hard qcd studies with the atlas detector · 25/02 26/03 24/04 23/05 21/06 20/07 19/08 total...
TRANSCRIPT
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
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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