Top Cross Section Measurement

Top Quarks in Year 1Akira Shibata

New York University ATLAS Workshop of the Americas @ NYU

5 Aug, 2009

[email protected] of the Americas ’09

Can Top change our perspective?• Why is top so heavy (10 water molecules)? Any

special role in EW symmetry breaking?

• Does it play even more fundamental role than Higgs mechanism + Yukawa coupling?

• If there is new physics signal lighter than top, does the top quark decay into them?

• Could non-SM physics first manifest itself in non-standard couplings of the top quark?

• Top quark can be measured at significant precision at the LHC to answer these questions.

• Top quark has been an extremely productive ground for speculation and searches at Tevatron.

2

mailto:[email protected]



New Physics via Top Decay

Dhiman Chakraborty The Top Quark

The fi nal state signature of tt events

• In the SM, each top quark decays into a Wboson and a b quark.

• The final state of a tt system is primarily clas-sified by the decaymodes of the twoW bosons:

9 Oct 2006 13

• Various decay modes make top physics interesting and useful

• The top interfere with a number of new physics signatures.

• Typical search modes:

• Lepton + jets (e/mu)

• Dileptonic• All hadronic

• Tau channels

• E.g. If mW<mH+<mt and tanβ>>1, top can decay into charged higgs, enhancing the τ lepton rate.

3




New Physics via Top Decay


The fi nal state signature of tt events

• In the SM, each top quark decays into a Wboson and a b quark.

• The final state of a tt system is primarily clas-sified by the decaymodes of the twoW bosons:

9 Oct 2006 13

• Various decay modes make top physics interesting and useful

• The top interfere with a number of new physics signatures.

• Typical search modes:

• Lepton + jets (e/mu)

• Dileptonic• All hadronic

• Tau channels

• E.g. If mW<mH+<mt and tanβ>>1, top can decay into charged higgs, enhancing the τ lepton rate.

Name Signature BR xsec at 10 TeV

Fully Hadronic jets 45.7% 191.5 pbLepton + Jets e + jets 17.2% 71.9 pb

µ + jets 17.2% 71.9 pbDilepton eµ + jets 3.18% 13.3 pb

µµ + jets 1.59% 6.67 pbee + jets 1.59% 6.67 pb

Tau + Jets ! + jets 9.49% 39.8 pbLepton + Tau ! + e/µ + jets 3.54% 14.8 pbTau + Tau ! + ! + jets 0.49% 2.06 pb

total all 100% 419 pb

• Diboson (ME: MC@NLO, PS: Herwig + Jimmy)

The MC samples were generated with 5 TeV against 5 TeV beam energy. CTEQ 6 PDF setwas used and the top mass was set to be 172.5 GeV. Alpgen MLM matching threshold was set to20 GeV and !R 0.4 (need to check.) Number of events analyzed for each sample is summarizedin the Appendix. The stacked histograms are normalized to 100 pb!1.

2.1 Definition

Etcone 20 (GeV)-2 0 2 4 6 8 10 12 14

En

trie

s

-210

-110

1

10

Isolated (truth match) Muon

Isolated

Non-Isolated

Background

Isolated (truth match) Muon

Etcone 20 (GeV)-2 0 2 4 6 8 10 12 14

En

trie

s

-210

-110

1

10

Isolated (truth match) Electron

Isolated

Non-Isolated

Background

Isolated (truth match) Electron

Figure 1: Isolation ET (cone 0.2) of muons and electrons in tt events matched to three types oftruth objects as described in the text.

Following the definition in the egamma group, leptons are classified into three types accordingto their true origin3):

1. Isolated: This includes leptons from the decay of W , Z and leptonic ! decay, which are notaccompanied by hadronic objects. Those originate from W , Z typically have high pT ofthe order of tens of GeV. These are the “signal” leptons that we aim to select while keepingthe contributions from the following two types that are referred to as “fake” leptons.

3)This follows from the definition implemented in egammaMCTruthClassifier tool, which was used to classifytrue leptons in this study. Use of this tool for muons is still experimental but Isolated and Non-Isolated componentsare consistent with electrons.

6

3




Search for Charged Higgs

12

[GeV]+HM

80 90 100 110 120 130 140 150 160

)!"

b

# +

b H

#B

r(t

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Expected limit 95% CLObserved limit 95% CL

=15$tan =25$tan =35$tan =45$tan =55$tan =65$tan =75$tan

DØ Run II Preliminary

-1L =1.0 fb

Expected limit 95% CLObserved limit 95% CL




FIG. 10: Observed (blue) and expected (red) limit with one standard deviation band (yellow) on Br(t ! H+b) as a functionof charged Higgs mass for simultaneous fit of Br(t ! H+b) and !tt in the tauonic model.

$tan 1 10

[G

eV

]+

HM

80

100

120

140

160

180

$tan 1 10

[G

eV

]+

HM

80

100

120

140

160

180! " # +H

Expected limit 95% CL

Excluded 95% CL

s c # +

H


Excluded 95% CL


leptophobictauonic

-1L =1.0 fb

FIG. 11: Observed (blue) and expected (red) limit with one standard deviation band (yellow) on charged Higgs mass as afunction of tan ".

Research Council and WestGrid Project (Canada), BMBF (Germany), A.P. Sloan Foundation, Civilian Researchand Development Foundation, Research Corporation, Texas Advanced Research Program, and the Alexander vonHumboldt Foundation.

[1] Comput. Phys. Commun. 156, 283 (2004).

DØ Note 5715-CONF

4




Search for Charged Higgs

12

[GeV]+HM

80 90 100 110 120 130 140 150 160

)!"

b

# +

b H

#B

r(t

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Expected limit 95% CLObserved limit 95% CL



-1L =1.0 fb





FIG. 10: Observed (blue) and expected (red) limit with one standard deviation band (yellow) on Br(t ! H+b) as a functionof charged Higgs mass for simultaneous fit of Br(t ! H+b) and !tt in the tauonic model.

$tan 1 10

[G

eV

]+

HM

80

100

120

140

160

180

$tan 1 10

[G

eV

]+

HM

80

100

120

140

160

180! " # +H


Excluded 95% CL

s c # +

H


Excluded 95% CL


leptophobictauonic

-1L =1.0 fb

FIG. 11: Observed (blue) and expected (red) limit with one standard deviation band (yellow) on charged Higgs mass as afunction of tan ".

Research Council and WestGrid Project (Canada), BMBF (Germany), A.P. Sloan Foundation, Civilian Researchand Development Foundation, Research Corporation, Texas Advanced Research Program, and the Alexander vonHumboldt Foundation.

[1] Comput. Phys. Commun. 156, 283 (2004).

DØ Note 5715-CONF

Good tau/jet calibration and background control is essential for this search. Not a “Day-1 physics”

4




4

[GeV]tt

M0 200 400 600 800 1000 1200

Fra

cti

on

/25

Ge

V

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

DØ Preliminary

(a) tSM t

=450GeVX

Mtt!X

=650GeVX

Mtt!X

=1000GeVX

Mtt!X

[GeV]tt

M0 200 400 600 800 1000 1200

Fra

cti

on

/25

Ge

V

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

DØ Preliminary

(b) tSM t

=450GeVX

Mtt!X

=650GeVX

Mtt!X

=1000GeVX

Mtt!X

FIG. 1: Shape comparison of expected tt invariant mass distribution for Run IIa data set for standard model top pair production(histogram) compared to resonant production from narrow-width resonances of mass MX = 450, 650 GeV, and MX = 1TeV,for (a) 3 jet events and (b) ! 4 jet events.

VI. SYSTEMATIC UNCERTAINTIES

The systematic uncertainties can be classified as those a!ecting only normalization and those a!ecting the shapeof any of the signal or background invariant mass distribution. The systematic uncertainties a!ecting only thenormalization include the theoretical uncertainty on the SM prediction for !tt, the uncertainty on the integratedluminosity (6.1%) [29] and the uncertainty of lepton identification e"ciencies.

The systematic uncertainties a!ecting the shape of the invariant mass distribution as well as the normalization havebeen determined for both signal and background samples. These include uncertainties on the jet energy calibration,the jet reconstruction e"ciency and b-tagging parametrizations for b, c and light quark jets. The central tt cross-section of 7.48 pb, appropriate for mt = 172.4GeV, is taken with an uncertainty of +0.56

!0.72 pb [30] to obtain thesystematic uncertainty on the tt background normalization. This includes the cross section variation due to a topmass uncertainty of ±1.2GeV [31]. The kinematic changes due to top mass uncertainty are evaluated by replacing thedefault SM background simulation with simulation done at top quark masses of 170 and 180GeV (170 and 175 GeV)for RunIIa (RunIIb) and taking half this variation to obtain the 1! errors for each of the two samples, correspondinggetting to a top mass uncertainty of 2.5GeV (1.25GeV) for RunIIa (RunIIb). Also the uncertainties of tuning theparameterization of the b-fragmentation function, the determination of the heavy flavor fraction in W+jets, and theuncertainties of the e"ciencies used in the Matrix Method were propagated to the limit setting.

Tables III and IV give a summary of the relative systematic uncertainties on the total SM background normalizationfor the combined "+jets channels in Run II. The e!ect of the di!erent systematic uncertainties on the shape of the ttinvariant mass distribution can not be inferred from this table.

VII. RESULT

After all selection cuts 1293 events remain in the e+jets channel and 1052 events in the µ+jets channel. The sums ofall standard model and multijet instrumental backgrounds are 1329±36 and 1053±32 events, respectively. The event

3 jets ! 4 jetstt 624 721Single top 47 13Diboson 32 8W+jets 592 129Z+jets 85 26Multijet 84 22Total background 1464 919Data 1411 934

TABLE I: Event yields from data and for the SM expectation.

Resonance Search

• Calculate the invariant mass for tt pairs ⇒

look for a bump!

• Now the dominant background is Standard

Model tt!

• New challenges for large masses (> 1 TeV)

• highly boosted top quarks

• overlapping decay products

• reconstruct “top quark jets”

• Mass limits depend on the theoretical model

• Systematic errors similar to those for tt

cross-section

8

t

W+

W!

t

e!

q

b

qb

X

boosted

boosted

overlapping

overlapping

New Physics into Top

pp! X ! t t

pp! b! b! !W"t W+t

pp! !g !g ! !gt !gt

2

pp! X ! t t

pp! b! b! !W"t W+t

pp! !g !g ! !gt !gt

2

Topcolor Z’ excluded < 800 GeV. Kaluza-Klein gluon excluded < 1TeVIf new physics is leptophobic,

they may couple strongly to top. Otherwise, dimuon is a

clearer signature.

5




4

[GeV]tt

M0 200 400 600 800 1000 1200

Fra

cti

on

/25

Ge

V

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

DØ Preliminary

(a) tSM t

=450GeVX

Mtt!X

=650GeVX

Mtt!X

=1000GeVX

Mtt!X

[GeV]tt

M0 200 400 600 800 1000 1200

Fra

cti

on

/25

Ge

V

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

DØ Preliminary

(b) tSM t

=450GeVX

Mtt!X

=650GeVX

Mtt!X

=1000GeVX

Mtt!X

FIG. 1: Shape comparison of expected tt invariant mass distribution for Run IIa data set for standard model top pair production(histogram) compared to resonant production from narrow-width resonances of mass MX = 450, 650 GeV, and MX = 1TeV,for (a) 3 jet events and (b) ! 4 jet events.

VI. SYSTEMATIC UNCERTAINTIES

The systematic uncertainties can be classified as those a!ecting only normalization and those a!ecting the shapeof any of the signal or background invariant mass distribution. The systematic uncertainties a!ecting only thenormalization include the theoretical uncertainty on the SM prediction for !tt, the uncertainty on the integratedluminosity (6.1%) [29] and the uncertainty of lepton identification e"ciencies.

The systematic uncertainties a!ecting the shape of the invariant mass distribution as well as the normalization havebeen determined for both signal and background samples. These include uncertainties on the jet energy calibration,the jet reconstruction e"ciency and b-tagging parametrizations for b, c and light quark jets. The central tt cross-section of 7.48 pb, appropriate for mt = 172.4GeV, is taken with an uncertainty of +0.56

!0.72 pb [30] to obtain thesystematic uncertainty on the tt background normalization. This includes the cross section variation due to a topmass uncertainty of ±1.2GeV [31]. The kinematic changes due to top mass uncertainty are evaluated by replacing thedefault SM background simulation with simulation done at top quark masses of 170 and 180GeV (170 and 175 GeV)for RunIIa (RunIIb) and taking half this variation to obtain the 1! errors for each of the two samples, correspondinggetting to a top mass uncertainty of 2.5GeV (1.25GeV) for RunIIa (RunIIb). Also the uncertainties of tuning theparameterization of the b-fragmentation function, the determination of the heavy flavor fraction in W+jets, and theuncertainties of the e"ciencies used in the Matrix Method were propagated to the limit setting.

Tables III and IV give a summary of the relative systematic uncertainties on the total SM background normalizationfor the combined "+jets channels in Run II. The e!ect of the di!erent systematic uncertainties on the shape of the ttinvariant mass distribution can not be inferred from this table.

VII. RESULT

After all selection cuts 1293 events remain in the e+jets channel and 1052 events in the µ+jets channel. The sums ofall standard model and multijet instrumental backgrounds are 1329±36 and 1053±32 events, respectively. The event

3 jets ! 4 jetstt 624 721Single top 47 13Diboson 32 8W+jets 592 129Z+jets 85 26Multijet 84 22Total background 1464 919Data 1411 934

TABLE I: Event yields from data and for the SM expectation.

Resonance Search

• Calculate the invariant mass for tt pairs ⇒

look for a bump!

• Now the dominant background is Standard

Model tt!

• New challenges for large masses (> 1 TeV)

• highly boosted top quarks

• overlapping decay products

• reconstruct “top quark jets”

• Mass limits depend on the theoretical model

• Systematic errors similar to those for tt

cross-section

8

t

W+

W!

t

e!

q

b

qb

X

boosted

boosted

overlapping

overlapping

New Physics into Top

pp! X ! t t

pp! b! b! !W"t W+t

pp! !g !g ! !gt !gt

2

pp! X ! t t

pp! b! b! !W"t W+t

pp! !g !g ! !gt !gt

2

Topcolor Z’ excluded < 800 GeV. Kaluza-Klein gluon excluded < 1TeVIf new physics is leptophobic,

they may couple strongly to top. Otherwise, dimuon is a

clearer signature.Good control of SM top and jet resolution & jet substructure. Not a “Day-1 physics”

5




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6

A HUGE amount of work is going

into understanding the potential of

first-year data. We will look at the

most promising tT cross-section

measurements in detail here.




State of the Art at Tevatron

8% precision all combined (summer 2008)

Cross-Section Measurement• Semileptonic channel

• High branching ratio (~36/81)• Event over-constrained• Manageable background

• Dileptonic channel• Low background• Low branching ratio (~9/81)• Event under-constrained

• Fully hadronic channel• Event fully constrained• Huge QCD and comb. background

• Lepton + Track• Highly inclusive• Different systematics for track

performance.

7




[TeV]s0 2 4 6 8 10 12 14

[p

b]

tt!

0

200

400

600

800

1000 CDF Results

Expectations (CSC)-1LHC 14 TeV @ 100pb

Theoretical NLO (pp)

)pTheoretical NLO (p

Theoretical NLO+NNLL

NLO+NNLL Scale Uncertainty

[hep-ph/0204244]

[arXiv:0907.2527]

Pro

duced b

y A

kira S

hib

ata

and U

lric

h H

usem

ann

Graph

1 1.5 2 2.5

[p

b]

tt!

2468

1012

Pro

duced b

y A

kira S

hib

ata

and U

lric

h H

usem

ann

Graph

ee/μμ/eμ

e/μ+jets

8

4

Some Perspective

! One can get a very good idea of production rates

just by looking at relative partonic luminosities

– Plot uses CTEQ6M

! Hardly a precision estimate, but good for “rules of thumb”

0 2 4 6 8 10 12 14 16

CM Energy (TeV)

0

0.2

0.4

0.6

0.8

1

1.2

Xsec r

ela

tive t

o 1

4 T

eV

W, Z

Top

Z' (2 TeV)

RULES OF THUMB

! Running at 10 TeV takes ~twice as

much data as 14 TeV for equivalent

sensitivity

! Running at 8 TeV takes ~twice as

much data as 10 TeV for equivalent

sensitivity

! Below 8 TeV things go “pear

shaped” quickly.

We know a lot about top already, from Tevatron and

theorists, but LHC will show us a LOT more. Priced

steeply in TeV, much more so than the background.NB: Large uncertainty in theory due to PDF not shown.

CSC assumed 5% luminosity




First year detector is less than idealNumerous uncertainties that affect measurements with the early data: • Trigger efficiency• Non-uniform detector• Lepton identification• Missing Et calibration and tails• Light/b jet energy scale• QCD activity (MI, ISR/FSR)• Beam related issues (Pile-up,

Luminosity)• PDF• Background normalization• other unknown unknowns


Top Physics Potential

e!/q

e + /q

t

W +

b

! +

"!

t

?

!

X

Productioncross section

Resonantproduction ?

Productionkinematics

Spinpolarization

Top mass, width, spin, charge

Wtb coupling, |Vtb|

Yukawa coupling ?

Anomalous couplings ?

Rare/non-SM decays ?

Branching fractions ?

9 Oct 2006 8

W mass constraint

Missing Et

Pile-up

Underlying event

Top mass constraint

Kinematic fit

Triggering

Lepton ID

Light jet e scale

Final state rad.

Jet reconstruction

B-tagging

Bjet energy scale

B fragmentation

Initial state rad.

Luminosity

PDF

Real performance need to be estimated from real data. Top is sensitive to a variety of effect but it will also provide means for calibration

based on constraints in masses and decays.9




• We will look into a very recent study on single and di-lepton cross section measurements.

• Much learned about the full approval process: INT approved, PUB approval in progress.

• Used the “MC08” Monte Carlo samples with full Geant 4 simulation (rel 14.2.20.x)

• Collision energy is 10 TeV. Plots and tables are normalized to 200 pb-1.

10

Timeline (Dilepton note)Apr 28. need for a summer note announced May 06. editors appointed May 12. discuss selection May 22. object and event selection agreed May 26. discuss systematics May 29. systematics treatment agreed June 02. note submitted and comments received June 05. top group approvalJuly 13. INT approvalJuly 28. Sent to collaboration review

cf: https://twiki.cern.ch/twiki/pub/Atlas/OperationModelOverviewDocument/physics_policy.pdf



https://twiki.cern.ch/twiki/pub/Atlas/OperationModelOverviewDocument/physics_policy.pdf

https://twiki.cern.ch/twiki/pub/Atlas/OperationModelOverviewDocument/physics_policy.pdf


Strategy with the First Data

Do the simplest thing we can do: “cut and count”

N = L σtT BR ϵtrigϵlep A + Bpray for a large number

but expect large uncertainty.May not be available quickly

well known in SM

to be estimated from data

sensitive to theoretical uncertainty

part data driven, part MC driven

Realistically, e and mu single lepton and dilepton channels only. Taus, too difficult to calibrate. Fully hadronic too

difficult to trigger.

11




Object Selection

12

• Trigger

• EF_e15_medium OR EF_mu15

• Electrons

• egamma isEM ElectronMedium

• pT > 20GeV

• |η| <1.37 or 1.52<|η| <2.47

• etcone20 < 6GeV

• Muons

• STACOmuons: isCombined.

• pT > 20GeV

• |η| < 2.5

• etcone20 < 6GeV

• No overlap with jets within ∆R = 0.3

• Jets

• Cone4TowerJets, with pT> 20GeV, |η| < 2.5.

• Overlap removal: no selected electrons within ∆R = 0.2

• No b-tagging• MEt

• MEt_RefFinal

Not fully optimized (ongoing study in Top Reconstruction Group) but emphasis on simplicity and robustness, relying only on the clearest feature of the events and objects.




Event Selection - Single Lepton

13

• One good lepton• MET > 20 GeV (reduces QCD and Z)• 3 Jets with Pt > 40 GeV (reduces W)• 4 Jets with Pt > 20 GeV (further reduces W)• Reconstruct top from 3 highest pt jet comb• Require there is one dijet comb with |Mw-Mdijet|

<10 GeV (further reduces W and single top)• Signal efficiency ~10%

July 27, 2009 – 22 : 56 DRAFT 14

Table 2: Number of events which pass the various electron selection criteria for the tt signal and for themost relevant backgrounds, at 10 TeV normalised to 200 pb!1 (left columns) and 14 TeV normalised to100 pb!1 (right columns). The statistical errors due to limited Monte Carlo Statistics are also shown.

Electron analysis

10TeV (200 pb!1 ) 14TeV (100 pb!1 )

Sample default +MW cut +mt cut default +MW cut +mt cut

ttbar 2600±15 1286±11 581±7 2555 1262 561W+jets 1305±33 448±20 108±9 761 241 60single top 210±9 81±6 27±3 183 67 23Z" ll +jets 148±4 43±2 11±1 115 35 8hadronic tt 16±3 10±2 2±1 11 4 0.0W bb 21±1 7±1 2±0 44 15 3W cc 19 6 1WW 11±2 6±1 2±1 7 4 0.4WZ 3±1 1±1 0±0 4 1 0.4ZZ 0.4±0.1 0.2±0.1 0.1±0.1 0.5 0.2 0.1

Signal 2600 1286 581 2555 1262 561Background 1715 598 154 1144 374 96S/B 1.5 2.1 3.8 2.2 3.4 5.8

Background evaluation

We considered a number of background processes. The dominant expected background is W+jets, but408

single top production, Z-boson+jets and Wbb are also significant. Tables 2 and 3 summarise the expectednumbers of signal and background events for the electron and muon analysis respectively. The first410

column of the two tables shows the event numbers obtained by applying the default selection, whilstthe second column gives the corresponding numbers with theMW cut. All numbers are normalised to412

200 pb!1 .The QCD production of jets is characterised by a cross-section many orders of magnitude larger than414

the tt signal and could therefore be a potentially important background. Requiring the presence of ahigh pT lepton and missing energy will reduce its contribution, butsince the cross-section enhancement416

relative to the signal is so large, there might be QCD events with a fake lepton and/or poor missingenergy reconstruction that pass these requirements as well. The QCD contamination of the tt signal will418

ultimately be determined from collision data and is not treated here. See [4] and Section 7 for furtherdetails.420

Hadronic top mass with backgrounds included

The distribution of the invariant mass of the three-jet combination that forms the hadronic top-quark422

candidate with the default selection and with the backgrounds added together, is shown in Fig. 5 (L).In Fig. 5 (R) the reconstructed three-jet mass after theMW cut is presented. The background is also424

shown. Both figures are taken from the electron channel analysis. The muon channel plots are verysimilar and not shown.426

Tables 2 and 3 show the number of signal and background events in a 200 pb!1 data samplecomparing the results for the cases of 200 pb!1 of 10 TeV and 100 pb!1 of 14 TeV data. These results428

show that the acceptance efficiency of tt events is similar at the two energies (while the cross-section

July 27, 2009 – 22 : 56 DRAFT 15

Table 3: Number of events which pass the various muon selection criteria for the tt signal and for themost relevant backgrounds, at 10 TeV normalised to 200 pb!1 (left columns) and 14 TeV normalised to100 pb!1 (right columns). The statistical errors due to limited Monte Carlo Statistics are also shown.

Muon analysis

10TeV (200 pb!1 ) 14TeV (100 pb!1 )

Sample default +MW cut +mt cut default +MW cut +mt cut

ttbar 3144±17 1584±12 712±8 3274 1606 755W+jets 1766±44 628±27 148±13 1052 319 98single top 227±9 98±6 33±4 227 99 25Z" ll +jets 144±4 49±2 13±1 84 23 3hadronic tt 11±2 5±1 2±1 35 17 7W bb 32±2 10±1 3±1 64 19 4W cc 26 9 3WW 14±2 7±1 2±1 7 3 0.7WZ 5±1 2±1 0.2±0.2 7 3 0.8ZZ 0.5±0.1 0.2±0.1 0.1±0.0 0.7 0.3 0.1

Signal 3144 1584 712 3274 1606 755Background 2199 799 201 1497 495 143S/B 1.4 2.0 3.5 2.2 3.2 5.3

going from 14 to 10 TeV decreases by a factor of two, the characteristics of the tt system and of the430

decay products at 10 and 14 TeV are very similar). However, the S/B ratio and signal significance isconsiderably lower at 10 TeV, largely due to an increase in the W+jets contribution. To give an indication432

of the signal purity in the top mass peak region, in the third column of Tables 2 and 3 we give the numberof events in a hadronic top mass region: 141 < mt< 189 GeV. Here the choice of window is about434

twice the width of the top mass distribution on both sides. Although not all signal events are correctlyreconstructed, in both the electron and muon analyses the purity of the signal in the top mass window is436

close to 80%.

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Table 1: Expected number of selected events for the ee channel selection for 200pb!1 . The errors areonly shown for the last two columns for readability, but they are fully taken into account in the summationand the S/B calculation. The column labeled ”true” represents the effect of applying truth-matching cuts.The column labeled ”fake” shows the number of events which failed the truth-matching cuts.

lepton sel. inv. mass cut EmissT cut jet cut trigger true fake

tt dilepton 351 322 261 220 214±6 209±6 4tt other 15 13 10 9 8±1 0+0.1!0 8single top 27 25 18 9 9±2 7±1 2Z! ee 68231 16283 24 14 13+2!1 11+2!1 2Z! !! 156 154 10 7 7+2!1 6+2!1 1W ! e" 126 118 56 7 7+4!2 0+4!0 4W ! !" 7 7 7 1 1+4!1 0+4!0 1diboson 145 73 33 3 3±1 2±1 1sum bkg 68707 16673 157 51 49+8!3 54+11!3

"

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"S+B 1.3 2.5 12.5 13.4 13.2 12.9

" Including all fakes.

4 Analysis Strategy

4.1 Cross-Section Determination

The expected integrated luminosity in the first year is 200pb!1 and it will be known with a relativelylarge (# 20%) uncertainty. It is important that in estimating the expected uncertainty on the cross-section,our approach incorporates various systematic uncertainties expected in the first year.The measurement is based on a simple counting experiment, thus we model the observed count N obs

as being Poisson distributed about some expectation N exptot . The tot subscript indicates that there are

several contributions: i. e. the signal and various backgrounds (indexed by k).

Pois(Nobs|Nexptot ) = Pois(Nobs| !

k#{sig,bkg}Nexpk ) (6)

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Figure 8: Missing transverse energy distribution (left) after requiring two opposite signed leptons andjet multiplicity distribution (right) after all cuts (except the N jets > 1 cut) for ee dilepton signal and MCbased background estimations after all cuts. The samples are normalized to 200pb!1.


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Figure 9: Missing transverse energy distribution (left) after requiring two opposite signed leptons and jetmultiplicity distribution (right) after all cuts (except the N jets > 1 cut) for µµ dilepton signal and MCbased background estimations after all cuts. The samples are normalized to 200pb!1.

The scale factors are defined as ! idata/! iMC, where ! idata are identification (ID) and trigger efficienciesmeasured in data from Z events, and ! iMC are efficiencies measured in inclusive Z MC. The acceptance isa convolution of kinematic, geometric and reconstruction efficiency contributions.Dilepton branching ratios include electrons, muons and leptonic tau decays of theW bosons in the

top pair decay. From MC@NLO t t MC we estimate BR(tt ! ee) = (1.67 ± 0.05)%, BR(t t ! µµ) =(1.64 ± 0.05)% and BR(tt ! eµ) = (3.40 ± 0.10)%. We also estimate top dilepton acceptances to beA(ee) = (16.5 ± 0.4)%, A(µµ) = (26.1 ± 0.4)% and A(eµ) = (26.5 ± 0.3)%.Based on MC simulation for signal and background, we present the expected kinematic distributions

for an integrated luminosity of 200 pb!1. Figures 8-10 show the purely MC based signal and backgroundEmissT and jet multiplicity distributions for each sub-channel. The corresponding Tables 1-3 contain thefull list of event types for each MC sample. The uncertainties are due to the limited number of events inthe MC samples. Statistical uncertainties for different luminosities are shown in Table 4. These statisticaluncertainties assume a Poissonian error on the total number of observed events whereas the error on thebackground is taken from the MC estimation. The table shows the combined statistical uncertainties.

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[email protected] of the Americas ’09 14

July 27, 2009 – 22 : 56 DRAFT 20

Table 5: Systematic uncertainties on the Cut and Count and Fit methods for the cross-section measure-ment, in percent, for electrons and muons. The entries in bold indicate the errors which are finally quotedand constitute the total estimates. The final summed totals do not include the luminosity error which isquoted separately.

Cut and Count method Fit method

Source e-analysis µ-analysis e-analysis µ-analysis

default +MW cut default +MW cut +MW cut +MW cut

(%) (%) (%) (%) (%) (%)

Stat. ± 2.5 ± 3.4 ±2.3 ±3.1 ± 14.1 ± 15.2Lepton ID eff. ±1.0 ±1.0 ±1.0 ±1.0 ± 1.0 ± 1.0Lepton trig. eff. ±1.0 ±1.0 ±1.0 ±1.0 ± 1.0 ± 1.050% W+jets ±25.1 ±17.4 ±28.1 ±19.8 ± 3.3 ± 5.620% W+jets ±10.0 ±7.0 ±11.2 ±7.9 ± 1.5 ± 2.6JES (10%,-10%) +24.8-23.4 +15.9-19.1 +20.5-22.3 +11.9-17.9 -14.4 -15.4JES (5%,-5%) +12.3-11.9 +8.6-9.3 +10.4-10.9 +6.1-8.4 -3.7 -3.9

PDFs ±1.6 ± 1.9 ±1.2 ± 1.4 ± 1.9 ± 1.4ISR/FSR +9.1-9.1 +7.6-8.2 +8.2-8.2 +5.2-8.3 -12.9 -12.9

Signal MC ±3.3 ±4.4 ±0.3 ±2.8 ± 4.5 ± 1.4Back. Uncertainty ±0.6 ±0.4 ±0.5 ±0.4 - -Fitting Model - - - - ± 3.3 ± 4.710% Lumi. ±11.6 ±11.2 ±11.4 ±11.1 ±10 ±1020% Lumi. ±23.2 ±22.3 ±22.8 ±22.2 ±20 ± 20Tot. without Lumi. +18.8-18.5 +14.4-15.2 +17.5-17.7 +11.9-14.7 +6.4 -14.9 +6.0 - 14.7

6.7 Cross-section Evaluation with the Baseline analysis538

With the first 200 pb!1 of data, we demonstrate that we can observe a tt signal and determine itsproduction cross-section. In the default scenario with 5% JES uncertainty and 20% uncertainty on the540

W+jets background this simple method yields the following uncertainty on the tt cross-section using thedefault lepton analysis plus the MW cut in the Cut and Count method:542

ElectronCutandcount!"

"= (3.4(stat)+14.4!15.2(syst)±22.3(lumi))% (4)

MuonCutandcount!"

"= (3.1(stat)+11.9!14.7(syst)±22.2(lumi))% (5)

where we added the systematic errors from the table in quadrature.544

Assuming that the lepton triggering system is functioning and calorimetry allows a 5% JES precisionin the central region of the detector, the Cut and Count method will allow a measurement of the tt546

cross-section with a less than 20% systematic error, excluding the luminosity uncertainty. The method ismainly sensitive to the JES, uncertainties in the modellingof ISR/FSR and the signal production process.548

With real data, these modelling errors are expected to be reduced significantly because the Monte Carlocan be tuned. Clearly, the analyses which include theMW cut yield a lower uncertainty, due to the higher550

S/B.The fact that the major background, W+jets, can be estimated from the data itself, is a major advan-552

tage to this analysis since its contribution is not succeptible to the uncertainties in the JES or luminosity.

• Lepton efficiency to be estimated using tag and probe

• Applicable after correction in η, pT and isolation.

• W+Jets is the dominant background

• Studied data-driven estimation based on Z/W ratio. Estimates 20% unc.

• The analysis rather sensitive to JES variation.

• Constant scaling of jet energy applied (MEt varied correspondingly).

• ISR/FSR effects evaluated using Pythia

• Variation of ΛQCD and cutoff leads to large variation.

• By far the dominant effect is the uncertainty on Luminosity.

Bold is used in final combination




Event Selection - Di-lepton

15

• Two good leptons, opposite charge (reduces most fake background)• MET > 35 GeV for ee/mumu, > 20 GeV for emu (reduces QCD and Z)• Z veto (for ee and mumu, |Mz-Mdilep|>10 GeV) (furthers reduce Z)• 2 or more jets with Pt > 20 GeV (reduces diboson)• Signal efficiency ~20%

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Table 6: The individual contributions to the relative uncertainty on the cross-section expected for each of

the channels individually and in combination for 200pb!1. The uncertainties are asymmetric and definea 68% confidence interval.

!"/" (%) ee channel !! channel e! channel combined

Stat only -7.5 / 7.8 -6.0 / 6.2 -4.0 / 4.1 -3.1 / 3.1

Luminosity -17.3 / 26.3 -17.4 / 26.2 -17.4 / 26.2 -17.4 / 26.2

Electron Efficiency -4.5 / 5.0 0.0 / 0.0 -2.2 / 2.4 -1.9 / 1.9

Muon Efficiency 0.0 / 0.0 -4.6 / 5.2 -2.1 / 2.2 -2.2 / 2.3

Lepton Energy Scale -0.3 / 1.6 -2.4 / 2.0 -0.5 / 0.5 -0.8 / 0.8

Jet Energy Scale -3.4 / 3.2 -3.0 / 4.5 -2.5 / 2.5 -2.8 / 3.0

PDF -2.1 / 2.3 -1.4 / 1.6 -1.6 / 1.8 -1.7 / 1.8

ISR FSR -4.0 / 4.2 -3.6 / 3.7 -3.5 / 3.5 -3.6 / 3.7

Signal Generator -4.7 / 5.4 -4.6 / 5.4 -4.7 / 5.3 -4.7 / 5.3

Cross-Sections -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3

Drell Yan -1.4 / 1.3 -2.2 / 2.2 -0.5 / 0.5 -0.8 / 0.9

Fake Rate -9.7 / 9.5 -1.1 / 1.1 -6.2 / 6.2 -4.0 / 4.0

All syst but Luminosity -12.7 / 13.9 -8.9 / 10.2 -9.4 / 10.2 -8.7 / 9.6

All systematics -21.0 / 30.3 -19.3 / 28.3 -19.5 / 28.5 -19.3 / 28.1

Stat + Syst -22.3 / 31.3 -20.2 / 29.0 -19.9 / 28.8 -19.5 / 28.3

higher and the jet energy scale is lower than their nominal values, then the expected number of events

may not be very different than the nominal prediction. The correlated effect on the measurement is

summarized by a correlation matrix in the fitted parameters of the model (see Table 7).

The log-likelihood curves obtained from fitting each channel individually and combined are shown

in Fig. 11. Note the asymmetric nature of the profile likelihood curve introduced by the systematics.

7 Conclusion

In this note, we studied the prospects for tt cross-section measurement using the dilepton final states.

The emphasis was on the analysis of early data and therefore the object and event selection strategy

was kept as simple as possible. The basic strategy was to use a well identified lepton pair and the

remaining background was removed using the dilepton invariant mass, the EmissT and the jet multiplicity.

The overall selection efficiency (S/B ratio) is 16.5%(4.1), 26.1%(3.8) and 26.5%(5.5) for the ee, !!and e! channels respectively.

We studied data-driven methods for the estimation of background. In particular, strategies to deter-

mine Drell-Yan and fake background were developed. Uncertainties related to the methods were also

estimated. The signal and Monte-Carlo based background were defined carefully to avoid any overlap

with the data-driven components.

A range of potential uncertainties were studied in addition to the ones related to the data-driven

methods. In particular, uncertainties on jet energy scale, lepton efficiency and Monte-Carlo model turned

out to be the largest contribution to the systematics after the uncertainty on luminosity, which is by far

the leading constraint on the measurement. On the other hand, with high selection efficiency and the

large expected cross-section, statistical uncertainty will not dominate the final uncertainty once several

tens of pb!1s of data will be accumulated.All uncertainties were combined by constructing a likelihood function for each channel. They were

fit on the nominal prediction from Monte-Carlo samples and the final sensitivity was obtained from a

profile likelihood ratio. The three channels were finally combined by performing a simultaneous fit

incorporating the correlations between uncertainties.

18

• Effect of lepton efficiency is larger in dilepton.

• Z+Jets is the main background in ee and μμ

• Studied data-driven estimation based on low MEt control region.

• Measurement robust against JES variation

• Signal acceptance is a remaining concern

• Compared MC@NLO and Alpgen

• Studied data-driven estimation of fake background

• Matrix method. Systematics estimated by comparing two control regions (low MEt and low ΔΦ)

• Again, luminosity is the dominant unc.




0

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Smaller statistics in dilepton but S/B significantly higher than single lepton. Systematics is dominant very quickly in both channels. Final sensitivity is much higher in dilepton. If 20% uncertainty on luminosity, it will totally dominate the measurement.

Assuming systematicsas constant

pb-1 pb-1




Single vs Di-lepton (visually)

18

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Fig. 476: Mass of 1st/2nd jet plus 1st/2nd lepton (all comb) [GeV] (can be found in../../../eps/default tmp/emu/trigger cum/Dilepton Mlb stack.eps)

cutting harder on /ET significance, for example, the signal can still be clearly seen over the estimated362

background. A comparison of the estimated signal yield to the true signal is shown in Figure 11 (right).363

The results for all the tested SUSY points are summarized in Table 13.364

Table 13: True and estimated background and signal, using the HT2 method, when the backgroundestimation is performed in the presence of SUSY signal. The numbers are for an integrated luminosityof 1 fb!1 except for the SU4 point where 100 pb!1 is used

SUSY /ET sig. True Estimatedpoint cut True bkg Est. bkg True signal Est. signal S/

!B S/

!B

SU1 16 39.2 ± 2.6 100.5 ± 10.4 219.7 ± 8.7 158.4 ± 13.8 35.1 15.820 15.1 ± 1.5 53.1 ± 7.8 167.0 ± 7.6 128.9 ± 11.0 43.0 17.724 6.2 ± 0.92 33.1 ± 6.5 120.8 ± 6.4 93.8 ± 9.2 48.6 16.3

SU2 14 60.6 ± 3.2 69.1 ± 6.4 30.4 ± 2.3 21.9 ± 7.5 3.9 2.616 39.2 ± 2.6 43.1 ± 5.3 24.0 ± 2.1 20.2 ± 6.2 3.8 3.118 23.6 ± 2.0 24.1 ± 3.7 18.3 ± 1.8 17.9 ± 4.6 3.8 3.620 15.1 ± 1.5 13.9 ± 2.7 13.5 ± 1.6 14.7 ± 3.5 3.5 3.9

SU3 16 39.2 ± 2.6 198.1 ± 22.5 328.1± 14.9 169.2 ± 27.2 52.4 12.020 15.1 ± 1.5 119.9 ± 18.5 228.9± 12.5 124.1 ± 22.4 59.0 11.324 6.2 ± 0.92 62.9 ± 13.7 144.7 ± 9.9 88.0 ± 16.9 58.3 11.1

SU4 16 3.92 ± 0.26 120.7± 8.7 76.4 ± 4.0 -40.4 ± 9.6 38.6 -3.720 1.51 ± 0.15 47.4 ± 5.5 37.4 ± 2.8 -8.5 ± 6.1 30.4 -1.224 0.62 ± 0.09 17.8 ± 3.3 18.8 ± 2.0 1.6 ± 3.9 23.9 0.4

SU6 16 39.2 ± 2.6 71.5 ± 7.2 140.5 ± 5.3 108.2 ± 9.3 22.4 12.820 15.1 ± 1.5 36.5 ± 5.0 108.8 ± 4.7 87.4 ± 7.0 28.0 14.524 6.2 ± 0.92 25.1 ± 4.3 79.3 ± 4.0 60.3 ± 6.0 31.9 12.0

1 TeV 16 39.2 ± 2.6 61.1 ± 6.8 155.0 ± 5.7 133.1 ± 9.2 24.7 17.020 15.1 ± 1.5 27.6 ± 4.4 118.1 ± 5.0 105.6 ± 6.8 30.4 20.124 6.2 ± 0.92 15.6 ± 3.5 84.5 ± 4.2 75.1 ± 5.6 34.0 19.0

2.3.5 Top background estimation with top redecay simulation365

Introduction It is possible to isolate a pure biased sample of fully-leptonic t t events by selecting low366

/ET (to reject SUSY signal) opposite sign dilepton events where one and only one pair of invariant mass367

combinationsm(l j) between the two leptons and two hardest jets (b-jets if tagging available) gives values368

below the expected endpoint from t!Wb! l!b decays: m(l j)max =!m2top"m2W (neglecting mb).369

A possible use of such a sample is to estimate the background of fully-leptonic t t events to SUSY370

searches. One can reconstruct the kinematics of the decaying particles (W ’s or top quarks), remove371

their inferred decay products from the reconstructed event (including from the event /ET ), redecay the372

reconstructedW ’s or top quarks using an event generator (e.g. PYTHIA) and then merge the simulated373

re-decay products back into the parent (‘seed’) event. By redecaying particles earlier in the decay chain374

(i.e. the top rather than theW ) the kinematic bias obtained from the event selection can be minimised.375

This technique has a number of advantages over conventional Monte Carlo techniques. In particular376

the event generator is used purely for modelling relatively well-understood decay and hadronisation377

processes – initially poorly understood aspects of process generation, such as parton distributions and the378

underlying event model, are effectively obtained from the data. In principle this technique is applicable379

also to other background processes such as Z ! "" , which could be modelled by replacing identified380

electrons or muons in Z! l+l! control sample events with redecayed taus.381

18

DATA-DRIVEN DETERMINATIONS OF W, Z, AND TOP BACKGROUND

29

7 4 jets 20GV cum 609

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Fig. 575: Hadronic Top Mass [GeV] (can be found in ../../../eps/default tmp/mujets/4 jets 20GV cum/HadronicTop

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10

Visible mass peak is an advantage of single lepton analysis (left). Currently studying variables to visibly confirm top quark in dilepton channel. M(lb) has an end point ~152.6 GeV (Mtop=172.5, Mw=80.4 GeV).




Statistical Formalism

19

!sig =Nobs!!k"{bkg} Nexp

kL " j " j sig

,

L(!sig,L ," j) = !l!{ee,µµ,eµ}

!

!i!bins

"Pois(Nobs

i |Nexpi,tot)Gaus(L |L ,!L ) !

j!systGaus(0|" j,1)

#$

Based on the principle:

Construct the likelihood function as product of PDFs of uncertainties:

Uniform approach for including systematics and combining the results from multiple measurements.

The log-likelihood curves obtained from fitting each channel individually and combined are shown

in Fig. 25. Note the asymmetric nature of the profile likelihood curve introduced by the systematics.

!"!#$#!

%&' %&( ) )&* )&+ )&' )&(

,-./#-012-03..4

%

%&*

%&+

%&'

%&(

)

)&*

)&+

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,-./#-012-03..4

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%&'

%&(

)

)&*

)&+

)&'

)&(

*

!"#!$%&'()*+*,-'.

!"#$%&'"()

(a) Likelihood Curves for the ee channel

!"!#$#!

%&' %&( ) )&* )&+ )&' )&(

,-./#-012-03..4

%

%&*

%&+

%&'

%&(

)

)&*

)&+

)&'

)&(

*

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,-./#-012-03..4

%

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%&+

%&'

%&(

)

)&*

)&+

)&'

)&(

*

!"#!$%&'()*+*,-'.

!"#$%&'"()

(b) Likelihood Curves for the !! channel

!"!#$#!

%&' %&( ) )&* )&+ )&' )&(

,-./#-012-03..4

%

%&*

%&+

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)

)&*

)&+

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,-./#-012-03..4

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%&+

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)

)&*

)&+

)&'

)&(

*

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!"#$%&'"()

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!"!#$#!

%&' %&( ) )&* )&+ )&' )&(

,-./#-012-03..4

%

%&*

%&+

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)

)&*

)&+

)&'

)&(

*

!"#!$%&'()*+*,-'.

!"#$%&'"()

(d) Likelihood Curves for the all channel combined

Figure 25: The solid blue curve is the log of the profile likelihood ratio ! log! ("sig), which includes allsources of systematics. The dotted red curve is the log of the likelihood ratio ! log r("sig), which can beconsidered as including only statistical errors. The horizontal green lines indicate 68%, 90%, and 95%

thresholds (from bottom to top).

44

5.1.3 Extending the Likelihood Function to include Multiple Bins or Channels

One may wish to extend the likelihood function in Eq. 15 to include multiple channels (e. g. ee/!!/e!)or several jet multiplicity bins. Formally, the extension looks very similar for both cases. Let us first

consider the case of multiple bins indexed by i. The expectation for the ith bin from the kth signal or

background contribution is

Nexpik = L !ik"

j

#i jk#i jk($ j)#i jk

= Nexpik "

j

#i jk($ j)#i jk

. (16)

Note, that we do not add the index to $j, because we see this as a common source of systematics which

is common for the different bins and the different signal and background contributions. The likelihood

function is now a product over these bins

L(!sig,L ,$ j) = "i!bins

!

Pois(Nobsi |Nexp

i,tot)"Gaus(L |L ,!L )"j

Gaus($ j = 0|$ j, %$ j= 1)

"

. (17)

The likelihood function for multiple channels is similar, with an additional product over the multiple

channels. The only subtlety is that k now runs over the set of signal and backgrounds specific to that

channel. Similarly, the sources of systematics might also be different for the different channels. Leaving

the range of the indices implicit, we arrive at

L(!sig,L ,$ j) = "l!{ee,!! ,e!}

#

"i!bins

!

Pois(Nobsi |Nexp

i,tot)Gaus(L |L ,!L ) "j!syst

Gaus(0|$ j,1)

"$

. (18)

5.2 Extracting Measurements from the Profile Likelihood Ratio

Armed with the final likelihood function in Eq. 18 and the Asimov dataset, we can now derive the ex-

pected uncertainty on the desired cross section measurement. The likelihood function can be maximized

to determine the maximum likelihood estimate of all the parameters !sig,L , $ j. One can then consider

the likelihood ratio

r(!sig) =L(!sig,L , $ j)L(!sig,L , $ j)

(19)

and the profile likelihood ratio:

& (!sig) =L(!sig, ˆL , ˆ$ j)L(!sig,L , $ j)

(20)

whereˆL and ˆ$ j represent the conditional maximum likelihood estimates ofL and $j holding !sig fixed.

Wilks’ theorem states that under certain conditions, which are satisfied in this case, the distribution

of #2log& (! truesig ) is asymptotically11) distributed as a '2 distribution with one degree of freedom. Wedo not know the value of !truesig , but the interval of points defined by #2log& (!sig) < 1 will cover the

true value 68% of the time. Similarly, the intervals defined by #2log& (!sig) < 2.71(3.84) will coverthe true value 90% (95%) of the time.

Note, the profile likelihood is always greater than the likelihood ratio, except at the maximum likeli-

hood estimate where they are equal. This means that the curve of #2log& is broader than #2log r, andthe difference in the intervals can be attributed to systematics.

11)Here, asymptotically means for sufficiently large sample sizes. In our case, the asymptotic limits are expected to be a verygood approximation.

28

Calculate profile likelihood:

Use Wilks’ theorem to extract confidence interval




Understanding correlation

20

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Stat only Stat only Stat only

Extract three dilepton cross sections in a simultaneous fit. This allows us to understand the correlation between the measurements. As

expected, smaller correlation is seen between ee/μμ than ee/eμ or μμ/emu. Shown here excluding luminosity uncertainty. Correlated

systematics will cancel out when we measure the ratio.




Ratio Measuremens

21

1resu

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results 200 40 0 1 mumu over emu.tableStatonly -7.1 / 7.5Luminosity -0.4 / nanJetEnergyScale -0.7 / 1.9ElectronE!ciency -2.3 / 2.3MuonE!ciency -2.7 / 2.8FakeRate -4.6 / 5.7Cross-Sections -0.2 / nanPDF nan / 0.6ISRFSR nan / 0.5SignalGenerator -0.7 / nanDrellYan -2.2 / 2.1LeptonEnergyScale -2.1 / 1.7AllsystbutLuminosity -6.5 / 7.6Allsystematics -6.5 / 7.7Stat + Syst -9.6 / 10.7

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07

results 200 40 0 1 ee over mumu.tableStatonly -9.4 / 10.2Luminosity -0.3 / nanJetEnergyScale -1.2 / nanElectronE!ciency -4.4 / 5.1MuonE!ciency -4.6 / 5.3FakeRate -8.6 / 8.5Cross-Sections nan / 0.8PDF -0.6 / 0.8ISRFSR -1.0 / nanSignalGenerator -0.3 / nanDrellYan -2.1 / 2.6LeptonEnergyScale -1.9 / 3.1AllsystbutLuminosity -11.0 / 12.2Allsystematics -10.9 / 12.3Stat + Syst -14.4 / 16.0

Simple variable substitution enables ratio measurements in likelihood formalism. Ratio measurements are not sensitive to luminosity at all,

making it an ideal early measurement. Many correlated systematics cancel out such as JES. On the other hand uncertainties that affect channels

differently can affect ratio measurement severely (such as MuonEff and FakeRate). Larger statistical unc. is now the leading effect.

ee/eμ μμ/eμ ee/μμ2 combined 200 40 0 1 AllsystbutLuminosity ee 15

SM! / !

0 0.5 1 1.5 2 2.5 3

-lo

g lik

elih

oo

d

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

SM! / !

0 0.5 1 1.5 2 2.5 3

-lo

g lik

elih

oo

d

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Fig. 4: profile likelihood ratio (can be found in in-clude/combined 200 40 0 1 AllsystbutLuminosity ee over mumu profileLR.eps)

ee/mumu

1resu

lts200

40

06

results 200 40 0 1 ee over emu.tableStatonly -8.4 / 8.9Luminosity -0.3 / nanJetEnergyScale -1.4 / nanElectronE!ciency -2.3 / 2.6MuonE!ciency -2.1 / 2.2FakeRate -4.0 / 2.7Cross-Sections nan / 0.3PDF nan / 1.0ISRFSR -0.6 / 0.6SignalGenerator -0.6 / nanDrellYan -1.3 / nanLeptonEnergyScale nan / 0.8AllsystbutLuminosity -5.3 / 4.8Allsystematics -5.2 / 4.7Stat + Syst -9.9 / 10.1




Result of Multibin Fit

22

1resu

lts200

40

04

results 200 40 0 1.tableStatonly -7.5 / 7.8 -6.0 / 6.2 -4.0 / 4.1 -3.1 / 3.1Luminosity -17.3 / 26.3 -17.4 / 26.1 -17.4 / 26.2 -17.4 / 26.2JetEnergyScale -3.4 / 3.2 -3.0 / 4.5 -2.5 / 2.5 -2.8 / 3.0ElectronE!ciency -4.5 / 5.0 0.0 / 0.0 -2.2 / 2.4 -1.9 / 1.9MuonE!ciency 0.0 / 0.0 -4.6 / 5.2 -2.1 / 2.2 -2.2 / 2.3FakeRate -9.7 / 9.5 -1.1 / 1.1 -6.2 / 6.2 -4.0 / 4.0Cross-Sections -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3PDF -2.1 / 2.3 -1.4 / 1.6 -1.6 / 1.8 -1.7 / 1.8ISRFSR -4.0 / 4.2 -3.6 / 3.7 -3.5 / 3.5 -3.6 / 3.7SignalGenerator -4.7 / 5.4 -4.6 / 5.4 -4.7 / 5.3 -4.7 / 5.3DrellYan -1.4 / 1.3 -2.2 / 2.2 -0.5 / 0.5 -0.8 / 0.9LeptonEnergyScale -0.3 / 1.6 -2.4 / 2.0 -0.5 / 0.5 -0.8 / 0.8AllsystbutLuminosity -12.7 / 13.9 -8.9 / 10.2 -9.4 / 10.2 -8.7 / 9.6Allsystematics -21.0 / 30.3 -19.3 / 28.3 -19.5 / 28.5 -19.3 / 28.1Stat + Syst -22.3 / 31.3 -20.2 / 29.0 -19.9 / 28.8 -19.5 / 28.3

Left, 2 or more jets in one bin. Right, 2/3/4/5/6(inclusive) bins, i.e. same statistics but measurements over multiple bins combined. Large reduction of systematics

that are treated separately in bins of jets such as jet energy scale, ISRFSR (Other systematics are treated as overall scaling.) Gain ~1% in precision!

ee μμ eμ comb ee μμ eμ comb

1resu

lts200

40

04

results 200 40 0 5.tableStatonly -7.4 / 7.7 -6.0 / 6.2 -4.0 / 4.1 -3.1 / 3.1Luminosity -16.9 / 25.8 -17.5 / 25.7 -17.5 / 25.3 -16.9 / 25.2JetEnergyScale -2.0 / 3.3 -2.5 / 3.9 -1.7 / 1.7 -1.6 / 1.8ElectronE!ciency -4.4 / 5.0 0.0 / 0.0 -2.2 / 2.4 -1.8 / 1.9MuonE!ciency 0.0 / 0.0 -4.6 / 5.2 -2.1 / 2.2 -2.2 / 2.3FakeRate -8.8 / 8.8 -1.1 / 1.1 -6.2 / 6.2 -3.8 / 3.9Cross-Sections -0.4 / 0.3 -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3PDF -2.0 / 2.3 -1.4 / 1.6 -1.6 / 1.8 -1.7 / 1.8ISRFSR -1.7 / 1.6 -1.8 / 1.8 -1.5 / 0.8 -0.9 / 0.8SignalGenerator -4.5 / 5.4 -4.6 / 5.4 -4.7 / 5.3 -4.7 / 5.3DrellYan -1.4 / 1.3 -2.2 / 2.2 -0.5 / 0.5 -0.8 / 0.8LeptonEnergyScale -0.5 / 1.3 -2.5 / 2.0 -0.4 / 0.5 -0.8 / 0.8AllsystbutLuminosity -11.3 / 12.7 -8.0 / 9.3 -8.6 / 9.4 -7.4 / 8.5Allsystematics -20.1 / 29.4 -18.9 / 28.1 -19.1 / 28.0 -18.6 / 27.4Stat + Syst -21.4 / 30.4 -19.8 / 28.8 -19.5 / 28.3 -18.9 / 27.6




Using multiple bins in 5-channels

23

Combining measurements from multiple jet bins for all 5 channels using the multiple bin measurement. As the effect of the systematics is rather different for single lepton and dilepton analyses, the combination needs to be done carefully.

1resu

lts200

40

05

results 200 40 0 5.tableStatonly -3.3 / 3.3 -3.0 / 3.0 -7.4 / 7.7 -6.0 / 6.2 -4.0 / 4.1 -1.8 / 1.8Luminosity -18.3 / 26.7 -18.0 / 26.4 -16.9 / 25.8 -17.5 / 25.7 -17.5 / 25.3 -16.5 / 24.1JetEnergyScale -4.9 / 8.0 -4.0 / 7.6 -2.0 / 3.3 -2.5 / 3.9 -1.7 / 1.7 -1.8 / 2.7ElectronE!ciency -2.1 / 2.3 0.0 / 0.0 -4.4 / 5.0 0.0 / 0.0 -2.2 / 2.4 -1.1 / 1.1MuonE!ciency 0.0 / 0.0 -2.1 / 2.2 0.0 / 0.0 -4.6 / 5.2 -2.1 / 2.2 -1.3 / 1.3FakeRate 0.0 / 0.0 0.0 / 0.0 -8.8 / 8.8 -1.1 / 1.1 -6.2 / 6.2 -0.9 / 0.9Cross-Sections -0.3 / 0.3 -0.3 / 0.3 -0.4 / 0.3 -0.3 / 0.3 -0.3 / 0.3 -0.3 / 0.3PDF -1.8 / 2.0 -1.4 / 1.4 -2.0 / 2.3 -1.4 / 1.6 -1.6 / 1.8 -1.6 / 1.7ISRFSR -3.4 / 3.5 -3.9 / 4.8 -1.7 / 1.6 -1.8 / 1.8 -1.5 / 0.8 -1.4 / 1.5SignalGenerator -4.0 / 4.4 -3.9 / 4.1 -4.5 / 5.4 -4.6 / 5.4 -4.7 / 5.3 -4.1 / 4.5DrellYan 0.0 / 0.0 0.0 / 0.0 -1.4 / 1.3 -2.2 / 2.2 -0.5 / 0.5 -0.3 / 0.3LeptonEnergyScale nan / 0.1 -0.6 / 0.5 -0.5 / 1.3 -2.5 / 2.0 -0.4 / 0.5 -0.5 / 0.5Westimate -5.4 / 5.4 -6.2 / 6.2 0.0 / 0.0 0.0 / 0.0 0.0 / 0.0 -1.8 / 1.8AllsystbutLuminosity -10.5 / 13.9 -10.8 / 14.9 -11.3 / 12.7 -8.0 / 9.3 -8.6 / 9.4 -6.4 / 8.0Allsystematics -20.9 / 32.0 -20.7 / 32.2 -20.1 / 29.4 -18.9 / 28.1 -19.1 / 28.0 -18.6 / 27.3Stat + Syst -21.2 / 32.2 -20.9 / 32.3 -21.4 / 30.4 -19.8 / 28.8 -19.5 / 28.3 -18.7 / 27.3

ejets μjets combee μμ eμ




[TeV]s0 2 4 6 8 10 12 14

[pb]

tt!

0

200

400

600

800

1000 CDF Results

Expectations-1LHC 10 TeV @ 200pb

Expectations (no Lumi)-1LHC 10 TeV @ 200pb

Expectations (CSC)-1LHC 14 TeV @ 100pb

Theoretical NLO (pp)

)pTheoretical NLO (p

Theoretical NLO+NNLLNLO+NNLL Scale Uncertainty

[hep-ph/0204244]

[arXiv:0907.2527]

Pro

duced b

y A

kira S

hib

ata

and U

lric

h H

usem

ann

Graph

1 1.5 2 2.5

[p

b]

tt!

2468

1012

Pro

duced b

y A

kira S

hib

ata

and U

lric

h H

usem

ann

Graph

ATLAS 2010?




Summary & Outlook• Simple and robust analysis has been established for the re-discovery of the top

quark with the LHC collisions in year one.

• Learning much about the strength and the weakness of the analyses fully including the systematic uncertainties.

• Studying methods to extract more information from the limited data. Profile likelihood method allow us to measure cross section in different channels and combine them.

• Established good communication and methods to collaborate with many groups. Had a useful experience with publication process.

• Once tT observation is established, implication is tremendous

• Indicates good understanding of the combined performance of the detector

• Powerful tool to understand JES, b-tagging, trigger efficiency and processes with more complicated event topology, new physics!

25




Back up

26




mujets:process all trigger one_muon met 3_jets_40GV 4_jets_20GV mW mtop---------------------------------------------------------------------------------------------------------------------------------------tbart onelepton 30164.70703 10220.02344 7827.36523 7104.29834 3658.92944 2978.71240 1562.58765 735.75818tbart other 13254.96484 3258.23901 1199.52881 1130.10925 265.68686 161.85925 55.48757 14.78055Ztautau 295499.84375 19437.99219 11480.51465 3712.91309 81.10379 48.38579 16.00406 3.29914Zll 588124.25000 211228.51562 90148.96094 49378.42578 172.53560 97.04016 33.73303 8.68211W 9686345.00000 1668069.75000 1447907.75000 1141870.50000 3450.38599 1806.83667 632.86090 162.10670Wbb 3572.16260 788.74146 624.22107 541.64465 60.04836 38.86301 12.19687 3.93595Diboson 7431.87793 2681.06226 1948.86670 1655.71423 39.05885 19.40950 9.52840 2.13509SingleTop 11488.52734 3060.96240 2326.75708 2091.45801 357.05862 221.21556 89.15331 27.45181

ejets:process all trigger one_elec met 3_jets_40GV 4_jets_20GV mW mtop---------------------------------------------------------------------------------------------------------------------------------------tbart onelepton 30164.70703 8187.08203 6535.53467 5883.80615 3019.78711 2448.61890 1266.27783 594.25214tbart other 13254.96484 2640.54346 1055.60046 995.50897 247.87746 157.25546 53.91260 16.11322Ztautau 295499.84375 15484.37500 8822.18066 2845.60278 65.04361 40.93086 13.04008 4.18833Zll 588124.25000 190923.15625 98764.57812 3779.99609 181.01596 107.35683 32.28349 7.56154W 9686345.00000 1509114.50000 1206295.00000 1084657.75000 2523.32275 1306.57996 453.86389 116.41811Wbb 3572.16333 611.40002 475.88712 407.85770 40.74120 27.14357 8.81521 2.67438Diboson 7431.87793 2281.33594 1604.79541 1294.66089 31.50813 14.76664 7.65403 2.21097SingleTop 11488.52734 2515.74512 1931.61731 1723.91162 316.31812 208.08707 76.92590 25.88531

Single lepton channels selection




mumu:process all mumu opposite mass met two_jets triggertbart dilepton 5353.67871 533.31244 531.85870 491.27258 401.49869 344.55710 334.13797tbart other 38078.43750 17.68820 9.08640 7.99603 6.17875 5.93645 5.57299Ztautau 295499.84375 262.88242 262.80087 261.73590 18.71443 10.69080 10.28443Zll 588124.25000 109379.18750 109376.47656 24725.50586 112.87164 49.45387 47.55950W 9686345.00000 20.15652 16.88178 16.45122 2.36805 0.21528 0.21528Wbb 3572.16187 1.90057 1.10806 0.79266 0.55384 0.47453 0.39377Diboson 7431.87793 215.32411 211.21104 100.62133 45.16145 5.34200 5.34200SingleTop 11488.52734 40.19653 36.81574 34.71317 26.80936 11.80857 11.46106

emu:process all emu opposite met two_jets triggertbart dilepton 5353.67871 913.48877 907.67346 845.52222 716.01025 696.38367tbart other 38078.43359 44.22046 24.35155 22.17082 21.32275 20.11124Ztautau 295499.84375 425.54050 421.09479 97.20030 28.25817 26.95681Zll 588124.25000 30.74260 14.51010 2.64526 0.66244 0.66244W 9686345.00000 295.31229 222.72047 187.26189 18.39359 15.87207Wbb 3572.16333 2.62599 1.42752 1.03316 0.39599 0.39599Diboson 7431.87793 143.22598 134.61403 110.98006 10.42713 10.00050SingleTop 11488.52734 65.17408 59.94395 56.24528 26.57366 25.67256

ee:process all ee opposite mass met two_jets triggertbart dilepton 5353.67871 352.43198 349.28201 320.20541 259.38693 218.55858 213.10672tbart other 38078.43750 24.35155 15.14400 13.20557 9.69216 8.84410 8.23834Ztautau 295499.84375 160.86870 156.66675 154.78156 10.04045 7.17354 7.01191Zll 588124.25000 68790.92969 68287.06250 16279.47852 23.59300 13.95325 13.38151W 9686345.00000 266.38525 171.44244 158.57240 80.94560 8.88327 8.35310Wbb 3572.16138 0.87689 0.47965 0.47965 0.00000 0.00000 0.00000Diboson 7431.87891 151.74471 145.05826 73.23516 32.85203 2.97346 2.97346SingleTop 11488.52734 29.65409 27.31582 24.91289 18.81128 9.19955 9.19955

Dilepton channels selection




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Top Cross Section Measurement

Documents

Transcript of Top Cross Section Measurement