Hard QCD studies with the ATLAS detectorQCD@LHC Workshop, St Andrews, Scotland

T. Spreitzer

University of Toronto

August 22, 2011

Introduction

Outline

The LHC has delivered an unexpected amount of data, and the ATLAScollaboration has produced an amazing number of results.

Events with jets in final state are copiously produced at hadron machines

LHC is new energy frontier and fertile ground for many tests of QCDtheory

Must be selective in what I present, apologies to those results notincluded here today

Jet cross sectionsAngular decorrelation, Dijets with jet vetoJet substructureb-jet cross sectionsW and Z production - see also talk by E. Devetak

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 2 / 44

Introduction

Unprecedented Luminosity

The LHC has outdone itself with the smooth running of the accelerator.

2010 Data Run45pb−1

Day in 2010

24/03 19/05 14/07 08/09 03/11

]­1

Tota

l In

tegra

ted L

um

inosity [pb

0

10

20

30

40

50

60

Day in 2010

24/03 19/05 14/07 08/09 03/11

]­1

Tota

l In

tegra

ted L

um

inosity [pb

0

10

20

30

40

50

60 = 7 TeVs ATLAS Online Luminosity

LHC Delivered

ATLAS Recorded

­1Total Delivered: 48.1 pb­1

Total Recorded: 45.0 pb

2011 Data Run> 2fb−1 and growing!

Day in 2011

25/02 26/03 24/04 23/05 21/06 20/07 19/08

]­1

Tota

l In

tegra

ted L

um

inosity [fb

0

0.5

1

1.5

2

2.5

3

Day in 2011

25/02 26/03 24/04 23/05 21/06 20/07 19/08

]­1

Tota

l In

tegra

ted L

um

inosity [fb

0

0.5

1

1.5

2

2.5

3 = 7 TeVs ATLAS Online Luminosity

LHC Delivered

ATLAS Recorded

­1Total Delivered: 2.55 fb­1

Total Recorded: 2.43 fb

ATLAS-CONF-2011-116 : Luminosity systematic uncertainty 3.7%

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 3 / 44

Introduction ATLAS Detector

ATLAS Trigger System

The ATLAS Detector isreally big!

Length : ∼ 46 m

Radius : ∼ 12 m

Weight : ∼ 7000 tons

∼ 108 electronicchannels

3000 km of cables

Trigger System

3-level trigger reducing therate from 20 MHz to ≈ 200Hz

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 4 / 44

Introduction ATLAS Detector

ATLAS Inner Detector and Muon System

The inner detector |η| < 2.5 consists ofPixel detectors, semi-conductortracker (SCT), transition radiationtracker

≈ 87 million readout channelsImmersed in 2T solenoidalmagnetic field

Resolution ofσ/pT = 5× 10−4 ⊕ 0.015

Muon Spectrometer (|η| < 2.7)

4T Air-core toroids with gas-basedmuon chambers

MDT, CSC for triggering, RPC,TGC for measurement

Muon measurement with designmomentum resolution ∆pT /pT <10% up to E ∼ 1 TeV

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 5 / 44

Introduction ATLAS Detector

ATLAS Calorimeters

Electromagnetic and hadronic calorimetersSubsystem technology and granularity ↔shower characteristics

Transverse and longitudinal sampling ≈200000 readout cells up to |η| < 4.9

ElectromagneticCalorimeters:

Fine granularity∆η ×∆φ =0.025× 0.025 incentral region

Energy resolution10%/

√E

Hadronic Calorimeters:

Granularity∆η ×∆φ = 0.1× 0.1in central region, lesssegmented in forwardregion

Energy resolution50%/

√E ⊕ 0.03

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 6 / 44

Introduction Jet Reconstruction and Calibration

Jet Reconstruction in ATLAS

|det

ηJet |

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Jet re

sponse a

t E

M s

cale

0.4

0.5

0.6

0.7

0.8

0.9

1

E = 30 GeV

E = 60 GeV

E = 110 GeV

E = 400 GeV

E = 2000 GeV

FCalHEC­FCalTransition

HECBarrel­EndcapTransition

Barrel

= 0.6, EM+JESR t

Anti­k

ATLAS Preliminary

Transitions between separate calorimeters evident.

η-dependent jet calib corrects for response diffs in η

Clustering seeded with|E| > 4σ cells

Iteratively addsneighbours withdifferent noise levels(4,2,0 scheme default)Use resulting 3Dtopological clusters asinput to jet clusteringalgorithm

Topo cluster massesassumed to be zero

Jet clustering uses theAntiKt algorithm withR = 0.6

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 7 / 44

Introduction Jet Reconstruction and Calibration

Jet Energy Calibration

Jet measurements require calibration of the jet energy

Derives a calibration which restore average JES with (η, E)-dependentcalibration constants from MC

Validated using in-situ techniques using γ-jet, track-jets, multi-jet events

[GeV]jet

Tp

30 40 210 210×23

103

10×2

Fra

ctional JE

S s

yste

matic u

ncert

ain

ty

0

0.02

0.04

0.06

0.08

0.1

0.12

ATLAS Preliminary

| < 0.8, Data 2010 + Monte Carlo QCD jetsη | ≤=0.6, EM+JES, 0.3 R t

Anti­k

ALPGEN + Herwig + Jimmy Noise Thresholds

JES calibration non­closure PYTHIA Perugia2010

Single particle (calorimeter) Additional dead material

Total JES uncertainty

[GeV]jet

Tp

2103

10

Data

/ M

C

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

Multi­jetTrack­jet ­jet direct balanceγ

­jet MPFγ

JES uncertainty =0.6, EM+JESR tanti­k

ATLAS Preliminary

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 8 / 44

Results Jet cross sections

Inclusive jet cross section

ATLAS-CONF-2011-047

Using 37 pb−1 pb of data,increasing the kinematic range ofprevious measurements

|yjet rapidity |

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

[G

eV

]T

pje

t

210

310

20

50

210×2

210×5

310×2

Inclusive jet cross section kinematic reach

Kinematic limit

­1 dt = 37 pbL∫Winter 2011,

­1 dt = 17 nbL∫Summer 2010,

= 0.6R jets, t

anti­k

= 7 TeVs

ATLAS Preliminary

Cross section out to |y| < 4.4

pT up to 1.5 TeV

[GeV]T

p

2103

10

[p

b/G

eV

]y

dT

p/d

σ2

d­9

10

­610

­310

1

310

610

910

1210

1510

1810

2110

2410

uncertainties

Systematic

Non­pert. corr.

×NLO pQCD (CTEQ 6.6)

)12 10×| < 0.3 (y|

)9

10×| < 0.8 (y0.3 < |

)6

10×| < 1.2 (y0.8 < |

)3

10×| < 2.1 (y1.2 < |

)0

10×| < 2.8 (y2.1 < |

)­3

10×| < 3.6 (y2.8 < |

)­6

10×| < 4.4 (y3.6 < |

PreliminaryATLAS

=7 TeVs, ­1

dt=37 pbL ∫ jets, R=0.4

tanti­k

[GeV]T

p

2103

10

[p

b/G

eV

]y

dT

p/d

σ2

d­9

10

­610

­310

1

310

610

910

1210

1510

1810

2110

2410

Comparison of data to NLO pQCDpredictions with CTEQ 6.6.

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 9 / 44

Results Jet cross sections

Inclusive jet cross section

| < 0.3y|1.5

1

0.5

[GeV]T

p

2103

10

| < 2.1y1.2 < |1.5

1

0.5

[GeV]T

p

2103

10

| < 1.2y0.8 < |1.5

1

0.5

| < 0.8y0.3 < |1.5

1

0.5

Ra

tio

wrt

NL

O p

QC

D

PreliminaryATLAS

statistical errorData with

uncertainties

Systematic

Non­pert. corr.

×(MSTW 2008)

NLO pQCD

(AMBT1)

Powheg + Pythia

(AUET1)

Powheg + Herwig

=7 TeVs

­1 dt=37 pbL ∫

jets, R=0.4t

anti­k

Prediction w.r.t NLOJet++ MC

| < 2.8y2.1 < |1.5

1

0.5

| < 3.6y2.8 < |1.5

1

0.5

[GeV]T

p

2103

10

| < 4.4y3.6 < |1.5

1

0.5

[GeV]T

p

2103

10

Ra

tio

wrt

NL

O p

QC

D

PreliminaryATLAS

statistical errorData with

uncertainties

Systematic

Non­pert. corr.

×(MSTW 2008)

NLO pQCD

(AMBT1)

Powheg + Pythia

(AUET1)

Powheg + Herwig

=7 TeVs

­1 dt=37 pbL ∫

jets, R=0.4t

anti­k

Powheg predictions are consistent withdata and NLOJet++, with presentuncertaintiesTrend for Powheg to predict differentslope to cross section

AMBT1, AUET1 are different detector tunes

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 10 / 44

Results Jet cross sections

Dijet cross section

R = 0.4

Observing masses up to 4.1 TeV, new energyrange!

Powheg systematically predicts higher cross

sections at low mass, and lower prediction at

high mass, than NLOJet++

[TeV]12

m

-210×7 -110 -110×2 -110×3 1 2

Ra

tio

wrt

NL

O p

QC

D

0.5

1

1.5

2 < 0.3max|y | ATLAS Preliminary

[TeV]12

m

-110×2 -110×3 1 2

Ra

tio

wrt

NL

O p

QC

D

0.5

1

1.5 < 0.8max|y0.3 < |

[TeV]12

m

-110×2 -110×3 1 2

Ra

tio

wrt

NL

O p

QC

D

0.5

1

1.5 < 1.2max|y0.8 < |

[TeV]12

m

-110×3 -110×4 1 2 3

Ra

tio

wrt

NL

O p

QC

D

0.5

1

1.5

2

2.5 < 2.1

max|y1.2 < |

[TeV]12

m

-110×6 1 2 3 4

Ra

tio

wrt

NL

O p

QC

D

1

2

< 2.8max|y2.1 < |

= 0.6R jets,t

anti-k

-1 dt = 37 pbL∫ = 7 TeVs

statistical errorData with

uncertainties

Systematic

Non-pert. corr.

×(MSTW 2008)

NLO pQCD

Pythia (AMBT1)

Powheg +

Herwig (AUET1)

Powheg +

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 11 / 44

Results Jet cross sections

Multijet cross section

CERN-PH-EP-2011-098,hep-ex/arXiv:1107.2092,Submitted to EurPhysJ C

Fundamental and direct test of QCD

2 3 4 5 6

[p

b]

σ

10

210

310

410

510

610

2 3 4 5 6

[p

b]

σ

10

210

310

410

510

610 ATLAS

­1 L dt=2.4 pb∫R=0.4,

=7 TeV)+syst.sData (

1.11×ALPGEN+HERWIG AUET1

0.65×PYTHIA AMBT1

1.22×ALPGEN+PYTHIA MC09’

1.06×SHERPA

Inclusive Jet Multiplicity2 3 4 5 6

MC

/Da

ta

0.5

1

1.5

Inclusive Jet Multiplicity2 3 4 5 6

MC

/Da

ta

0.5

1

1.5

Find ALPGEN better describes data

100 200 300 400 500 600 700 800

2≥ ]

Tlea

d/d

pσ

/[d

3

≥ ]Tle

ad

/d p

σ[d

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

ATLAS

­1 L dt=2.4 pb∫R=0.6,

=7 TeV)+syst.sData (

NLO+non.pert.+syst

(leading jet) [GeV]T

p100 200 300 400 500 600 700 800T

he

ory

/Da

ta

0.5

1

1.5

100 200 300 400 500 600 700 8000.5

1

1.5

200 400 600 800 1000 1200

2≥ ]

T(2)

/d H

σ/[

d

3≥ ]

T(2)

/d H

σ[d

0.1

0.2

0.3

0.4

0.5ATLAS

­1 L dt=2.4 pb∫R=0.6,

=7 TeV)+syst.sData (

NLO+non.pert.+syst

[GeV]T

(2)H

200 400 600 800 1000 1200Th

eo

ry/D

ata

0.5

1

200 400 600 800 1000 12000.5

1

H(2)T is the XXXX

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 12 / 44

Results Jet cross sections

ATLAS High Mass Dijet Event

2011 data event with dijet mass of 4040 GeV

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 13 / 44

Results Jet Angular Distributions

Dijet Azimuthal DecorrelationCERN-PH-EP-2011, Submitted

to PRL

Testing pQCD for multijet productionwithout measuring additional jets

[radians]φ∆/2π /3π2 /6π5 π

]­1

[ra

dia

ns

φ∆

/dσ

dσ

1/

­310

­210

­110

1

10

210

310

410

510

610

710

810

910

­1=36 pbdtL∫Data

)8

10× 800 GeV (>max

Tp

)710× 800 GeV (≤max

Tp<600

)6

10× 600 GeV (≤max

Tp<500

)5

10× 500 GeV (≤max

Tp<400

)410× 400 GeV (≤max

Tp<310

)3

10× 310 GeV (≤max

Tp<260

)210× 260 GeV (≤max

Tp<210

)110× 210 GeV (≤max

Tp<160

)0

10× 160 GeV (≤max

Tp<110

)4sαO(NLO pQCD

unc.sαPDF &

scale unc.

ATLAS =7 TeVs|<0.8y>100 GeV |

Tp jets R=0.6 tanti­k

160 GeV≤max

Tp<110

ATLAS =7 TeVs­1=36 pbdtL∫

310 GeV≤max

Tp<260

/2π /3π2 /6π5 π

600 GeV≤max

Tp<500

210 GeV≤max

Tp<160

PYTHIAHERWIGSHERPA stat. unc.

400 GeV≤max

Tp<310

/3π2 /6π5 π

800 GeV≤max

Tp<600

260 GeV≤max

Tp<210

jets R=0.6tanti­k|<0.8y>100 GeV |

Tp

500 GeV≤max

Tp<400

[radians]φ∆/3π2 /6π5 π

800 GeV>max

Tp

ratio

to S

HE

RP

A φ

∆/d

σ d

σ1

/

2.0

1.0

0.5

2.0

1.0

0.5

2.0

1.0

0.5

Ratio of data to PYTHIA, HERWIG,SHERPA

All describe data, SHERPA does best,

expected result as SHERPA explicitly

includes higher-order tree level diagrams

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 14 / 44

Results Dijet with Veto

Dijet with Jet Veto

CERN-PH-EP-2011-100,Submission to JHEP

Rapidity gap measurement, no colorflow between jetsfgap = NpassGapV eto/Nall

POWHEG + PYTHIA Generallyagree with dataHEJ Agreement better in some placesthan othersQuick theory reaction R.M. DuranDelgado at al, arXiv:1107.2084:”The message is clear: the accuracy ofthe ATLAS data already demandsbetter theoretical calculations”

< 270 GeV (+3)T

p ≤240

< 240 GeV (+2.5)T

p ≤210

< 210 GeV (+2)T

p ≤180

< 180 GeV (+1.5)T

p ≤150

< 150 GeV (+1)T

p ≤120

< 120 GeV (+0.5)T

p ≤90

< 90 GeV (+0)T

p ≤70

Data 2010

HEJ (parton level)

POWHEG + PYTHIA

POWHEG + HERWIG

dijet selectionT

Leading p

= 20 GeV0

Q

ATLAS

y∆

0 1 2 3 4 5 6

Gap f

ractio

n

0

1

2

3

4

5

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 15 / 44

Results Jet Substructure

Jet Substructure, Shapes

ATL-PHYS-PUB-2011-010,Phys. Rev. D 83, 052003 (2011)

0 0.1 0.2 0.3 0.4 0.5 0.6

(r)

ρ

­110

1

10

ATLAS jets R = 0.6tanti­k

< 40 GeVT

30 GeV < p

| y | < 2.8 (a)

­1 ­ 3 pb­1 dt = 0.7 nbL∫Data

PYTHIA­Perugia2010

HERWIG++

ALPGEN

PYTHIA­MC09

r0 0.1 0.2 0.3 0.4 0.5 0.6

DA

TA

/ M

C

0.8

1

1.2

Differential jet shape

ρ(r) =1

∆r

1

Njet

ΣjetspT (r − ∆r/2, r + ∆r/2)

pT (0, R)

Differential jet shape

demonstrates clear jet-like

structure. Data/MC shows

different levels of agreement,

but no clear outliers

Integral jet shape

Ψ(r) =1

Njet

ΣjetpT (0, r)

pT (0, R)

(GeV)T

p

0 100 200 300 400 500 600

(r =

0.3

)Ψ

1 ­

0

0.05

0.1

0.15

0.2

0.25

0.3

jets R = 0.6t

anti­k

| y | < 2.8

(a)

ATLAS

­1 ­ 3 pb­1 dt = 0.7 nbL∫Data

PYTHIA­Perugia2010

HERWIG++

ALPGEN

PYTHIA­MC09

Integral jet shape shows an

underestimate of the amount of

soft, wide-angle contributions to

the jet by PYTHIA, ALPGEN.

Herwig overestimates.

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 16 / 44

Results Jet Substructure

Jet Substructure, “Fat” Jets

ATLAS-CONF-2011-073Jets are both complete 4-vectors and complex composite objects.At LHC, decays of t, W , H, etc decays can be collimated into a jetKnowledge of the internal jet substructure is important in distinguishingthese decays from gluon or light-quark initiated jetsJet mass encodes information about both the parton shower and thepotential presence of heavy particle decays within the jet.

50 100 150 200 250 300

dm

[G

eV

]σ

d

σ1

0

0.002

0.004

0.006

0.008

0.01

0.012 ­1ATLAS 2010 Data, L = 35pb

Pythia

HerwigJimmy

Herwig++

Cambridge­Aachen R=1.2 jets

> 300 GeV, |y| < 2T

= 1, pPVN

PreliminaryATLAS

Jet Mass [GeV]50 100 150 200 250 300

MC

/ D

ata

0.20.40.60.8

11.21.41.61.8

Jet Mass [GeV]50 100 150 200 250 300

MC

/ D

ata

0.20.40.60.8

11.21.41.61.8

[GeV]jm

50 100 150 200 250 300

d M

[G

eV

]σ

d

σ1

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018ATLAS Preliminary ­1ATLAS 2010 data: 35 pb

Pythia MC10

Herwig/Jimmy

Herwig++

jets with R=1.0T

Anti k

> 300 GeV, | y | < 2T

= 1, pPVN

jet mass [GeV]50 100 150 200 250 300

MC

/Da

ta

0.5

1

1.5

Jet mass is unfolded to the particle level to correct for detector effects.T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 17 / 44

Results Jet Substructure

Candidate Boosted Top Decay

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 18 / 44

Results Jet Substructure

Jet Substructure, “Fat” Jets

ATLAS-CONF-2011-073√d1,2 = min(p1

T , p2T )δR1,2,

δR1,2 =

√dφ1,2

2 + dy1,22

Splitting scale is threshold atwhich jets can be broken intosub-components, structure startsto form

Expected to be different forhadronic jets and boosted signal(V , t, etc.)

Heavy particle decays expectedto be reasonably symmetricQCD splittings generallyasymmetric

[GeV]12

d

0 20 40 60 80 100 120 140 160 180 200

[G

eV

]1

2d

d

σ d

σ1

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04 ATLAS Preliminary ­1ATLAS 2010 data: 35 pb

Pythia MC10

Herwig/Jimmy

Herwig++

jets with R=1.0T

Anti k

> 300 GeV, | y | < 2T

= 1, pPVN

[GeV]12

d0 20 40 60 80 100 120 140 160 180 200

MC

/Data

0.5

1

1.5

Corrected to particle level fordetector effects

Data well predicted byMC+detector simulation

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 19 / 44

Results Jet Substructure

Jet Substructure, “Fat” Jets

ATLAS-CONF-2011-073

Currently work ongoing in H → bb̄ channel, studying jet structure, waysto distinguish H decay from QCDBy applying the jet filtering algorithm generator differences are reducedand impact of pile-up is removed.Data/MC agreement very good after filtering

PVN

1 2 3 4 5 6 7 8 9

Me

an

Je

t M

ass [

Ge

V]

80

100

120

140

160

180

200

220

240

260 Before Splitting/Filtering

After Splitting/Filtering

Jets which will pass Splitting

Cambridge­Aachen R=1.2 jets

> 0.3qq

Split/Filtered with R

> 300 GeV, |y| < 2T

p

0.3 GeV± = 2.9 PV

dNdm

0.1 GeV± = 4.2 PV

dNdm

0.2 GeV± = 0.1 PV

dNdm

PreliminaryATLAS

50 100 150 200 250 300

dm

[G

eV

]σ

d

σ1

0

0.002

0.004

0.006

0.008

0.01 ­1ATLAS 2010 Data, L = 35pb

Pythia

HerwigJimmy

Herwig++

Cambridge­Aachen R=1.2 jets

> 0.3qq

Split/Filtered with R

> 300 GeV, |y| < 2T

= 1, pPVN

PreliminaryATLAS

Jet Mass [GeV]50 100 150 200 250 300

MC

/ D

ata

0.20.40.60.8

11.21.41.61.8

Jet Mass [GeV]50 100 150 200 250 300

MC

/ D

ata

0.20.40.60.8

11.21.41.61.8

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 20 / 44

Results Heavy Flavour Production

b-jet Cross Section

b-jet tagger “SV0”Iterative secondary vertex seedingfrom track pairs

separation power from decaylength significance

Also tag b-jets with muon decay

Determine the relative distancebetween jet axis and muon

Fit templates for b-jet contribution

050100150200250300350400b

-ta

gg

ing

eff

icie

ncy

0.2

0.4

0.6

0.8 Stat. error

Stat. + syst. error

|y| < 0.3≤0.0

050100150200250300350400

0.2

0.4

0.6

0.8

|y| < 0.8≤0.3

050100150200250300350400

0.2

0.4

0.6

0.8

|y| < 1.2≤0.8

[GeV]T

Jet p

0 50 100 150 200 250 300 350 400

0.2

0.4

0.6

0.8

|y| < 2.1≤1.2

ATLAS Preliminary

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 21 / 44

Results Heavy Flavour Production

b-jet Cross Section

Good agreement with Powheg+PYTHIA

MC@NLO+Herwig predicts too fewcentral jets, too many forward jets

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 22 / 44

Results Heavy Flavour Production

Inclusive bb̄ Jet Cross Section

PYTHIA MC10 and Powheg showgood agreement

MC@NLO does not model thedata, especially at high dijet mass

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 23 / 44

Results W and Z Production

W and Z Production

Precision studies of W and Z production at LHC serve several purposes:

Important to re-establish the standard model

Understand main backgrounds to searches for new physics, Higgs sector

Although these points are often quoted and true, they ignore the reasons whyW and Z are important in their own right

Leptonic decay channels are among thecleanest final states that we can exploit athadron colliders

Higher order QCD corrections modify thecross sections by 30-40% and have a visibleeffect on kinematics.

Experimentally able to test new toolsavailable:NLO QCD MC generators (MC@NLO, POWHEG, )

LO-matched multi-jet generators (ALPGEN, MADGRAPH, SHERPA), which willbecome NLO-matched in the near future.

NNLO Drell Yan predictions with FEWZ, DYNNLO

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 24 / 44

Results W and Z Production

W and Z Cross Sections

ATLAS-CONF-2011-041

Fiducial cross section corrected forefficiency factor, adjusted to data/MCdifferences

Comparing the fiducial regiondisentangles theor. and exp. effectsProvides most precise comparison withtheory

Total cross section corrected foracceptance based on MC

Comparing total cross sections,acceptance uncertainty accounts foreffects of different PDFs on unmeasuredphase space

) [nb]­l

+ l→*γ BR(Z/⋅ Z

fidσ

0.4 0.45 0.5 0.55

) [n

b]

ν l→

± B

R(W

⋅ ±

Wfid

σ

4

4.5

5

5.5

= 7 TeV)sData 2010 (

MSTW08

ABKM09

JR09

total uncertainty

sys⊕sta

uncertainty

68.3% CL ellipse area

­1 L dt = 33­36 pb∫

ATLAS Preliminary

) [nb]­l

+ l→*γ BR(Z/⋅ Z

fidσ

0.4 0.45 0.5 0.55

) [n

b]

ν l→

± B

R(W

⋅ ±

Wfid

σ

4

4.5

5

5.5

) [nb]­l

+ l→*γ BR(Z/⋅ Z

totσ

0.8 0.9 1

) [n

b]

ν l→

± B

R(W

⋅ ±

Wtot

σ8

9

10

11

= 7 TeV)sData 2010 (

MSTW08

ABKM09

JR09

total uncertainty

acc⊕ sys ⊕sta

uncertainty

68.3% CL ellipse area

­1 L dt = 33­36 pb∫

ATLAS Preliminary

) [nb]­l

+ l→*γ BR(Z/⋅ Z

totσ

0.8 0.9 1

) [n

b]

ν l→

± B

R(W

⋅ ±

Wtot

σ8

9

10

11

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 25 / 44

Results W and Z Production

W+jets Production

ATLAS-CONF-2011-060

W+jets, sensitive to pQCD predictions

Inclusive Jet Multiplicity Ratio

0≥1/≥ 1≥2/≥ 2≥3/≥ 3≥4/≥ 4≥5/≥

­ 1

je

ts)

jet

N≥

(W +

σ

je

ts)/

je

tN

≥(W

+

σ

0.1

0.2

0.3

0.4

0.5 + jetsνe→W

=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT­SHERPAMCFM

­1Ldt=33 pb∫

ATLAS Preliminary

200 400 600

[pb/G

eV

]T

/dH

σd

­610

­510

­410

­310

­210

­110

1

10

210

310 + jetsνe→W

=7 TeVsData 2010,

ALPGEN

SHERPA

BLACKHAT­SHERPA

­1Ldt=33 pb∫

ATLAS Preliminary

1 jets≥

W +

­1

2 jets, x10

≥W +

­23 jets, x10

≥W +

­34 jets, x10

≥W +

200 400 600

Th

eo

ry/D

ata

0.5

1

1.51 jet≥W +

[GeV]TH

200 400 600

Theory

/Data

0

0.5

1

1.52 jets≥W +

HT is a common choice for factorization and

renormalization scale, important variableT. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 26 / 44

Results W and Z Production

Z+jets Production

ATLAS-CONF-2011-042

­1)

jet

N≥)+­

µ+

µ →

*(γ(Z

/σ

)/je

t N≥

)+­µ

+µ

→*(γ

(Z/

σ 0.1

0.15

0.2

0.25

0.3

0.35

0.4

­1)

jet

N≥)+­

µ+

µ →

*(γ(Z

/σ

)/je

t N≥

)+­µ

+µ

→*(γ

(Z/

σ 0.1

0.15

0.2

0.25

0.3

0.35

0.4

= 7 TeV)sData 2010 (

Alpgen

Sherpa

Pythia

MCFM

)+jets­µ+µ →*(γZ/Preliminary ATLAS

­1 L dt = 33 pb∫

jets, R = 0.4,tanti­k

> 30 GeVjets

Tp

Da

ta /

NL

O

0.60.8

11.21.41.6

Da

ta /

NL

O

0.60.8

11.21.41.6 = 7 TeV)sData 2010 (

theoretical uncertainties

Da

ta /

MC

0.60.8

11.21.41.6

Da

ta /

MC

0.60.8

11.21.41.6 Data 2010 / Alpgen

Data 2010 / Sherpa

­1jet N≥/jet N≥1 2 3

Data

/ M

C

0.60.8

11.21.4

0je

ts)

[1/G

eV

] ≥

)+­µ

+µ

→*(γ

(Z/

σ/T

/dp

σd

­310

­210

­110

0je

ts)

[1/G

eV

] ≥

)+­µ

+µ

→*(γ

(Z/

σ/T

/dp

σd

­310

­210

­110

= 7 TeV)sData 2010 (

Alpgen

Sherpa

MCFM

)+jets­µ+µ →*(γZ/Preliminary ATLAS

­1 L dt = 33 pb∫

1 jet,≥* +γZ/

jets, R = 0.4,tanti­k

> 30 GeVjets

Tp

Da

ta /

NL

O

0.60.8

11.21.41.61.8

Da

ta /

NL

O

0.60.8

11.21.41.61.8 = 7 TeV)sData 2010 (

theoretical uncertainties

Da

ta /

MC

0.60.8

11.21.41.61.8

Da

ta /

MC

0.60.8

11.21.41.61.8 Data 2010 / Alpgen

Data 2010 / Sherpa

[GeV]jet

T p

30 40 50 60 70 80 90 100 110 120

Data

/ M

C

0.60.8

11.21.4

Good agreement with matched LO prediction fromALPGEN and Sherpa.

Poor agreement with LO PYTHIA at high jet

multiplicity

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 27 / 44

Results W and Z Production

W and Z pT

The measurement of the boson pT is sensitive to dynamic effects of stronginteraction, complementary to associated production of bosons with jets.

CERN-PH-EP-2011-095

[GeV]ee

Tp

1 10 210

Da

ta (

Pre

dic

tio

n)

/ R

ES

BO

S

0.6

0.8

1

1.2

1.4

1.6

­e+ e→*γZ/

Data 2010RESBOSPYTHIAMC@NLOPOWHEGALPGENSHERPA

| < 2.4eη|

> 20 GeVe

Tp

< 116 GeVee66 GeV < m

ATLAS

­1 L dt = 35 pb∫

Resbos shows good agreement with data,

indicating importance of resummation.

[GeV]W

Tp

0 50 100 150 200 250 300

(Da

ta,P

red

ictio

n)

/ R

ES

BO

S

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

ATLAS Preliminary

­1 31 pb≈Ldt ∫

Combined Data

Stat. Uncert.

ALPGENMC@NLO

POWHEG

PYTHIARESBOS

For pWT > 120 GeV Pythia and Resbos agreein predicting a softer spectrum than Alpgenand Sherpa

First corrected pWT measurement, precision

comparable to pZTT. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 28 / 44

Summary

Summary

As promised, the LHC era has allowed us to test QCD in new kinematicregimes, good testing ground for predictions.

Already ATLAS data is able to discriminate between different MCpredictions.

Theorists are eager to compare predictions against our data, and arewriting papers about our data!

Breaking new ground in techniques to identify boosted final states.

There is LOTS more data to analyse, more to learn.

Looking forward to fruitful interactions between the theory andexperimental viewpoints

Exciting times are upon us!

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 29 / 44

Backup

Additional Material

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 30 / 44

Backup

A word on simulation

Jet production is the most common process at the LHC, and leads to anenormous number of diagrams at higher orders.

LO generators like Pythia and Herwig have 2→ 2 process at matrixelement, plus some leading logs terms to match with parton shower.

A full NLO calculation for 2→ 2 and 2→ 3 parton processes availablewith NLO++, but no matching with PS. Partons are however clusteredinto jets, then soft corrections coming from unfolding

ALPGEN contains 2→ n LO matrix elements, so it should be well-suitedfor multiple final states

POWHEG box now includes 2 parton final states at NLO, with matchingto both Pythia and Herwig PS.

HEJ is a new fully-resummed MonteCarlo for wide-angle emission ofsimilar momentum partons. Recently interfaced to ARIADNE + Pythia,just used at parton level here

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 31 / 44

Backup

Pileup in 2011 data

Mean number of interactions per bunch crossing in 2011 data run

Mean Number of Interactions per Crossing

0 2 4 6 8 10 12 14

]­1

Record

ed L

um

inosity [pb

­410

­310

­210

­110

1

10

210=7 TeVsATLAS Online 2011,

­1 Ldt=1.25 fb∫

> = 5.7µ<

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 32 / 44

Backup

In-situ Jet Calibration with 2011 data

In 2011 with additional event activity (pileup) the jet calibration using theMPF technique appears robust

100 200 300 400 500 600

MP

FR

0.60

0.65

0.70

0.75

0.80

0.85

­1 L dt = 38 pb∫2010 data:

­1 L dt = 786 pb∫2011 data:

ATLAS preliminary

= 7 TeVs

MPF EM scale, all jet algorithms

[GeV]γ

Tp

100 200 300 400 500 600

20

11

da

ta /

20

10

da

ta

0.96

0.98

1.00

1.02

1.04100 200 300 400 500 600

MP

FR

0.60

0.65

0.70

0.75

0.80

0.85

Data

+jetγPYTHIA

ATLAS preliminary

∫ ­1L dt = 786 pb

= 7 TeVs

MPF EM scale, all jet algorithms

[GeV]γ

Tp

100 200 300 400 500 600

Da

ta /

MC

0.96

0.98

1.00

1.02

1.04

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 33 / 44

Backup

Inclusive jet cross section - PDF variation

| < 0.3y|1.5

1

0.5

[GeV]T

p

2103

10

| < 2.1y1.2 < |1.5

1

0.5

[GeV]T

p

2103

10

| < 1.2y0.8 < |1.5

1

0.5

| < 0.8y0.3 < |1.5

1

0.5

Ratio w

rt C

TE

Q 6

.6 PreliminaryATLAS

statistical errorData with

uncertainties

Systematic

=7 TeVs

­1 dt=37 pbL ∫

jets, R=0.4t

anti­k

Non­pert. corr.

×NLO pQCD

CTEQ 6.6

MSTW 2008

NNPDF 2.1

HERAPDF 1.5

| < 2.8y2.1 < |1.5

1

0.5

| < 3.6y2.8 < |1.5

1

0.5

[GeV]T

p

2103

10

| < 4.4y3.6 < |1.5

1

0.5

[GeV]T

p

2103

10

Ratio w

rt C

TE

Q 6

.6 PreliminaryATLAS

statistical errorData with

uncertainties

Systematic

=7 TeVs

­1 dt=37 pbL ∫

jets, R=0.4t

anti­k

Non­pert. corr.

×NLO pQCD

CTEQ 6.6

MSTW 2008

NNPDF 2.1

HERAPDF 1.5

Ratio of inclusive jet cross section measurement in data and MC, with variousPDFs

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 34 / 44

Backup

Dijet with Jet Veto

CERN-PH-EP-2011-100,Submission to JHEP

Historically measurement hasbeen a search for coloursinglet exchange

Increased c.m. energy of LHC

Testing ground forexperimental techniques tosearch for VBF Higgs

Ga

p f

ractio

n

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

dijet selectionT

Leading p

= 20 GeV0

Q

< 120 GeVT

p ≤90

Data 2010

PYTHIA 6 AMBT1

HERWIG++

ALPGEN +

HERWIG/JIMMY

y∆

0 1 2 3 4 5 6

MC

/Da

ta

0.6

0.8

1

1.2

1.4ATLAS

Gap fraction= (# events passing the Gapveto) / (all events)

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 35 / 44

Backup

Dijet with Jet Veto

POWHEG + PYTHIA

Generally agree with data

HEJ

Agreement better in someplaces than othersQuick theory reaction R.M.Duran Delgado at al,arXiv:1107.2084:”The message is clear: theaccuracy of the ATLAS dataalready demands bettertheoretical calculations”

< 270 GeV (+3)T

p ≤240

< 240 GeV (+2.5)T

p ≤210

< 210 GeV (+2)T

p ≤180

< 180 GeV (+1.5)T

p ≤150

< 150 GeV (+1)T

p ≤120

< 120 GeV (+0.5)T

p ≤90

< 90 GeV (+0)T

p ≤70

Data 2010

HEJ (parton level)

POWHEG + PYTHIA

POWHEG + HERWIG

dijet selectionT

Leading p

= 20 GeV0

Q

ATLAS

y∆

0 1 2 3 4 5 6

Gap fra

ction

0

1

2

3

4

5

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 36 / 44

Backup

Dijet with Jet Veto

y < 5 (+3)∆ ≤4

y < 4 (+2)∆ ≤3

y < 3 (+1)∆ ≤2

y < 2 (+0)∆ ≤1

Data 2010

HEJ (parton level)

POWHEG + PYTHIA

POWHEG + HERWIG

dijet selectionT

Leading p

= 20 GeV0

Q

ATLAS

[GeV]T

p50 100 150 200 250 300 350 400 450 500

Me

an

nu

mb

er

of

jets

in

th

e g

ap

0

1

2

3

4

5

6

7

Data 2010

HEJ (parton level)

POWHEG + PYTHIA

POWHEG + HERWIG

ATLAS

dijet selectionT

Leading p

= 20 GeV0

Q

y < 5∆ ≤4

0.5

1

y < 4∆ ≤3

0.5

1

1.5

y < 3∆ ≤2

0.6

0.8

1

1.2

1.4

y < 2∆ ≤1

[GeV]T

p

50 100 150 200 250 300 350 400 450 500

0.6

0.8

1

1.2

Th

eo

ry /

Da

ta

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 37 / 44

Backup

Dijet Azimuthal Decorrelation

CERN-PH-EP-2011, Submitted to PRL

Testing pQCD for multijet productionwithout measuring additional jets

[radians]φ∆/2π /3π2 /6π5 π

Num

ber

of E

vents

1

10

210

310

410

510

jets R=0.6t

anti­k

|<2.8y>100 GeV |T

p

|<0.8yLeading two jets: |>110 GeVmax

Tp

­1=36 pbdtL∫Data

2 jets≥3 jets≥4 jets≥5 jets≥

PYTHIA

ATLAS =7 TeVs

Decorrelation increases whenadditional jets are required

[radians]φ∆/2π /3π2 /6π5 π

]­1

[ra

dia

ns

φ∆

/dσ

dσ

1/

­310

­210

­110

1

10

210

310

410

510

610

710

810

910

­1=36 pbdtL∫Data

)8

10× 800 GeV (>max

Tp

)710× 800 GeV (≤max

Tp<600

)6

10× 600 GeV (≤max

Tp<500

)5

10× 500 GeV (≤max

Tp<400

)410× 400 GeV (≤max

Tp<310

)3

10× 310 GeV (≤max

Tp<260

)210× 260 GeV (≤max

Tp<210

)110× 210 GeV (≤max

Tp<160

)0

10× 160 GeV (≤max

Tp<110

)4sαO(NLO pQCD

unc.sαPDF &

scale unc.

ATLAS =7 TeVs|<0.8y>100 GeV |

Tp jets R=0.6 tanti­k

Differential cross section in data andNLO pQCD (NLOJET++, MSTW2008 PDF)

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 38 / 44

Backup

Dijet Azimuthal Decorrelation

160 GeV≤max

Tp<110

ATLAS =7 TeVs­1=36 pbdtL∫

310 GeV≤max

Tp<260

/2π /3π2 /6π5 π

600 GeV≤max

Tp<500

210 GeV≤max

Tp<160

)4sαO(NLO pQCD

scale unc. unc.sαPDF &

400 GeV≤max

Tp<310

/3π2 /6π5 π

800 GeV≤max

Tp<600

260 GeV≤max

Tp<210

jets R=0.6tanti­k|<0.8y>100 GeV |

Tp

500 GeV≤max

Tp<400

[radians]φ∆/3π2 /6π5 π

800 GeV>max

Tp

ratio

to

NL

O p

QC

D φ

∆/d

σ d

σ1

/

2.0

1.0

0.5

2.0

1.0

0.5

2.0

1.0

0.5

Ratio of data to NLO pQCDTheory consistent with data

160 GeV≤max

Tp<110

ATLAS =7 TeVs­1=36 pbdtL∫

310 GeV≤max

Tp<260

/2π /3π2 /6π5 π

600 GeV≤max

Tp<500

210 GeV≤max

Tp<160

PYTHIAHERWIGSHERPA stat. unc.

400 GeV≤max

Tp<310

/3π2 /6π5 π

800 GeV≤max

Tp<600

260 GeV≤max

Tp<210

jets R=0.6tanti­k|<0.8y>100 GeV |

Tp

500 GeV≤max

Tp<400

[radians]φ∆/3π2 /6π5 π

800 GeV>max

Tp

ratio

to S

HE

RP

A φ

∆/d

σ d

σ1

/

2.0

1.0

0.5

2.0

1.0

0.5

2.0

1.0

0.5

Ratio of data to PYTHIA, HERWIG,SHERPAAll describe data, SHERPA does best

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 39 / 44

Backup

Jet Substructure

ATL-PHYS-PUB-2011-010Jets are both complete 4-vectors and complex compositeobjects.

At LHC energy decays of top, W , etc decays can becollimated into one jet

Knowledge of the internal jet substructure is important indistinguishing these decays from gluon or quark initiatedjets

Internal structure of energetic jets is mainly dictated by

emission of multiple gluons from primary parton

Calculable in pQCD

Differential jet shape

ρ(r) =1

∆r

1

NjetΣjets

pT (r −∆r/2, r + ∆r/2)

pT (0, R),∆r/2 ≤ r ≤ R−∆r/2 (1)

Integral jet shape

Ψ(r) =1

NjetΣjet

pT (0, r)

pT (0, R), 0 ≤ r ≤ R (2)

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 40 / 44

Backup

Jet Substructure

0 0.1 0.2 0.3 0.4 0.5 0.6

(r)

ρ

­110

1

10

ATLAS jets R = 0.6tanti­k

< 40 GeVT

30 GeV < p

| y | < 2.8 (a)

­1 ­ 3 pb­1 dt = 0.7 nbL∫Data

PYTHIA­Perugia2010

HERWIG++

ALPGEN

PYTHIA­MC09

r0 0.1 0.2 0.3 0.4 0.5 0.6

DA

TA

/ M

C

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6

(r)

ρ

­110

1

10

ATLAS

jets R = 0.6tanti­k

< 310 GeVT

260 GeV < p

| y | < 2.8

(d)

­1 ­ 3 pb­1 dt = 0.7 nbL∫Data

PYTHIA­Perugia2010

HERWIG++

ALPGEN

PYTHIA­MC09

r0 0.1 0.2 0.3 0.4 0.5 0.6

DA

TA

/ M

C

0.8

1

1.2

Differential jet shapes vs jet pT ,integrated over |y| < 2.8

As expected, jet narrows withincreasing pT

Data compared to various MCpredictions

PYTHIA-Perugia2010PYTHIA-MC09Herwig++Alpgen (withHerwig+Jimmy)

General agreement, althoughHerwig++ predicts jets toonarrow

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 41 / 44

Backup

Jet Substructure

(GeV)T

p

0 100 200 300 400 500 600

(r =

0.3

)Ψ

1 ­

0

0.05

0.1

0.15

0.2

0.25

0.3

jets R = 0.6t

anti­k

| y | < 2.8

(a)

ATLAS

­1 ­ 3 pb­1 dt = 0.7 nbL∫Data

PYTHIA­Perugia2010

HERWIG++

ALPGEN

PYTHIA­MC09

PYTHIA-Perugia 2010 provides reasonable description of data

Herwig++ broader than data

Alpgen predictions too narrow at high pT

PYTHIA-MC09 produces jets which are too narrow in the whole kinematic range(possibly due to soft QCD mismodeling)

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 42 / 44

Backup

W+jets Production, muon channel

ATLAS-CONF-2011-060

W+jets, sensitive to pQCD predictions

Inclusive Jet Multiplicity Ratio

0≥1/≥ 1≥2/≥ 2≥3/≥ 3≥4/≥ 4≥5/≥

­1 je

ts)

jet

N≥(W

+

σ je

ts)

/ je

t N≥

(W +

σ

0.1

0.2

0.3

0.4

0.5 + jetsνµ→W

=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT­SHERPAMCFM

­1Ldt=33 pb∫

ATLAS Preliminary

200 400 600

[pb/G

eV

]T

/dH

σd

­610

­510

­410

­310

­210

­110

1

10

210

310 + jetsνµ→W

=7 TeVsData 2010,

ALPGEN

SHERPA

BLACKHAT­SHERPA

­1Ldt=33 pb∫

ATLAS Preliminary

1 jets≥

W +

­1

2 jets, x10

≥W +

­23 jets, x10

≥W +

­34 jets, x10

≥W +

200 400 600

Th

eo

ry/D

ata

0.5

1

1.51 jet≥W +

[GeV]TH

200 400 600

Theory

/Data

0

0.5

1

1.52 jets≥W +

HT is a common choice for factorization and

renormalization scale, important variableT. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 43 / 44

Backup

Z+jets Production, muon channel

ATLAS-CONF-2011-042

­1)

jet

N≥)+­

e+

e→

*(γ(Z

/σ

) /

jet

N≥)+­

e+

e→

*(γ(Z

/σ

0.1

0.15

0.2

0.25

0.3

0.35

0.4ATLAS Preliminary

­1 L dt = 33 pb∫

jets, R = 0.4,t

anti­k

> 30 GeVjets

Tp

) + jets­e+ e→*(γZ/

= 7 TeV)sData 2010 (

Alpgen

Sherpa

Pythia

MCFM

1 2 3

Da

ta /

NL

O

0.60.8

11.21.41.6 = 7 TeV)sData 2010 (

theoretical uncertainties

Da

ta /

MC

0.60.8

11.21.41.6 Data 2010 / Alpgen

Data 2010 / Sherpa

­1jet N≥ / jet N≥1 2 3

Da

ta /

MC

0.60.8

11.21.4

0 jets

) [1

/GeV

]≥

)+­e

+ e

→*(γ

(Z/

σ /

T/d

pσ

d

­310

­210

­110ATLAS Preliminary

­1 L dt = 33 pb∫

1 jet,≥*+ γZ/

jets, R = 0.4,t

anti­k

> 30 GeVjets

Tp

) + jets­e+ e→*(γZ/

= 7 TeV)sData 2010 (

Alpgen

Sherpa

MCFM

30 40 50 60 70 80 90 100 110 120

Data

/ N

LO

0.60.8

11.21.41.61.8 = 7 TeV)sData 2010 (

theoretical uncertainties

Data

/ M

C

0.60.8

11.21.41.61.8 Data 2010 / Alpgen

Data 2010 / Sherpa

[GeV]jet

Tp

30 40 50 60 70 80 90 100 110 120

Data

/ M

C

0.60.8

11.21.4

Good agreement with matched LO prediction fromALPGEN and Sherpa.

Poor agreement with LO PYTHIA at high jet

multiplicity

T. Spreitzer (University of Toronto) Hard QCD studies with the ATLAS detector August 22, 2011 44 / 44