Top quark pair production cross section using the ATLAS detector at the LHC

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Top quark pair production cross section using the ATLAS detector at the LHC

Timothée Theveneaux-Pelzer

on behalf of the ATLAS collaboration

Laboratoire de Physique Corpusculaire de Clermont-Ferrand (LPC)

37th International Conference on High Energy Physics Valencia, Thursday, July 3rd 2014

T. Theveneaux-Pelzer LPC - Clermont-Ferand t¯tcross section in ATLAS Thursday, July 3rd 2014 1 / 20

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Introduction

Motivations forσ_tt measurements :

pp→ttis main production mode of the heaviest known particle σ_tt is an input for many analyses

Experimental test of NNLO SM predictions Probe for new physics

Latest available ATLAS results :

√s[TeV] L[fb⁻¹] Reference

Inclusive, l+jets 8 5.8 ATLAS-CONF-2012-149

Inclusive, dilepton 7 4.6

arXiv :1406.5375 New !

8 20.3

Differential, l+jets 7 4.6 arXiv :1407.0371 New !

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Inclusive cross-section, l+jets

Inclusive cross-section in l+jets channel

ATLAS-CONF-2012-149

Event selection and measurement method

UsingL=5.8fb⁻¹of 2012√

s=8 TeV data Event selection :

→ Single isolated lepton trigger

→ 1eorµ,pT>40 GeV and|η|<2.5

→ ≥3 jets,pT>25 GeV and|η|<2.5

→ ≥1 b-tagged jet, MVA tagger with 70% efficiency

→ e+jets :E^miss_T >30 GeV andmT(W)>30 GeV

→ µ+jets :E^miss_T >20 GeV andE^miss_T +m_T(W)>60 GeV Measurement method :

→ Likelihood based on event aplanarity and leptonη

→ Fit of likelihood distribution to extractN_t¯_t

→ σ_tt=R ^N^t^¯^t Ldt×BR×εsig

Events / 0.1

0 2000 4000 6000 8000 10000 12000 14000 16000

3 jets

≥ e+

ATLAS Preliminary -1 Ldt = 5.8 fb

∫

_s_{= 8 TeV}

Data t

t W+Jets Multijet

Z+Jets Single Top Dibosons

Likelihood 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Data / Expectation

1 1.5

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Inclusive cross-section, l+jets

Inclusive cross-section in l+jets channel

ATLAS-CONF-2012-149

Results

Number of fittedttevents and corresponding cross-sections : Channel N_t¯_t(stat) σ_t¯_t[pb] (stat) e+jets 31050±350 239±3 µ+jets 45000±400 242±2 combined 76000±500 241±2 Summary of relative systematic uncertainties onσt¯_t :

Source e+jets µ+jets combined

Jet/MET reconstruction, calibration ⁺₋₆⁶_.^.₃⁷ ⁺₋⁵₄^._.⁴₆ ⁺₋₅⁵_.^.₂⁹ Lepton trigger, identification and reconstruction ⁺₋₂²_.^.₇⁴ ⁺₋⁴₄^._.⁷₂ ⁺₋₂²_.^.₈⁷ Background normalization and composition ⁺₋₂¹_.^.₂⁹ ⁺₋¹₁^._.⁶₅ ⁺₋₁¹_.^.₉⁸ b-tagging efficiency ⁺₋₁¹_.^.₃⁷ ⁺₋¹₁^._.⁹₁ ⁺₋₁¹_.^.₂⁸

MC modelling of the signal ±12 ±11 ±11

Total ±14 ±13 ±13

Result (assumingm_t=172.5 GeV) :

σ_tt=241±2(stat.)±31(syst.)±9(lumi.)pb NNLO+NNLL prediction :

σ_tt =252.9±11.7(PDF)⁶₈^._.⁴₆(QCD scale)pb

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Inclusive cross-section, dilepton

Inclusive cross-section in e µ channel

arXiv :1406.5375

New !

Two measurements, same method

→ UsingL=4.6fb⁻¹of 2011√

s=7 TeV data

→ UsingL=20.3fb⁻¹of 2012√

s=8 TeV data (update of ATLAS-CONF-2013-097)

Event selection :

→ 1eand 1µ, OS,pT>25 GeV and|η|<2.5

→ ≥1 b-tagged jets, MVA tagger with 70% efficiency Measurement method :

→ Count events with 1 or 2 b-tagged jets N_1tag=Lσ_ttε_eµ2ε_b(1−C_bε_b) +N_1tag^bkg N_2tag=Lσ_ttεeµC_bε²_b+N_2tag^bkg

→ Solve equations to obtainσ_ttandεb

→ εb: reconstruction and b-tagging of jets from top decays

→ CorrelationC_bbetween b-tagged jets measured on MC

→ Signal pre-selection efficiencyεeµmeasured on MC

b-tag N

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

Events

0 1000 2000 3000 4000 5000

6000 ATLAS

= 7 TeV, 4.6 fb-1 s

Data 2011 Powheg+PY t t Wt Z+jets Diboson Mis-ID lepton Powheg+PY MC@NLO+HW Alpgen+HW

b-tag N

0 1 2 ≥ 3

MC/Data

0.5 1 1.5

b-tag N

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

Events

0 5000 10000 15000 20000 25000

30000 ATLAS _-1

= 8 TeV, 20.3 fb s

Data 2012 Powheg+PY t t Wt Z+jets Diboson Mis-ID lepton Powheg+PY MC@NLO+HW Alpgen+HW

b-tag N

0 1 2 ≥ 3

MC/Data

0.5 1 1.5

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Inclusive cross-section in e µ channel

arXiv :1406.5375

New !

Systematic uncertainties

Uncertainties onC_b,εeµandN₁^bkg_,_2tagpropagated toσ_tt Summary of relative uncertainties onσ_tt (in %) :

Source √

s=7 TeV √

s=8 TeV ttmodelling and QCD scale ±1.46 ±1.26

PDF ±1.04 ±1.13

Background modelling ±0.83 ±0.83 Lepton efficiencies ±0.87 ±0.88

Jets and b-tag ±0.58 ±0.82

Misidentified leptons ±0.41 ±0.34 Total of above sources ±2.27 ±2.26

Data statistics ±1.69 ±0.71

Luminosity ±1.98 ±3.10

Beam energy ±1.79 ±1.72

Total ±3.89 ±4.27

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Inclusive cross-section in e µ channel

arXiv :1406.5375

New !

Results

Number of observed events and of expected background events :

√s=7 TeV √

s=8 TeV Number of events N_1tag N_2tag N_1tag N_2tag

Data 3527 2073 21666 11739

Total background 400±70 70±16 2590±230 460±130 Result (assumingm_t=172.5 GeV) :

σ_tt(7TeV) =182.9±3.1(stat.)±4.2(syst.)±3.6(lumi.)±3.3(beam en.)pb σ_tt(8TeV) =242.4±1.7(stat.)±5.5(syst.)±7.5(lumi.)±4.2(beam en.)pb Also providing fiducial cross-section measurements

→ Reduction of systematic uncertainties (PDF, QCD scale)

RatioR_tt=σ_tt(8TeV)/σ_tt(7TeV):

measured:1.326±0.024(stat.)±0.015(syst.)±0.049(lumi.)±0.001(beam en.) NNLO+NNLL:1.430±0.013(PDF+α_s)±0.001(QCD scale)

Determination of top-quark mass combining 7 and 8 TeV :m^pole_t =172.9⁺₋²₂^._.⁵₆GeV

(see talk by K. Yau Wong)

95% CL exclusion ofmt<m˜_t<177GeV(assumingBR(˜t1→t˜

χ⁰₁) =100%)

(see talk by J. Firmino da Costa)

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Summary plot for inclusive cross-sections

Summary of ATLAS results @7 TeV for σt¯t

[pb]

t

σ

t

50 100 150 200 250 300 350 ATLAS Preliminary

= 7 TeV s Data 2011,

Effect of LHC beam energy uncertainty: 3.3 pb

Channel & Luminosity

July 2014

scale uncertainty scale+PDF uncertainty NNLO+NNLL (top++ 2.0)

= 172.5 GeV PDF4LHC mtop

stat. uncertainty total uncertainty

(lumi) (syst) ± (stat) ±

t± σt

Single lepton 0.70 fb^-1 179 ± 4 ± 9 ± 7 pb

Dilepton 0.70 fb^-1 - 7 pb

+ 8 - 11

6 + 14

173 ± ν

µ X

→ Single lepton, b 4.66 fb-1

3 pb

± 17

± 2

± 165 + jets

τhad 1.67 fb^-1 194 ± 18 ± 46 pb

+ lepton

τhad 2.05 fb^-1 186 ± 13 ± 20 ± 7 pb

All hadronic 4.7 fb-1

7 pb

±

- 57

12 + 60

168 ± , b-tag

Dilepton, eµ 4.6 fb^-1 182.9 ± 3.1 ± 4.2 ± 3.6 pb

miss

- ET

, Njets

Dilepton, eµ 4.6 fb^-1 181.2 ± 2.8 _{- 9.5}^{+ 9.7} ± 3.3 pb

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Summary of ATLAS results @8 TeV for σt¯t

[pb]

t

σ

t

100 150 200 250 300 350 400 ATLAS Preliminary

= 8 TeV s Data 2012,

Channel & Luminosity

July 2014

scale uncertainty scale+PDF uncertainty NNLO+NNLL (top++ 2.0)

= 172.5 GeV PDF4LHC mtop

stat. uncertainty total uncertainty (lumi) (syst) ± (stat) ±

t± σt

Single lepton 5.8 fb-1

9 pb 31 ± 2 ± 241 ±

, b-tag Dilepton, eµ

20.3 fb-1 242.4 ± 1.7 ± 5.5 ± 7.5 pb

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Summary of ATLAS results vs. √

s for σ

_t

¯

_t

[TeV]

s

7 7.5 8

cross section [pb]t Inclusive t

100 150 200 250 300

ATLAS Preliminary

* Preliminary

dilepton L = 4.6 fb-1

t

t -1

lepton+jets* L = 0.7 fb t

t -1

dilepton L = 20.3 fb t

t -1

lepton+jets* L = 5.8 fb t

t

NNLO+NNLL (pp)

Czakon, Fiedler, Mitov, PRL 110 (2013) 252004

uncertainties according to PDF4LHC αS

= 172.5 GeV, PDF ⊕ mtop

[TeV]

s

7 7.5 8

100 150 200 250 300

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Summary of ATLAS and CMS results @7 TeV for σt¯t

[pb]

t

σt

50 100 150 200 250 300 350

(*) Superseded by results shown below the line

July 2014

scale uncertainty uncertainty αS

⊕ PDF

⊕ scale

= 172.5 GeV

NNLO+NNLL (top++ 2.0), PDF4LHC, mtop stat. uncertainty

total uncertainty (lumi)

± (syst)

± (stat) t± σt

ATLAS, l+jets 179 ± 4 ± 9 ± 7 pb Lint=0.7 fb^-1

ATLAS, dilepton (*) − 7 pb Lint=0.7 fb^-1

8 + 11

− 14 6 + 173 ±

ATLAS, all jets (*) 167 ± 18 ± 78 ± 6 pb Lint=1.0 fb^-1

ATLAS combined − 7± 7 pb Lint=0.7-1.0 fb^-1

8 3 + 177 ±

CMS, l+jets (*) 164 ± 3 ± 12 ± 7 pb Lint=0.8-1.1 fb^-1

CMS, dilepton (*) 170 ± 4 ± 16 ± 8 pb Lint=1.1 fb^-1

(*) µ had+ τ

CMS, 149 ± 24 ± 26 ± 9 pb Lint=1.1 fb^-1

CMS, all jets (*) 136 ± 20 ± 40 ± 8 pb Lint=1.1 fb^-1

CMS combined 166 ± 2 ± 11 ± 8 pb Lint=0.8-1.1 fb^-1 LHC combined (Sep 2012) 173 ± 2 ± 8 ± 6 pb Lint=0.7-1.1 fb^-1

ν Xµ

ATLAS, l+jets, b→ 165 ± 2 ± 17 ± 3 pb Lint=4.7 fb^-1

, b-tag

ATLAS, dilepton eµ 182.9 ± 3.1 ± 4.2 ± 3.6 pbLint=4.6 fb^-1 miss

-ET , Njets µ

ATLAS, dilepton e − 9.5± 3.3 pb Lint=4.6 fb^-1

9.7 2.8 +

± 181.2 +l

τhad

ATLAS, 186 ± 13 ± 20 ± 7 pb Lint=2.1 fb^-1

+jets τhad

ATLAS, 194 ± 18 ± 46 pb Lint=1.7 fb^-1

ATLAS, all jets − 57± 7 pb Lint=4.7 fb^-1

+ 60 12

± 168

CMS, l+jets 158 ± 2 ± 10 ± 4 pb Lint=2.2-2.3 fb^-1

CMS, dilepton 162 ± 2 ± 5 ± 4 pb Lint=2.3 fb^-1

had+l

CMS, τ 143 ± 14 ± 22 ± 3 pb Lint=2.2 fb^-1

+jets τhad

CMS, 152 ± 12 ± 32 ± 3 pb Lint=3.9 fb^-1

CMS, all jets 139 ± 10 ± 26 ± 3 pb Lint=3.5 fb^-1

= 7 TeV TOPLHCWG s

summary, t σt ATLAS+CMS Preliminary

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Summary of ATLAS and CMS results @8 TeV for σt¯t

[pb]

t

σt

150 200 250 300 350

July 2014

scale uncertainty uncertainty αS

⊕ PDF

⊕ scale

NNLO+NNLL (top++ 2.0), PDF4LHC = 172.5 GeV

mtop stat. uncertainty

total uncertainty (lumi)

± (syst)

± (stat) t± σt

Effect of LHC beam energy uncertainty: 4.2 pb +jets

µ ATLAS prel., e/

=5.8 fb-1 ATLAS-CONF-2012-149, Lint

9 pb

±

± 31

± 2 241

, b-tag µ ATLAS, dilepton e

=20.3 fb-1 arXiv:1406.5375, Lint

7.5 pb

± 5.5

± 1.7

± 242.4

+jets µ CMS prel., e/

=2.8 fb-1 CMS-PAS TOP-12-006, Lint

10 pb

26±

− + 29

± 9 228

CMS, dilepton

=5.3 fb-1 JHEP 02 (2014) 024, Lint

6 pb

±

± 11

± 2 239

τh µ+ CMS prel. e/

=19.6 fb-1 CMS-PAS TOP-12-026, Lint

7 pb

±

± 24

± 3 257 = 8 TeV TOPLHCWG s

summary, t σt ATLAS+CMS Preliminary

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Summary of ATLAS and CMS results vs. √

s for σ

_t

¯

_t

[TeV]

s

2 3 4 5 6 7 8 9

1 10 10

2

ATLAS+CMS Preliminary TOPLHCWG

July 2014

* Preliminary

Tevatron combination* L = 8.8 fb-1

ATLAS dilepton L = 4.6 fb-1

CMS dilepton L = 2.3 fb-1

ATLAS lepton+jets* L = 0.7 fb-1

CMS lepton+jets L = 2.3 fb-1

ATLAS dilepton L = 20.3 fb-1

CMS dilepton L = 5.3 fb-1

ATLAS lepton+jets* L = 5.8 fb-1

CMS lepton+jets* L = 2.8 fb-1

NNLO+NNLL (pp) ) p NNLO+NNLL (p

Czakon, Fiedler, Mitov, PRL 110 (2013) 252004

uncertainties according to PDF4LHC αS

⊕ = 172.5 GeV, PDF mtop

7 8

150 200 250

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Differential cross-section, l+jets

Differential cross-section in l+jets channel

arXiv :1407.0371

New !

UsingL=4.6fb⁻¹of 2011√

s=7 TeV data

(update of ATLAS-CONF-2013-09)

σ_tt vs. kinematic variables : test of the SM

→ p^t_T: sensitive to higher order corrections at high values

→ m_tt: sensitive to newttresonances

→ p^tt_T: sensitive to extra radiation

→ y_tt: test of PDF sets Event selection :

→ 1eorµ,pT>25 GeV and|η|<2.5

→ ≥1 b-tagged jet, MVA tagger with 70% efficiency

→ E^miss_T >30 GeV andmT(W)>35 GeV

Events/Unitlog(ℒ)

1000 2000 3000 4000

5000 ^ATLAS M+jets

L dt = 4.6 fb-1

=7 TeV s

Data (l+jets) t t

(dilepton) t t Single top W+jets Multijet Other

log(ℒ) -80 -75 -70 -65 -60 -55 -50 -45

Data/Prediction

0.5 1 1.5

Measurement method :

→ Likelihood fitter to reconstructttkinematics

→ Kinematic variables are then unfolded to derive differential cross-sections

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Differential cross-section in l+jets channel

arXiv :1407.0371

New !

Event reconstruction and unfolding

Likelihood fitter :

→ Associates reconstructed object kinematics tottsystem

→ Using LO representation of thettdecay

→ Maximization of likelihood by testing all permutations

→ log

L

^>−50 to remove poorly fitted solutions Data is unfold to derive parton-level kinematics

→ Regularized SVD method

→ Migration matrices from LO simulatedttsample Computation of 1/σdσ_tt/dX whereX=p_T^t,m_tt,p_T^tt,y_tt

→ eandµchannels combined with BLUE method Main systematic uncertainties :

→ Signal MC generator (2-7%), JES (1-7%), b-tag (1-4%)

Events / GeV

10-3 10-2 10-1 1 10 102 103 104

+jets µ ATLAS

L dt = 4.6 fb-1

∫

=7 TeV

s ^Data

(l+jets) t t

[GeV]

Hadronic top pT 0 100 200 300 400 500 600 700 800

Data/Prediction

0.5 1 1.5

Events / Unit rapidity

2000 4000 6000 8000 10000 12000

+jets µ ATLAS

L dt = 4.6 fb-1

∫

=7 TeV

s ^Data

(l+jets) t t

t yt

-2 -1 0 1 2

Data/Prediction

0.5 1 1.5

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Differential cross-section in l+jets channel

arXiv :1407.0371

New !

Comparison of unfolded data and generators

Softerp_T^t on unfolded data above 200 GeV

p^tt_Tandm_tt well described Alpgen gives best description ofy_tt

0 100 200 300 400 500 600 700 800

-1GeVt Tdpσ

d 1σ

10-4 10-3

Data ALPGEN+HERWIG MC@NLO+HERWIG POWHEG+HERWIG POWHEG+PYTHIA

ATLAS L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

T pt 0 100 200 300 400 500 600 700 800

DataMC

0.5 1

1.5 0 100 200 300 400500 600 700 800 9001000

-1GeV Tttdpσ

d 1σ

10-5 10-4 10-3

10-2 ^Data

ALPGEN+HERWIG MC@NLO+HERWIG POWHEG+HERWIG POWHEG+PYTHIA

ATLAS L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV] T t pt 0 100 200 300 400 500 600 700 800 900 1000

DataMC

0.5 1 1.5

0 500 1000 1500 2000 2500

-1GeV ttdmσdσ1

10-5 10-4 10-3

ATLAS L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

t mt

0 500 1000 1500 2000 2500

DataMC

0.8 1

1.2 ⁰ ^0.5 ¹ ^1.5 ² ^2.5

|ttd|yσdσ1

10-1 1

ATLAS L dt = 4.6 fb-1

∫

= 7 TeV s

t|

|yt

0 0.5 1 1.5 2 2.5

DataMC

0.9 1 1.1

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Differential cross-section in l+jets channel

arXiv :1407.0371

New !

Comparison of unfolded data and NLO QCD calculations

NLO QCD predictions with MCFM, using CT10

→ Scale : 172.5 GeV - uncertainty from 2-1/2 up-down variation

→ PDF uncertainty from error-PDF set

Softer unfolded distributions at high p^t_Tandm_tt

0 100 200 300 400 500 600 700 800

-1GeVt Tdpσ

d 1σ

10-4 10-3

Data NLO QCD ATLAS

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

T pt 0 100 200 300 400 500 600 700 800

DataNLO

0.5 1

1.5 0 100 200 300 400500 600 700 800 9001000

-1GeV Tttdpσ

d 1σ

10-5 10-4 10-3

10-2 Data

NLO QCD ATLAS

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV] T t pt 0 100 200 300 400 500 600 700 800 900 1000

DataNLO

0.5 1 1.5

0 500 1000 1500 2000 2500

-1GeV ttdmσdσ1

10-5 10-4 10-3

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

t mt

0 500 1000 1500 2000 2500

DataNLO

0.5 1

1.5 0 0.5 1 1.5 2 2.5

|ttd|yσdσ1

10-1 1

L dt = 4.6 fb-1

∫

= 7 TeV s

t|

|yt

0 0.5 1 1.5 2 2.5

DataNLO

0.5 1 1.5

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Differential cross-section in l+jets channel

arXiv :1407.0371

New !

Comparison of unfolded data and NLO+NNLL QCD calculations

NLO+NNLL QCD predictions using MSTW2008NNLO

→ Scales set atm_tt Uncertainties :

→ PDF uncertainty from error-PDF set except forp_T^t

→ Scale uncertainty from 2-1/2 up-down variation

→ Forp^t_T: also contribution of dynamic scale uncertainty µ=

q m²_t+p_T^t² Softer distributions at highp^t_T,m_tt

0 100 200 300 400 500 600 700 800

-1GeVt Tdpσ

d 1σ

10-4 10-3

Data NLO+NNLL ATLAS

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

T pt 0 100 200 300 400 500 600 700 800 DataNLO+NNLL 0.5

1

1.5 0 100 200 300 400 500 600 700 800 9001000

-1GeV Tttdpσ

d 1σ

10-5 10-4 10-3

10-2 Data

NLO+NNLL ATLAS

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV] T t pt 0 100 200 300 400 500 600 700 800 900 1000 DataNLO+NNLL 0.5

1 1.5

0 500 1000 1500 2000 2500

-1GeV ttdmσdσ1

10-5 10-4 10-3

Data NLO+NNLL ATLAS

L dt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

t mt

0 500 1000 1500 2000 2500

DataNLO+NNLL 0.5 1 1.5

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Differential cross-section in l+jets channel

arXiv :1407.0371

New !

Comparison of unfolded data and PDF sets

MCFM predictions using 4 PDF sets Tension at highp_T^t for all PDF sets

HERAPDF1.5 gives best description ofm_tt HERAPDF1.5 and NNPDF2.3 give best description ofy_tt

[GeV]

t pT

Theory / Data

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Data CT10NLO MSTW2008NLO NNPDF 2.3 HERAPDF 1.5

0 50 100 150 200 250 350 800 ATLAS

Ldt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

t t pT

Theory / Data

0.5 1 1.5 2 2.5

0 40 170 340 1000

ATLAS Ldt = 4.6 fb-1

∫

= 7 TeV s

[GeV]

t mt

Theory / Data

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

250 450 550 700 960 2700

∫

= 7 TeV s

t|

|yt

Theory / Data

0.8 0.9 1 1.1 1.2 1.3

1.4 Data

CT10NLO MSTW2008NLO NNPDF 2.3 HERAPDF 1.5

0.0 0.5 1.0 2.5

∫

= 7 TeV s

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Conclusions

Measurements in good agreement with SM theory predictions Preliminary result for inclusive cross-section inl+jets at 8 TeV

→ Using 5.8 fb⁻¹of 2012 data

→ Result dominated by systematic uncertainties

New result for inclusive cross-section ineµchannel at 7 and 8 TeV

→ Using the full run-I dataset

→ Most precise measurement so far (4.3% @8 TeV)

→ Also providing fiducial cross-sections, pole-mass, limit onm_˜_t New result for differential cross-section inl+jets at 7 TeV

→ Using the full 2011 dataset

→ Comparison with MC generators, PDF sets, NLO (+NNLL) QCD computation

→ Data softer than predictions at highp^t_Tvalues Not yet the end of run-I measurements !