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Top-quark production in ATLASTo cite this article Danilo Enoque Ferreira de Lima 2013 J Phys Conf Ser 455 012014

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Top-quark production in ATLAS

Danilo Enoque Ferreira de Lima

Kelvin Building University of Glasgow Glasgow G12 8QQ UK

E-mail dferreirmailcernch

Abstract Measurements of the top-quark production cross-sections in proton-protoncollisions with the ATLAS detector at the Large Hadron Collider are presented Themeasurement require no one or two electrons or muons in the final state (hadronic singlelepton or dilepton channel) In addition the decay modes with tau leptons are presented(channels with tau leptons) Differential measurements of tt final states are presented inparticular measurements that are able to constrain the modelling of additional parton radiationMeasurements of single top-quark production in the t- and Wt-channels are presented anddetermination of the CKM matrix element |Vtb| is discussed In addition the s-channelproduction is explored and limits on exotic production in single top-quark processes arediscussed This also includes the search for flavour changing neutral currents and the search foradditional W prime bosons in the s-channel

1 Introduction

The top quark is the heaviest particle in the Standard Model [1] It decays into a W -boson anda b-quark almost exclusively This characteristic has been explored in the measurement of itsproduction cross-section by analysing the final states produced by the W -boson and the b-quark

The Large Hadron Collider (LHC) [2] is a 27 km circumference synchroton which has a setof particle detectors which measure the final state particles in proton-proton collisions TheATLAS [3] experiment is a general purpose detector in the LHC which has many subdetectorssuch as the Inner Detector the Calorimeters and the Muon Spectrometer A 2 T magnetic fieldbends the outgoing charged particlesrsquo trajectory The Inner Detector reconstructs the tracksof charged particles which are used to measure their momentum and charge with an |η| lt 25coverage The Calorimeter measures the energy of the particles with a coverage of |η| lt 49and in the region of overlap with the Inner Detector the electromagnetic calorimeter is finelygranular aiming at precision measurements for electrons and photons The Muon Spectrometeris specifically designed to measure the momenta of muon tracks with a coverage of |η| lt 27The ATLAS detector has many physics goals such as testing the Standard Model and searchingfor new physics It is designed to have 4π solid angle coverage

In ATLAS the top quark may be produced in top-antitop pairs or without its antiparticle inthe ldquosingle-top channelrdquo through dominant strong and electroweak production In the top-pairchannel the final states can be classified into dilepton if both top-quarks generate leptons in thefinal state single-lepton if only one top-quark decay product includes a lepton or hadronic ifboth top-quarks final state particles are quarks

Results for the single-top production are also shown with a measurement of its productionin the t-channel evidence for Wt-channel and a search for the s-channel

International Workshop on Discovery Physics at the LHC (Kruger2012) IOP PublishingJournal of Physics Conference Series 455 (2013) 012014 doi1010881742-65964551012014

Content from this work may be used under the terms of the Creative Commons Attribution 30 licence Any further distributionof this work must maintain attribution to the author(s) and the title of the work journal citation and DOI

Published under licence by IOP Publishing Ltd 1

Events

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3 Jets

4 Jets

3 Jets

4 Jets ge5 Jets

e + Jets micro + JetsR

atio D

ata

Fit

15

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05

Likelihood Discriminant0 20 40 60 80 100

ge5 Jets

L dt = 070 fbndash1intATLAS Preliminary

ttW+Jets

Data 2011 radics = 7 TeV

Other EWQCD Multijet

Figure 1 Likelihood discriminant distributions in data compared to simulation for thesemileptonic inclusive cross-section measurement at

radics = 7 TeV Error bars shown in data

refer to the statistical uncertainties [4]

Measurements of the top-quark production in ATLAS are shown in this document as afunction of a set of observables in different channels

2 Top pair production

In the semileptonic final state of the tt system the inclusive cross-section has been measured bybuilding a likelihood-ratio discriminant Di for the signal and each background [4] The likelihoodfunctions were calculated using the lepton η exp(minus8timesA) (where A is the aplanarity) the pT ofthe leading jet and the HT3p (a ratio of transverse to longitudinal momenta) This was carriedout for the electron and muon channels The likelihood discriminant for data and simulation isgiven in Figure 1 for the

radics = 7 TeV analysis To enhance heavy flavour content at least one

b-jet was required using multivariate techniques for the b-tagging algorithm In this analysisinformation from the 3 4 and ge 5 jet multiplicities was used and a cut on the transverse massof the missing transverse energy and the lepton transverse momentum was applied to reduce theQCD-multijet background

SM

T E

ffic

ien

cy

2

Ma

tch

06

07

08

09

1

For ApprovalATLAS

-1 L dt = 47 pb

| lt 1101 lt | probe

) $ MC simulation (J

Uncertainty candidates) $ Data (J

Uncertainty

[GeV]T

probe muon p4 5 6 7 8 9 10 11 12

Da

taM

C S

ca

le F

acto

r

095

1

105

-Internal

-

-66 fb

P

-1

i

IIIIIIIII

scale

facto

re

IIIIIIIIIIIIIIIIIIIIIIIIIPreliminary

Figure 2 Soft Muon Tagger efficiency in the7 TeV lepton + jets analysis with semileptonicb decays [6]

The tt cross-section was extracted through amaximum-likelihood fit using templates for thediscriminant Di which resulted in a measure-ment of σtt = 1790 plusmn 39(stat) plusmn 90(syst) plusmn66(lumi) pb Systematic uncertainties werefound to be mainly due to the generator vari-ations (3) followed by the jet-energy scale(24)

A similar analysis was carried out using theradics = 8 TeV data [5] but building a likelihood

discriminator using only the lepton η and thetransformed aplanarity Furthermore hardercuts on leptons (pT gt 40 GeV) were used toreduce the fake leptons contributions With thissetup a likelihood fit was done to the likelihooddiscriminant resulting in an inclusive cross-section measurement of σtt = 241 plusmn 2(stat) plusmn

31(syst) plusmn 9(lumi) pb The measurement is in good agreement with the approximate NNLO

calculations from HATHOR σtheorytt

= 238+22minus24 pb for a top-quark mass of mt = 1725 GeV


2

An alternative b-tagging algorithm using leptonically decaying b-jets has been used to performa single-lepton channel analysis [6] It takes advantage of the smaller systematic uncertaintyassociated with this b-tagging algorithm which demands a match between a muon and a b-jetcandidate The muon must satisfy quality cuts including pT gt 4 GeV a match criteria basedon a χ2 between the Muon Spectrometer and the Inner Detector that it has ∆R gt 001 fromthe micro coming from the W -boson decay and that it has ∆R lt 05 to the b-jet candidate decayThe efficiency of this b-jet selection is shown in Figure 2 The final measured cross-section wasextracted by subtracting the background estimate and correcting for the selection efficiencywhich results in a cross-section of σtt = 165plusmn 2(stat)plusmn 17(syst)plusmn 3(lumi) pb at

radics = 7 TeV

jet) [GeV]τm(0 20 40 60 80 100 120 140 160 180 200

OS

- S

S E

vent

s 5

GeV

0

10

20

30

40

50

60

gt 07jBDT

ATLAS

-1 L dt = 205 fbint

Data

hadτ l + rarr tt bkgtt

other EWuncertainty

(b)

(a) Mass of the tagged τ and jet for selected events inthe τ + emicro + jets analysis

trackn

0 2 4 6 8 10 12 14 16 18

Eve

nts

0

20

40

60

80

100

120

140

160

180

200

Data 2011Fit [All]Fit [TauElectron]Fit [Quark-jets]Fit [Gluon-jets]

ATLAS

= 7 TeVs -1

L dt = 167 fbint

(b) Track multiplicity for fitted backgrounds in the τ +jets analysis at 7 TeV

Figure 3 Mass of the tagged τ and jet for the τ + emicro analysis and fitted track multiplicity forthe τ+ jets analysis Statistical uncertainties are shown for data and systematic uncertaintiesare shown on the left for simulation The solid circles indicate data points and the histogramsrepresent the simulation expectation [7] [8]

[ pb ] t t

σ0 50 100 150 200 250 300

Combination - 11+ 141 8plusmn5 plusmn176

w b-taggingmicroe - 12+ 171 8plusmn7 plusmn192

w b-taggingmicromicro - 13+ 171 - 7

+ 81 11plusmn175 ee w b-tagging - 25

+ 281 - 6+ 81 15plusmn184

TLmicro - 40+ 451 - 7

+ 91 24plusmn168 eTL - 33

+ 451 - 7+ 81 23plusmn161

microe - 12+ 151 8plusmn7 plusmn177

micromicro - 11+ 151 - 7

+ 81 12plusmn167 ee - 26

+ 311 - 7+ 91 17plusmn186

-1 Ldt = 070 fbint

Theory (approx NNLO)

= 1725 GeVtm

(lumi)plusmn(syst)plusmn(stat)plusmn

ATLAS

Figure 4 Summary of the tt cross-sectionmeasurements in the dilepton (emicro) channelat 7 TeV for each channel and the combinedmeasurement [9]

A special treatment is given to final statesincluding the τ -lepton for the τ hadronic decayFor the τ + emicro+ jets final state [7] at

radics = 7

TeV a set of Boosted Decision Trees are used toidentify the τ lepton separate it from electrons(BDTe) and separate it from other jets (BDTj)Since some backgrounds are charge symmetricit is possible to cancel them by subtracting a setof Opposite Sign (OS) selected sample from aSame Sign (SS) sample The final cross-sectionis then extracted from the (OS minus SS) yieldsafter a χ2 fit to the BDTj output The mass ofthe tagged τ + jet system is shown for selectedevents in Figure 3(a) The final measured cross-section is σtt = 186plusmn13(stat)20(syst)plusmn7(lumi)pb

A τ + jet analysis [8] was also performedat

radics = 7 TeV from hadronic τ decays using

a one dimensional fit to the number of tracksassociated with the τ candidate taking advantage of the fact that a τ decays preferentiallyinto one or three charged particles An extended binned likelihood fit was applied to fit the


3

observed events to the number of τ + e Figure 3(b) shows the track multiplicity distributionand associated fit for signal and backgrounds The number of τ + e events were then scaledto the fraction of τ events within this selection to extract the number of τ + jets events Atleast two b-tagged particles five jets and a veto on electrons or muons were applied to suppressthe backgrounds The final cross-section was measured by correcting for the efficiency of theselection which results in σtt = 194plusmn 18(stat) plusmn 46(syst) pb

A cross-section measurement has been performed using the dilepton final state [9] by selectingelectrons and muons using the

radics = 7 TeV data It requires two oppositely-charged lepton

candidates two high-pT central jets and applies constraints on the mass of the two leptonsystem to reduce the background contribution A profile likelihood fit was used to estimate thecross-section in different channels and combine the results A missing transverse energy cut wasapplied in the ee and micromicro channels while an HT cut was used in the emicro channel to suppress theZγlowast+ jets contribution1 A summary of cross-section measurements in the dilepton final stateis shown in Figure 4

3 Relative tt differential cross-section measurements

Besides inclusive cross-section measurements which have been shown before differential cross-section measurements [10] were also done at centre-of-mass energy of

radics = 7 TeV In this

context the measurement is normalised by the inclusive cross-section and showed as a functionof X using 1σttdσttdX where X is the mass mtt the transverse momentum pT or the rapidityy of the tt system

The analyses were performed in the semileptonic final state which includes one emicro leptona neutrino and at least four jets Accordingly the selection requires at least four jets and largemissing transverse energy Furthermore a likelihood fit of the measured kinematic variables toa lowest order representation of the tt decay was used to reconstruct the tt system with theW -boson mass and the top-quark mass contraints An extra requirement was applied in thelikelihood to select events which are consistent with the tt decay hypothesis (on the final stateof interest)

[1

Te

V]

ttd

mtt

d tt

1

-310

-210

-110

1

10data

NLO (MCFM)

ALPGEN

MCNLO

ATLAS

-1 L dt = 205 fb

[GeV]tt

m300 1000 2000

Th

eo

ryD

ata

081

1214

(b)(a) Unfolded relative cross-section binned in mtt

[1

Te

V]

tTt

dp

tt

d tt

1

-210

-110

1

10

210data

NLO (MCFM)

ALPGEN

MCNLO

ATLAS

-1 L dt = 205 fb

[GeV]tTt

p7 10 20 100 200 1000

Th

eo

ryD

ata

05

1

15

(c)(b) Unfolded relative tt cross-section binned in pT

Figure 5 Unfolded distributions for the relative tt cross-section comparing data and simulationpredictions The shaded band represents the systematic uncertainties on the simulation [10]1 HT is defined as the scalar sum of all objectsrsquo transverse momentum


4

Eve

nts

1

10

210

310

410

510

610

Data ttW+jets QCD multijet

Single top Z+jets

Diboson

ATLAS Preliminary-1

L dt = 47 fbint = 7 TeVs e + jets

| lt 25η| gt 25 GeVT

p

jetsn3 4 5 6 7 8geD

ata

Exp

ec

05

1

15

(a) DataMC comparison for the tt jet multiplicity inthe e + jets channel

Eve

nts

1

10

210

310

410

510

610

710Data ttW+jets QCD multijet

Single top Z+jets

Diboson

ATLAS Preliminary-1

L dt = 47 fbint = 7 TeVs + jetsmicro


p

jetsn3 4 5 6 7 8geD

ata

Exp

ec

05

1

15

(b) DataMC comparison for tt jet multiplicity in the micro+ jets channel

Figure 6 DataExpectation comparison for tt jet multiplicity in the semileptonic final stateData points include the statistical uncertainties as error bars and the simulation histogramshave the systematic uncertainty as the shaded band [11]

The measurement is performed using an unregularised unfolding procedure subtracting theestimated background Bi correcting for the acceptance Aj and migration between bins (Mminus1)jiand using the luminosity L according to Equation 1 The effect of the systematic uncertaintiesfrom the differential cross-section is reduced by the normalisation to the inclusive cross-sectionbut the final result is still dominated by systematic uncertainties as it can be seen in Figures 5(a)and 5(b)

σj =

sumi(M

minus1)ji(Ni minusBi)

AjL(1)

4 tt jet multiplicity and jet veto gap fraction

Analyses that work as a test of QCD are very important to study final state radiation Onesuch analysis is the measurement of the tt jet multiplicity in the semileptonic final state [11]which is particularly important since this final state is a significant background for ttH bprimebprime andother resonance searches As a first step at least three jets at least one b-tagged jet and oneelectron or muon with pT gt 25 GeV were required for a first data to simulation comparisonas shown in Figures 6(a) and 6(b) for the electron and muon channels respectively Missingtransverse energy greater than 30 GeV and transverse mass between2 greater than 35 GeVwere also required to reduce the backgrounds contribution A veto was applied in the secondlepton with pT gt 20 GeV to reduce the tt dilepton background contribution The electronswere required to be in the region given by |η| lt 247 excluding the 137 lt |η| lt 152 regionwhile muons used had the requirement |η| lt 25 Isolation requirements were also applied tothe leptons

2 In this context the transverse mass is defined using the lepton transverse momentum and the missing transverseenergy


5

Eve

nts

210

310

410

Data

ALPGEN+HERWIGMCNLO+HERWIG

Down)sαALPGEN+PYTHIA (

POWHEG+PYTHIA

ATLAS Preliminary

-1 L dt = 47 fbint

= 7 TeVs

R=04tanti k

| lt 25η| gt 25 GeV

Tp

e+jets

jetsn3 4 5 6 7 8ge

MC

Dat

a

05

1

15

(a) Unfolded tt jet multiplicity in the e + jets channel

Eve

nts

210

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410

Data



POWHEG+PYTHIA

ATLAS Preliminary

-1 L dt = 47 fbint

= 7 TeVs

R=04tanti k


Tp

+jetsmicro

jetsn3 4 5 6 7 8ge

MC

Dat

a05

1

15

(b) Unfolded tt jet multiplicity in the micro + jets channel

Figure 7 Unfolded tt jet multiplicity for the semileptonic tt decay The shaded band on the datapoints represents the systematic uncertainty propagated through the unfolding procedure [11]

The result is unfolded to account for detector effects by subtracting the background estimate(~fbngd) and correcting for the acceptance difference in the reconstruction-level and particle-level

simulations except for the jet multiplicity requirement (~faccpt) Corrections are also appliedfor events that pass the jet multiplicity requirements at reconstruction-level but not at particle-level (~frecopart) and events that satisfy the particle-level jet multiplicity requirement but failthe reconstruction-level requirement The migration between bins (Mpart) is taken into accountusing an iterative unfolding procedure [13] The unfolding procedure can be summarised inEquation 2

~Npart = ~fpartreco middotMpart middot ~frecopart middot ~faccpt middot ( ~Nreco minus ~fbngd) (2)

The unfolded result is shown in Figures 7(a) and 7(b) It can be seen thatMCNLO+HERWIG underestimates the data for bins with ge 6 jets while ALPGEN+PYTHIAwith the downward αS variation and POWHEG+PYTHIA describe the data very well

Another important test of extra radiation in the final state is the measurement of the ldquojet vetogap fractionrdquo defined as in Equation 3 in which σ(Q0) is the cross-section for the productionof tt events with no additional jet with pT gt Q0 The final state analysed in this study is thedilepton decay of the tt system which includes neutrinos two leptons (in this study e or micro)and two b-jets with extra radiation This particular final state is used to have a clean eventselection The results profit from a reduced systematic uncertainty because the ldquojet veto gapfractionrdquo is a ratio

f(Q0) =σ(Q0)

σ(3)

The jet veto gap fraction for jets within |y| lt 08 for different generators is shown inFigure 8(a) and it can be seen that the MCNLO simulation tends to produce fewer jets Also itcan be seen from Figure 8(b) that this measurement constrains systematic uncertainties comingfrom ISRFSR modelling


6

[GeV]0Q50 100 150 200 250 300

The

ory

Dat

a

096

098

1

102

104 50 100 150 200 250 300

Gap

frac

tion

075

08

085

09

095

1

Data + stat

Syst + stat

MCNLO

ERWIG+HOWHEGP

YTHIA+POWHEGP

HERPAS

ERWIG+HLPGENA

veto region |y| lt 08

-1 L dt = 205 fbint

ATLAS

(a) Jet veto gap fraction for jets in |y| lt 08

[GeV]0Q50 100 150 200 250 300

The

ory

Dat

a09

095

1

105 50 100 150 200 250 300

Gap

frac

tion

07

075

08

085

09

095

1

Data + stat

Syst + stat

MC nominalCERA

Increased ISR

Decreased ISR


-1 L dt = 205 fbint

ATLAS

(b) Jet veto gap fraction for jets in |y| lt 08 comparedto AcerMC with ISRFSR systematic variations

Figure 8 Jet veto gap fraction measured in |y| lt 08 rapidity range including the systematicand statistical uncertainties from the measurement in data in the yellow band [12]

5 Single top-quark production

The single top-quark production cross-section in the LHC is smaller than the dominant strongforce mediated production of tt It can be produced through a t-channel Wt-channel ands-channel diagrams but only the t-channel has been observed so far

[TeV]Wrsquom06 08 1 12 14 16 18 2

tb

) [p

b]

B

(Wrsquo

W

rsquo)

(pp

-110

1

10

210 95 CL Expected limit

1plusmnExpected

2plusmnExpected

95 CL Observed limit

TheoryR

Wrsquo

ATLAS

1-tag and 2-tag

-1 L dt = 104 fb$ = 7 TeVs

Figure 9 Limit on the mass of W prime

R rarr tbThe systematic uncertainty variations for theexpected limits for 1σ and 2σ are shown as thegreen and yellow bands [19]

The t-channel production generates a topquark with an extra quark and it has theinteresting feature that it allows one to measurethe production cross-section for top eventsand antitop events Furthermore the ratiobetween these cross-sections Rt is sensitive tothe extra quarkrsquos (u or d) Parton Distributionfunction This measurement was carried out atradics = 7 TeV finding Rt = 181 plusmn 010(stat)

+021minus020(syst) [14] The measurement of the singletop t-channel at

radics = 8 TeV was also done

without the topantitop separation finding afinal cross-section of σt = 951 plusmn 24(stat) plusmn180(syst) pb [15] in agreement with the872+28

minus10+20minus22 pb approximate NNLO theoretical

calculations [16]The analysis in the Wt-channel at

radics = 7

TeV was also performed [17] and the background-only hypothesis was excluded at 33σ witha cross-section measurement obtained through a maximum likelihood estimate of σWt =168plusmn29(stat)plusmn49(syst) pb in agreement with the approximate NNLO theoretical calculationsof 156 plusmn 04 plusmn 11 pb [16] This allows for an estimate of the |Vtb| CKM matrix element of|Vtb| = 103+016

minus019The s-channel search [18] was performed at

radics = 7 TeV and it leads to an observed cross-


7

section upper limit of 265 pb while the predicted Standard Model cross-section is 456plusmn007+018minus017

pb [16] Using this result a tb-resonance search [19] was also done for a model of a right-handedW prime

R with Standard Model-like couplings Limits are set for this exotic particle as shown in theFigure 9 for mW prime

Rgt 113 TeV at 95 Confidence Level

6 Summary

The ATLAS Collaboration has used different techniques to estimate the top-quark productioncross-section in different scenarios The cross-section of the dominant tt production process hasbeen measured for dileptonic and semileptonic final states which also included a cross-checkstudy using an uncorrelated tagger (Soft Muon Tagger)

Extra radiation in the semileptonic final state and the dileptonic final states are studied inthe tt+ jets analysis and the jet veto gap fraction study The jet veto gap fraction analysis wasused to constrain the AcerMC variations Smaller variations were used to compare with the tt+jets analysis The results of the tt+ jets analysis are consistent with the jet veto gap fractionanalysis The former is also important to understand the main background for many searcheswhile the latter constrains the ISRFSR systematic uncertainty in the generators by sim 50

The single top-quark production has also been studied in the t- Wt- and s-channels whichwere used to estimate the |Vtb| CKM matrix element in agreement to the world average of|Vtb| = 0999146+0000021

minus0000046 [20] and set limits for the s-channel cross-section and for a model fora right-handed W prime

R rarr tb The cross-section in the t-channel was measured and evidence for theWt-channel was found

Other recent results from ATLAS complementary to the ones shown include themeasurement of the tt all hadronic cross section [21] and a search for single top-quark FlavourChanging Neutral Currents [22]

References[1] J R Incandela et al Prog Part Nucl Phys 63 (2009) 239-292[2] O S Bruning (Ed) P Collier (Ed) P Lebrun (Ed) S Myers (Ed) R Ostojic (Ed) J Poole (Ed)

and P Proudlock (Ed) CERN-2004-003-V-1[3] The ATLAS Collaboration ATLAS Detector Status and Physics Startup Plans JINST 3 S08003 (2008)[4] The ATLAS Collaboration ATLAS-CONF-2011-121[5] The ATLAS Collaboration ATLAS-CONF-2012-149[6] The ATLAS Collaboration ATLAS-CONF-2012-131[7] The ATLAS Collaboration Phys Lett B 717 (2012) 89-108[8] The ATLAS Collaboration arXiv12117205[9] The ATLAS Collaboration JHEP 1205 (2012) 059

[10] The ATLAS Collaboration arXiv 12075644[11] The ATLAS Collaboration ATLAS-CONF-2012-155[12] The ATLAS Collaboration Eur Phys J C72 (2012) 2043[13] G DrsquoAgostini Nucl Instr and Meth A 362 (1995) 487[14] The ATLAS Collaboration ATLAS-CONF-2012-132[15] The ATLAS Collaboration ATLAS-CONF-2012-056[16] N Kidonakis arXiv 12107813[17] The ATLAS Collaboration Phys Lett B 716 (2012) 142-159[18] The ATLAS Collaboration ATLAS-CONF-2011-118[19] The ATLAS Collaboration Phys Rev Lett 109 (2012) 081801[20] J Beringer et al (PDG) Phys Rev D86 010001 (2012)[21] The ATLAS Collaboration ATLAS-CONF-2012-031[22] The ATLAS Collaboration arXiv 12030529


8

Top-quark production in ATLAS

Danilo Enoque Ferreira de Lima

Kelvin Building University of Glasgow Glasgow G12 8QQ UK

E-mail dferreirmailcernch

Abstract Measurements of the top-quark production cross-sections in proton-protoncollisions with the ATLAS detector at the Large Hadron Collider are presented Themeasurement require no one or two electrons or muons in the final state (hadronic singlelepton or dilepton channel) In addition the decay modes with tau leptons are presented(channels with tau leptons) Differential measurements of tt final states are presented inparticular measurements that are able to constrain the modelling of additional parton radiationMeasurements of single top-quark production in the t- and Wt-channels are presented anddetermination of the CKM matrix element |Vtb| is discussed In addition the s-channelproduction is explored and limits on exotic production in single top-quark processes arediscussed This also includes the search for flavour changing neutral currents and the search foradditional W prime bosons in the s-channel

1 Introduction

The top quark is the heaviest particle in the Standard Model [1] It decays into a W -boson anda b-quark almost exclusively This characteristic has been explored in the measurement of itsproduction cross-section by analysing the final states produced by the W -boson and the b-quark

The Large Hadron Collider (LHC) [2] is a 27 km circumference synchroton which has a setof particle detectors which measure the final state particles in proton-proton collisions TheATLAS [3] experiment is a general purpose detector in the LHC which has many subdetectorssuch as the Inner Detector the Calorimeters and the Muon Spectrometer A 2 T magnetic fieldbends the outgoing charged particlesrsquo trajectory The Inner Detector reconstructs the tracksof charged particles which are used to measure their momentum and charge with an |η| lt 25coverage The Calorimeter measures the energy of the particles with a coverage of |η| lt 49and in the region of overlap with the Inner Detector the electromagnetic calorimeter is finelygranular aiming at precision measurements for electrons and photons The Muon Spectrometeris specifically designed to measure the momenta of muon tracks with a coverage of |η| lt 27The ATLAS detector has many physics goals such as testing the Standard Model and searchingfor new physics It is designed to have 4π solid angle coverage

In ATLAS the top quark may be produced in top-antitop pairs or without its antiparticle inthe ldquosingle-top channelrdquo through dominant strong and electroweak production In the top-pairchannel the final states can be classified into dilepton if both top-quarks generate leptons in thefinal state single-lepton if only one top-quark decay product includes a lepton or hadronic ifboth top-quarks final state particles are quarks

Results for the single-top production are also shown with a measurement of its productionin the t-channel evidence for Wt-channel and a search for the s-channel


Content from this work may be used under the terms of the Creative Commons Attribution 30 licence Any further distributionof this work must maintain attribution to the author(s) and the title of the work journal citation and DOI

Published under licence by IOP Publishing Ltd 1

Events

400

800

1200

1600

2000

2400

3 Jets

4 Jets

3 Jets

4 Jets ge5 Jets


atio D

ata

Fit

15

10

05


ge5 Jets


ttW+Jets











SM

T E

ffic

ien

cy

2

Ma

tch

06

07

08

09

1

For ApprovalATLAS

-1 L dt = 47 pb




Uncertainty

[GeV]T

probe muon p4 5 6 7 8 9 10 11 12

Da

taM

C S

ca

le F

acto

r

095

1

105

-Internal

-

-66 fb

P

-1

i

IIIIIIIII

scale

facto

re










2


radics = 7 TeV

jet) [GeV]τm(0 20 40 60 80 100 120 140 160 180 200

OS

- S

S E

vent

s 5

GeV

0

10

20

30

40

50

60

gt 07jBDT

ATLAS

-1 L dt = 205 fbint

Data


other EWuncertainty

(b)


trackn

0 2 4 6 8 10 12 14 16 18

Eve

nts

0

20

40

60

80

100

120

140

160

180

200


ATLAS

= 7 TeVs -1

L dt = 167 fbint



[ pb ] t t

σ0 50 100 150 200 250 300





+ 281 - 6+ 81 15plusmn184

TLmicro - 40+ 451 - 7

+ 91 24plusmn168 eTL - 33

+ 451 - 7+ 81 23plusmn161



+ 81 12plusmn167 ee - 26

+ 311 - 7+ 91 17plusmn186

-1 Ldt = 070 fbint


= 1725 GeVtm


ATLAS



radics = 7






3










[1

Te

V]

ttd

mtt

d tt

1

-310

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6 Summary











8

Documents

Top-quark production in ATLAS - IOPscience