di erential top cross-section...

Differential Top Cross-section MeasurementsALPS2017 Obergurgl

Michael Fentonon behalf of the ATLAS Collaboration

University of [email protected]

April 18, 2017

Michael Fenton Differential Top Cross-section Measurements April 18, 2017 1 / 16

Outline

1 Motivation2 Analysis Strategy3 Uncertainties4 Results5 Summary


Why do differential measurements of tt̄ processes? [1/2]

The top quark is unique in the SM due to it’s large mass:

decay before hadronisation

→ only quark that can be studied in isolationprecision QCD test

same order as V.E.V in SM

→ mt ' 173 GeV, v = 246 GeV↪→ direct sensitivity to new physics

mt = ytv/√

2

∆mth ∼ −

m2

v 2

Λ

4π2

Top properties at the LHC

Top production & decay• Top production dominated by QCD production. EW

production provides direct access to Wtb vertex:

• In SM top decays to Wb:

4


Why do differential measurements of tt̄ processes? [2/2]

Major background to many interesting searches like tt̄H or SUSY

Not always well described in current MC generators→ Differential measurements crucial input to MC tuning efforts!

TOPQ-2014-15

Differential data very useful to theorists:

constraining EFT operators: TopFitter

gluon PDF at large x: Czakon et al

highly sensitive to NNLO effects Czakon et al


https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2014-15/

https://arxiv.org/abs/1512.03360



Outline



Analysis Strategy

1 Select events

Lepton+jets channelUtilize both resolved andboosted topologies

2 Estimate backgrounds

3 Reconstruct tops and tt̄ system

4 Unfold data

ICHEP Conference Note


https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2016-040/

Analysis Strategy

1 Select events


Data-driven QCD estimate andW+jetsMC based signal and otherbackgrounds


4 Unfold dataE

vent

s / G

eV

500

1000

1500

2000

2500

3000

3500 DatattSingle topW+jetsZ+jetsDibosonVtt

MultijetsStat.+Syst. Unc.

-1 = 13 TeV, 3.2 fbsATLAS Preliminary

Resolved

[GeV]missTE

0 50 100 150 200

Dat

a/P

red.

0.81

1.2




Analysis Strategy

1 Select events



Measure pt,hadT ,

∣∣y t,had∣∣

ptt̄T , mtt̄ ,

∣∣∣y tt̄∣∣∣ (resolved only)

4 Unfold data




Analysis Strategy

1 Select events



4 Unfold data

Iterative Bayesian Unfolding(4 iterations)Unfold to particle level

Mig

ratio

n [%

]

0

10

20

30

40

50

60

70

80

90

100

[GeV] (detector level)t,had

Tp

[GeV

] (pa

rtic

le le

vel)

t,had

Tp 70 28 2

11 71 18 1

1 15 69 15

1 15 71 12

1 18 69 12

2 20 65 12

2 22 62 13

1 3 21 64 11

1 3 24 61 12

1 4 24 59 13

1 4 20 65 9

1 3 23 63 10

1 4 24 59 11

2 5 23 59 12

1 4 14 81

0 0

25

25

50

50

75

75

105

105

135

135

165

165

195

195

230

230

265

265

300

300

350

350

400

400

450

450

500

500

1000

1000

ATLAS SimulationPreliminaryResolved




Resolved Selection

t tW

b

q

q

W

`

ν

b

= 1 lepton≥ 4 R=0.4 jet, ≥ 2 b-jet

No explicit requirement

pT > 25 GeV|η| < 2.5

pT > 25 GeV|η| < 2.5

pT > 25 GeV|η| < 2.5

pT > 25 GeV|η| < 2.5

pT > 25 GeV|η| < 2.5

Eve

nts

20

40

60

80

100

120

140

310×DatattSingle topW+jetsZ+jetsDibosonVtt



Resolved

Jet multiplicity

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Dat

a/P

red.

0.81

1.2

Eve

nts

/ GeV

500

1000

1500

2000

2500

3000




Resolved

[GeV]missTE

0 50 100 150 200

Dat

a/P

red.

0.81

1.2

Eve

nts

/ GeV

200

400

600

800

1000

1200

1400

1600




Resolved

[GeV]t,had

Tp

0 200 400 600 800 1000 D

ata/

Pre

d.0.8

11.2


Boosted Selection

t t

b

W q

q

W

`

ν

b

∆R < 2.0

∆R > 1.5

∆φ(`, thad) > 1.0

= 1 lepton≥ 1 R=1.0 jet, ≥ 1 top tag, ≥ 1 R=0.4 jet, ≥ 1 b-jet

EmissT +mW

T > 60 GeV

EmissT > 20 GeV

pT > 25 GeV|η| < 2.5

pT > 25 GeV|η| < 2.5

pT > 300 GeV

|η| < 2.0

lep_pt

0 50 100 150 200 250 300 350 400 450 500

Eve

nts

/ GeV

20

40

60

80

100DatattSingle topW+jetsZ+jetsDibosonVtt



Boosted

[GeV]T

Lepton p

0 100 200 300 400 500

Dat

a/P

red.

0.60.8

11.21.4

ljet_m

0 50 100 150 200 250 300

Eve

nts

/ GeV

20

40

60

80

100

120

140

160DatattSingle topW+jetsZ+jetsDibosonVtt



Boosted

Large-R jet mass [GeV]

0 100 200 300

Dat

a/P

red.

0.60.8

11.21.4

hadtop_pt_varbin_logscale

400 600 800 1000 1200 1400

Eve

nts

/ GeV

2−10

1−10

1

10

210




Boosted

[GeV]t,had

Tp

500 1000 1500 D

ata/

Pre

d.0.60.8

11.21.4


Outline



Uncertainties

[GeV]t,had

Tp

0 100 200 300 400 500 600 700 800 900 1000

Fra

ctio

nal U

ncer

tain

ty [%

]

60−

40−

20−

0

20

40

60

Stat.+Syst. Unc. Stat. Unc.Flavor Tagging JES/JERBackgrounds IFSR, PDF, MC Stat.Hadronisation Hard Scattering

ATLAS Preliminary-1 = 13 TeV, 3.2 fbs

Resolved

Fiducial phase-space

Relative cross-section

[GeV]t,had

Tp

400 600 800 1000 1200 1400

Fra

ctio

nal U

ncer

tain

ty [%

]

80−

60−

40−

20−

0

20

40

60

80

Stat.+Syst. Unc. Stat. Unc.Large-R jet , Small-R jet

T

missLepton, EBackgrounds IFSR, PDF, MC Stat.Hadronisation Hard Scattering


Boosted


Relative cross-section

Jet energy scale and b-tagging uncertainties largest in resolved

Large-R jets systematics are dominant in boosted analysis

Generator systematics important in both analyses


Outline



Top pT /

GeV

t,had

T /

d p

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10

1

Data

t=mdampPWG+PY6 h radHit=2mdampPWG+PY6 h

radLot=mdampPWG+PY6 h

t=mdampPWG+PY8 h

t=mdampPWG+H++ haMC@NLO+H++Stat. unc.Stat.+Syst. unc.

ATLAS Preliminary Fiducial phase-space-1 = 13 TeV, 3.2 fbs

Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

1.4

[GeV]t,had

Tp

0 200 400 600 800 1000

Dat

aP

redi

ctio

n

0.81

1.21.4

/ G

eVt,h

ad

T /

d p

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10

1

Data



t=mdampPWG+PY8 h



Boosted

Dat

aP

redi

ctio

n

1

2

[GeV]t,had

Tp

400 600 800 1000 1200 1400D

ata

Pre

dict

ion

0

1

2

MC predicts harder spectrum than observed in data

Similar slope seen in both regions, as has been observed previously


tt̄ kinematics /

GeV

tt /

d m

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10 Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.5

1

1.5

[GeV]ttm

500 1000 1500 2000 2500 3000

Dat

aP

redi

ctio

n

0.5

1

1.5

/ G

eVtt T

/ d

pttσ

d

⋅ ttσ1/

4−10

3−10

2−10

1−10

1

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

[GeV]ttT

p

0 100 200 300 400 500 600 700 800

Dat

aP

redi

ctio

n

0.8

1

1.2

|tt| /

Uni

t |y

tt /

d |y

ttσ d

⋅ ttσ

1/

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

|tt|y

0 0.5 1 1.5 2 2.5

Dat

aP

redi

ctio

n

0.8

1

1.2

Powheg gives best description of ptt̄T and mtt̄

Expected high dependence of ptt̄T on hdamp parameter and radiation settings

Powheg+Herwig++ much better at high∣∣y tt̄

∣∣


Outline



Summary

Differential cross sections of tt̄ production important in SM and BSMphysics, experiment and theory, both as a signal and a background

/ G

eVt,h

ad

T /

d p

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10

1

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

1.4

[GeV]t,had

Tp

0 200 400 600 800 1000

Dat

aP

redi

ctio

n

0.81

1.21.4

Modelling of tt̄ process is generallygood

But top pT slope from Run1 stillremainsNNLO corrections may accountfor this

Biggest systematic is often the MCrelated ones

Can use these results to improvethe MC going forward


BACKUP


Run 1 NNLO Comparison


Resolved 8TeV vs 13TeV /

GeV

t,had

T /

d p

ttσ d

⋅ ttσ

1/

-510

-410

-310

-210

-110

1

Data

t=mdampPWG+PY6 h

t=mdampPWG+PY8 hMC@NLO+HW AUET2MadGraph+PY6 P2011CPWG+HW6 AUET2Stat. unc.Stat.+Syst. unc.

ATLAS Fiducial phase-space-1 = 8 TeV, 20.3 fbs

[GeV]t,had

Tp

0 200 400 600 800 1000

Dat

aP

redi

ctio

n

0.81

1.21.4

/ G

eVt,h

ad

T /

d p

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10

1

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n0.8

1

1.2

1.4

[GeV]t,had

Tp

0 200 400 600 800 1000

Dat

aP

redi

ctio

n

0.81

1.21.4


Boosted 8TeV vs 13TeV

/ G

eVt,h

ad

T /

d p

ttσ d

⋅ ttσ

1/

5−10

4−10

3−10

2−10

1−10

1

Data



t=mdampPWG+PY8 h



Boosted

Dat

aP

redi

ctio

n

1

2

[GeV]t,had

Tp

400 600 800 1000 1200 1400

Dat

aP

redi

ctio

n

0

1

2


Common Object Definitions

Electrons; gradient isolation, tight, |η| < 2.47 excluding crack, pT > 25GeV,|dBL

0 sig | < 5, |zBL0 sin θ| < 0.5mm

Muons; gradient isolation, medium, |η| < 2.5, pT > 25GeV, |dBL0 sig | < 3,

|zBL0 sin θ| < 0.5mm

Small Jets; anti-kT R=0.4, |η| < 2.5, pT > 25GeV, with JVT cut andcalibration

Large Jets; anti-kT R=1.0, |η| < 2.0, pT > 300GeV, trimmed (Rsub = 0.2 &fcut = 0.05), m > 50 GeV, pT < 1500 GeV

b-tags; MV2c20 > −0.4434 ( 77% working point )

Top-tags; pT dependent large jet mass and τ32 cuts, with 80% efficiencyworking point ATL-PHYS-PUB-2015-053

At particle level flat m > 100 GeV, τ32 < 0.75 is used to define fiducial volume

Overlap removal: Jets within ∆R < 0.2 of electron; Electrons within∆R < 0.4 of surviving jets; Jets with ∆R < 0.4 of a muon removed if < 3tracks; otherwise muon removed


https://cds.cern.ch/record/2116351

Samples

Physics process Generator Cross-section PDF set for Parton shower Tunenormalisation hard process

tt̄ Signal Powheg-Box v2 NNLO+NNLL CT10 Pythia 6.428 Perugia2012tt̄ PS syst. Powheg-Box v2 NNLO+NNLL CTEQ6L1 Herwig++2.7.1 UE-EE-5tt̄ ME syst. MadGraph5_ NLO CT10 Herwig++2.7.1 UE-EE-5

aMC@NLOtt̄ rad. syst. Powheg-Box v2 NNLO+NNLL CT10 Pythia 6.428 ’radHi/Lo’s top t-channel Powheg-Box v1 NLO CT10f4 Pythia 6.428 Perugia2012s top s-channel Powheg-Box v2 NLO CT10 Pythia 6.428 Perugia2012s top Wt-channel Powheg-Box v2 NLO+NNLL CT10 Pythia 6.428 Perugia2012tt̄+W/Z/WW MadGraph5_ NLO NNPDF2.3LO Pythia 8.186 A14

aMC@NLOW(→ `ν)+ jets Sherpa 2.1.1 NNLO CT10 Sherpa SherpaZ(→ ` ¯̀)+ jets Sherpa 2.1.1 NNLO CT10 Sherpa SherpaWW,WZ,ZZ Sherpa 2.1.1 NLO CT10 Sherpa Sherpa

Table 1: Summary of MC samples, showing the hard process generator, cross-section normalisation precision,PDF choice as well as the parton shower and the corresponding tune used in the analysis. The Pythia andHerwig++ models use the CTEQ6L1 PDFs for their respective parton shower PDFs.

• The factorisation and hadronisation scales are varied down by a factor of 0.5, simultaneously thehdamp parameter is increased to 2mtop. Furthermore the ’radHi’ tune variation from the P2012tune set is used.

• The factorisation and hadronisation scales are varied up by a factor of 2.0 while the hdamp param-eter is unchanged. Furthermore the ’radLo’ tune variation from the P2012 tune set is used.

The tt̄ samples are normalised to σtt̄ = 831.76+46.45−50.85 pb (scale, PDF and αS ), evaluated using the

Top++2.0 program [34]. The calculation includes next-to-next-to-leading-order (NNLO) QCD correc-tions and resums next-to-next-to-leading logarithmic (NNLL) soft gluon terms [35–40].

Events containing W or Z bosons in association with jets are simulated using the Sherpa 2.1.1 [41]generator. Matrix elements are calculated for up to two partons at NLO and four partons at LO usingthe Comix [42] and OpenLoop [43] matrix element generators and merged with the Sherpa partonshower [44] using the ME+PS@NLO prescription [45]. The CT10 PDF set is used in conjunctionwith dedicated parton shower tuning developed by the authors of Sherpa. The W/Z+jets events arenormalised to the NNLO cross-sections [46].

Diboson processes with one of the bosons decaying hadronically and the other leptonically are sim-ulated using the Sherpa 2.1.1 generator [41, 47]. They are calculated for up to one (ZZ) or 0 (WW,WZ) additional partons at NLO and up to three additional partons at LO using the Comix [42] andOpenLoops [43] matrix element generators and merged with the Sherpa parton shower [44] using theME+PS@NLO prescription [45]. The CT10 PDF set is used in conjunction with dedicated partonshower tuning developed by the Sherpa authors. The generator cross-sections are used in this case(already at NLO).

The tt̄ + electroweak processes (tt̄ + W/Z/WW) are simulated using the MadGraph5_aMC@NLOgenerator at LO interfaced to the Pythia 8.186 parton shower model [48]. The matrix elements aresimulated with up to two (tt̄ + W), one (tt̄ + Z) or no (tt̄ + WW) extra partons. The ATLAS underlyingevent tune A14 is used together with the NNPDF2.3LO PDF set. The events are normalised to theirrespective NLO cross-sections [49].

A summary of the MC samples used in this analysis is shown in Tab. 1.

5


Unfolding procedure

Using the Iterative Bayesian method in RooUnfold with 4 iterations

Master formula:

dσfid

dX i≡ 1

L ·∆X i· f ieff ·

∑j

M−1ij · f

jacc ·

(N j

reco − N jbkg

)(1)

f ieff ≡(

Npart

Nreco&part

)i

,f jacc ≡(Nreco&part

Nreco

)j


Uncertainties for Absolute Spectra

[GeV]t,had

Tp

0 100 200 300 400 500 600 700 800 900 1000

Fra

ctio

nal U

ncer

tain

ty [%

]

60−

40−

20−

0

20

40

60

Stat.+Syst. Unc. Stat. Unc.Flavor Tagging JES/JERBackgrounds IFSR, PDF, MC Stat.Hadronisation Hard Scattering


Resolved


Absolute cross-section

[GeV]t,had

Tp

400 600 800 1000 1200 1400

Fra

ctio

nal U

ncer

tain

ty [%

]80−

60−

40−

20−

0

20

40

60

80

Stat.+Syst. Unc. Stat. Unc.Large-R jet , Small-R jet

T

missLepton, EBackgrounds IFSR, PDF, MC Stat.Hadronisation Hard Scattering


Boosted


Absolute cross-section


Top pT (Absolute) /

GeV

[pb]

t,had

T /

d p

ttσd

3−10

2−10

1−10

1

10

210

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

[GeV]t,had

Tp

0 200 400 600 800 1000

Dat

aP

redi

ctio

n

0.8

1

1.2 /

GeV

[pb]

t,had

T /

d p

ttσd

4−10

3−10

2−10

1−10

1Data



t=mdampPWG+PY8 h



Boosted

Dat

aP

redi

ctio

n

1

2

[GeV]t,had

Tp

400 600 800 1000 1200 1400

Dat

aP

redi

ctio

n

0

1

2


tt̄ kinematics (Absolute) /

GeV

[pb]

tt /

d m

ttσd

3−10

2−10

1−10

1

10 Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

1

1.5

[GeV]ttm

500 1000 1500 2000 2500 3000

Dat

aP

redi

ctio

n

1

1.5

/ G

eV [p

b]tt T

/ d

pttσ

d

2−10

1−10

1

10

210

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

[GeV]ttT

p

0 100 200 300 400 500 600 700 800

Dat

aP

redi

ctio

n

0.8

1

1.2

| [pb

]tt

| / U

nit |

ytt

/ d

|yttσ

d

20

40

60

80

100

120

140

160

Data



t=mdampPWG+PY8 h



Resolved

Dat

aP

redi

ctio

n

0.8

1

1.2

|tt|y

0 0.5 1 1.5 2 2.5

Dat

aP

redi

ctio

n

0.8

1

1.2


di erential top cross-section...

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