS) or cross-checks (CMS)

I Theory: Large perturbative ttbbuncertainties even increased byNLO+PS algorithms

I Both: More rigorous combination ofinclusive tt+jets and ttbb predictions.

Why do we care so much about ttbb?

‣ ATLAS and CMS ttH(bb) analyses rely on MC

modelling for irreducible ttbb background

• included as template in profile likelihood fit

‣ Largest sources of uncertainty on extracted

signal strength related to tt+HF MC modelling!

‣ What can we improve?

• ATLAS & CMS: relied on NLO+PS ttbar so far!

More accurate theory with NLO ttbb used only to

reweight HF fractions (ATLAS) or cross-checks (CMS)

• Theory: Large perturbative ttbb uncertainties

even enlarged by NLO+PS algorithms

• Both: More rigorous combination of inclusive

tt+jets and ttbb predictions.

2

Status of ttbb

Traditional MC simulation approaches to tt+HF

I Five-flavor scheme:I Inclusive NLO+PS tt sample

with HF from parton shower g → bbI Multi-leg merged tt+jets sample

with HF from higher-order MEs (hard b)or parton shower g → bb (soft/coll b)

Surprising feature:

Sherpa

MEPS@LO tt+0,1,2jtt+0btt+1btt+2b

10−3

10−2

10−1

1

10 1

10 2

10 3Inclusive B-jet multiplicity distribution

σ[p

b]

0 1 2 3 40

0.20.40.60.8

11.21.4

NB−jets

dσ/

dσre

f

arXiv:1802.00426

I Jet production described by hard MEs,but b-jets not always from b-MEs!

I soft/collinear g → bb from PScan transform light jets into b-jets

I Four-flavor scheme:I NLO+PS ttbb using matrix elements

with massive b-quarks

Surprising feature:

Sherpa+OpenLoops

LO

NLO

MC@NLO

MC@NLO2b

10−1

1

10 1

Mass of first two b-jets (ttbb cuts)

dσ/dm

bb[fb/GeV

]

0 50 100 150 200 250 300 350 4000.5

1

1.5

2

2.5

mbb [GeV]

dσ/d

σLO

arXiv:1309.5912

I Secondary bb from g → bb in PScan convert light jet into b-jet→ even interpretation changes

Status of ttbb

I Several tools on the marketI Sherpa + OpenLoops [Cascioli,Maierhofer,

Moretti,Pozzorini,Siegert] arXiv:1309.5912

I PowHel + Pythia/Herwig[Bevilacqua,Garzelli,Kardos] arXiv:1709.06915

I PowhegBox + OpenLoops +Pythia/Herwig [Jezo,Lindert,Moretti,Pozzorini]

arXiv:1802.00426

I MG5 aMC + Pythia/HerwigI Herwig7 + OpenLoops

I History of out-of-the-box comparisons:I Large discrepanciesI Due in part to pQCD uncertaintiesI But also beyond: Parton Shower,

NLO+PS matching algorithm

I Ongoing: Tuned comparison

[HXSWG] arXiv:1610.07922

pp → ttbb@ 13TeV

LHCHIG

GSXSW

G20

16

Sherpa+OpenLoops

MG5aMC@NLO

PowHel+PY8NLO

0 50 100 150 200 250 300 350 400

10−3

10−2

10−1

Invariant mass of the 1st light-jet and 1st b-jet system (ttb cuts)

m [GeV]

dσ/dm

[pb/GeV

]

0 50 100 150 200 250 300 350 4000.60.8

11.21.41.61.8

2

m [GeV]

σ/

σSherpa+Open

Loops

0 50 100 150 200 250 300 350 4000.60.8

11.21.41.61.8

2

m [GeV]

σ/

σMG5aMC@NLO

0 50 100 150 200 250 300 350 4000.60.8

11.21.41.61.8

2

m [GeV]

σ/

σPowHel+

PY8

I Fixed-order studies of ttbbj at NLO showstabilization of K-factor for µR = (ET,tET,tET,bET,b)

1/4

→ New benchmark for NLO+PS programs! [Buccioni,Pozzorini,Zoller ’19]

Matching X+jets & Xbb

. . .

(b1)

(b)

b

b

. . .. . .. . .

(b2)

b

b

b

b

. . .

(b3)

b

b

(b4)

b

b

. . .

[Krause,Siegert,SH] arXiv:1904.09382I Interpret Xbb as part of Xjj

1. Cluster to obtain parton shower history2. Apply αs(µ

2R)→ αs(p

2T ) reweighting

3. Apply Sudakov factors ∆(t, t′) (trial showers)

I Remove double-counting

1. Cluster PS-level event using inverse PS2. Look at leading two emissions

I Heavy Flavour → keep from Xbb(“direct component”)

I Light Flavour → keep from X+jets(“fragmentation component”)

I Subleading g → bb splittingsnot from Xbb ME, but X4j ME+PS

I Match 5F→4F in PDFs and αs

1. Use 5F PDF / αs to be consistent with Xjj2. Use matching coefficients to correct to 4F scheme

[Buza,Matiounine,Smith,van Neerven] hep-ph/9612398, [Forte,Napoletano,Ubiali] arXiv:1607.00389

→ Coefficients up to (N)LL generated by (N)LO parton shower!3. Reweighting needed only for αs in hard ME

Can be applied to LO and NLO merging!

Example: Z+jets & Zbb

[Krause,Siegert,SH] arXiv:1904.09382I Validation with LHC data

bb

b

b

b

b

b

b

bb

bb

bATLAS JHEP07(2013)032Z+jets ⊕ Zbb

direct componentfragmentation component

Z+jetsZbb, 4FS

10−5

10−4

10−3

10−2

10−1

1Transverse momentum of Z-boson

dσ

/dp ⊥

[pb/

GeV

]

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

0 50 100 150 200 250 300 350 400 4500

0.20.40.60.8

11.21.4

p⊥(Z) [GeV]

MC

/Dat

a

Data [pb] Fusing [pb]

Z+ ≥ 1b 3.55± 0.24comb 3.80(5)± 0.830.33

Z+ ≥ 2b 0.331± 0.037comb 0.282(4)± 0.0270.022

Sherpa+OpenLoops

b

b

b

b

b

b

b

bb

bb

b

b

b

bCMS Eur.Phys.J. C77 (2017) no.11, 751Z+jets ⊕ Zbb

7 point uncert. PS+MEdirect component

fragmentation componentZbb, 4FS

Z+jets

Qcut = 15 GeV Qcut = 30 GeV

10−4

10−3

10−2

10−1

CMS, 8 TeV, Leading b-jet transverse momentum, at least one b-jet

dσ

/d

p ⊥[p

b/G

eV]

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

00.20.40.60.8

11.21.4

MC

/Dat

a

bb b b b

b bb b

bb b

bb

50 100 150 200 250 3000.20.40.60.8

11.21.4

p⊥(b) [GeV]

Rat

io

Matching tt+jets & ttbb

[Katzy,Krause,Pollard,Siegert] in preparation

I Combination of tt+0,1j@NLO+2,3j@LO and massive ttbb@NLO

I 2-bjet production dominated by direct component, but 1-bjet observableswith equal contributions from direct and fragmentation configurations!

[F.Siegert SM@LHC ’19]

Sneak preview: Application to ttbb

‣ Application to fusion of MEPS@NLO tt + 0,1j@NLO + 2,3j@LO

and massive ttbb@NLO

‣ 2-bjet production dominated by direct component, but 1-bjet observables with

equal contributions from direct and fragmentation configurations!

20

[K

atzy, K

rau

se, P

ollard

, F

S in

p

rep

]

[Preliminary] [Preliminary]

stable tops stable tops

Summary

Active research directly related to tops:

I Matching for resonant processesproduction, production × decay, full final state

I Relation between MC masses and pole mass

I 4F vs. 5F scheme and VFNS

I Phenomenology . . .

Other topics of current interest:

I Full color parton showers

I Extension of parton showers to NLO

I Benchmarking of parton showers with resummation

I Phenomenology . . .