Measurements of the ttbb cross-sectionfrom Run 1

based on arXiv:1508.06868

Spyros Argyropoulos

MCnet meetingCERN, 25/9/2015

Measurements of the ttbb cross-sectionfrom Run 1

talk recru

ited by c. R

asmussen

based on arXiv:1508.06868

Spyros Argyropoulos

MCnet meetingCERN, 25/9/2015

The topic of this talk was not is not directly related to my affiliation with MCnet

but some of the results that I will show came out of the discussion session of the MCnet meeting in Karlsruhe!

for that look at my presentation in the previous MCnet meeting

Why we measured ttbb...

Why we measured ttbb...

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b

b

t

t

b

b

H

t

t

because we wanted to measure ttH

which looks a lot like ttbb...b

b

t

t

b

b

H

t

t

and is therefore very hard to measure. So...

Why we measured ttbb...

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every once in a while...

organized with theorists...

Copyright 2004-2014 XanderNatas from DeviantArt

Why we measured ttbb...

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every once in a while...

organized with theorists...

Copyright 2004-2014 XanderNatas from DeviantArt a common ttH meeting

Why we measured ttbb...

8

every once in a while...

organized with theorists...

Copyright 2004-2014 XanderNatas from DeviantArt a common ttH meeting

Reason #1 : it

’s the do

minant irre

ducible ba

ckground

to ttH

Why to measure ttbb (2)

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b

b

t

t

b

b

H

t

t

mt

mb

- large uncertainties - αs4 ⇒ calculations extremely sensitive to choice of scale- multi-scale process ⇒ large logarithms can arise

[Maltoni, Ridolfi, Ubiali arXiv:1203.6393]

Reason #2: test and constrain QCD predictions

Why to measure ttbb (3)

Reason #3 : it is sensitive to g → bb splitting

Constraints with 25-30% precision mostly from LEP/SLC

10

b

b

t

t

b

b

H

t

t

It was a measurement with many challenges...

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i. b-tagging efficiency 70% ⇒ kills 75% of statistics

ii. backgrounds tough to suppress/model, e.g. ttcc/ttjj

iii. lots of jets, low pT jets ⇒ big JES uncertainty

iv. close-by jets from g→bb splitting

v. trying to be model independentand many more

I will not go over the experimental details, but I invite you to look at the paper

What we did

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• 4 fiducial measurements: - tt+b lepton+jets- tt+b di-lepton- tt+bb di-lepton (2 different methods)- tt+bb/tt+jj

• fiducial means: avoiding extrapolations, measuring well-defined objects (Nb-jets, Nleptons)• total uncertainties: 25-35%

{3 resolvedb-jets

{4 resolvedb-jets

ttbb/ttjj ratio

One step beyond...

• fiducial means measuring everything that has the same final state: ttbb, ttZ, ttH, ... and their interferences

• to compare with theory we can subtract the contributions from ttZ and ttH using MC

[fb]ttbb dileptonfid!

5 10 15 20 25 30 35

ttbb dilepton

fit-basedcut-based

[fb]ttb dileptonfid!

20 40 60 80

ttb dilepton

[fb]ttb lepton-plus-jetsfid!

500 1000 1500

ttb lepton-plus-jets

aMC@NLO+Pythia8 (BDDP)

/4)T

aMC@NLO+Pythia8 (H

/2)T

Powhel+Pythia8 (H

MadGraph+Pythia

Pythia8 (wgtq3)

Pythia8 (wgtq5)

Pythia8 (wgtq6, sgtq=0.25)

)tPowheg+Pythia6 (inclusive t

Measurement results

stat. syst. "stat.

ATLAS-1=8 TeV, 20.3 fbs

QCD production of ttbb

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[fb]ttbb dileptonfid!

5 10 15 20 25 30 35

ttbb dilepton

fit-basedcut-based

[fb]ttb dileptonfid!

20 40 60 80

ttb dilepton

[fb]ttb lepton-plus-jetsfid!

500 1000 1500

ttb lepton-plus-jets

aMC@NLO+Pythia8 (BDDP)

/4)T

aMC@NLO+Pythia8 (H

/2)T

Powhel+Pythia8 (H

MadGraph+Pythia

Pythia8 (wgtq3)

Pythia8 (wgtq5)

Pythia8 (wgtq6, sgtq=0.25)

)tPowheg+Pythia6 (inclusive t

Measurement results

stat. syst. "stat.

ATLAS-1=8 TeV, 20.3 fbs

NLOttbb

4F4F5F

merged tt+≤3 jets

LO ttdifferent

splitting kernels

NLO tt

One step beyond...

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Conclusions• data favor predictions with soft scales (μ2=mtop(pT(b)pT(b))1/2)

[Bredenstein, Denner, Dittmaier, Pozzorini, 1001.4006]• σ(5F) > σ(4F), with the difference being smaller than the respective scale uncertainties• extreme Pythia 8 model for g→bb splitting disfavored

[confirms comparison with LEP data: see thesis F. Jiménez]

One last remark

• many improvements expected in Run 2 - cross-section goes up by a factor of 5 with 100/fb ⇒ expect 5 times smaller statistical error- more powerful b-tagging algorithm

• inclusive cross-section will be much more tightly constrained but more importantly...

• we should be able to do differential measurements

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Thank you for your attention!

For the more curious...

Data-theory comparison in numbers

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Uncertainties

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ttZ and ttH data vs theory

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W cross section [fb]tt

0 100 200 300 400 500 600

Z c

ross

sect

ion [fb

]tt

0

100

200

300

400

500

600ATLAS ATLAS Best Fit

ATLAS 68% CL

ATLAS 95% CL

NLO prediction*

Z Theory uncertaintytt

W Theory uncertaintytt

-1 = 8 TeV, 20.3 fbs

* Madgraph5_aMC@NLO calculation

Parameter value0 0.5 1 1.5 2 2.5 3 3.5 4

µ

ttHµ

ZHµ

WHµ

VBFµ

ggFµ

Run 1LHC PreliminaryCMS and ATLAS ATLAS

CMSATLAS+CMS

! 1±! 2±

• removing/doubling ttZ/ttH is considered as an uncertainty on the measurement

ATLAS-CONF-2015-044

arXiv:1509.05276

CMS measurements

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• theory NLO here means HELAC-PHEGAS matched to Pythia 6 • parton-level result with stable tops (no hadronization/MPI/UE)

[Bevilacqua, Worek, 1403.2046]• measurements are higher than theory

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g→bb splitting kernels

dPg→qq̄ ∝ αs(Q2)

2π

1

2

�z2 + (1− z)2

�dz

Parton showers usually provide massless splitting kernels:

In Pythia 8 mass dependence can be included:

dPg→QQ̄ ∝ αs(Q2)

2π

βQ

2

�z2 + (1− z)2+2(1− β2

Q)z(1− z)�dz

Several options in Pythia - TimeShower:weightGluonToQuark• 1 (default): neglect mass ⇒ low splitting probability in threshold region• 2: include mass dependence • 3: “DGLAP form” ⇒ high splitting probability out to large masses (upper bound) • 4: “ME form” ⇒ like 3 but with a phase-space suppression factor (lower bound)• 5-8: like 1-4 but using αs(m2) instead of αs(pT2)

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ttbb production - QCD and EW

Denner, Feger, Scharf [1412.5290]

Interferenceincluded

• pure EW production negligible• Mixed QCD/EW terms significant (∼40%)• Destructive interference ∼4%