cteq07 michigan may 2007 benchmark qcd measurements and tools at atlas craig buttar university of...
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CTEQ07 Michigan May 2007
Benchmark QCD Measurements and Tools at ATLAS
Craig Buttar
University of Glasgow
CTEQ07 Michigan May 2007
Outline
• Going from the Tevatron to the LHC– Pdfs
– Total xsect vs 2 2 xsect
– Multijets sudakov form factors
• These considerations define measurements that are needed to define the hadronic environment at the LHC– Measurements of PDFs with inclusive jet xsect
– Underlying event and minimum bias
– Multijet rates
• Implications for physics– PDFs identifying new physics
– Multijets MET?
– Underlying event Higgs; different processes
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Tevatron LHC
Q2
(GeV
)
tot
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Low pt physics
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Minimum bias and Underlying Event: LHC predictions
Tevatron
● CDF 1.8 TeV
PYTHIA6.214 - tuned PYTHIA predictions
dN/dη (η=0)
Nch jet-pt=20GeV
1.8TeV (pp) 4.1 2.3
14TeV (pp) 7.0 7.0
increase ~x1.8 ~x3
~80%~200%
LHC prediction
Tevatron
PYTHIA6.214 - tuned
● CDF 1.8 TeV
LHC
MB onlyUE includes radiation and small impact parameter bias
dN/d in minimum bias events
Minimum bias = inelastic pp interactionUnderlying event = hadronic environment not part of the hard scatter
Pt leading jet (GeV)
Particle
density
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The underlying eventTra
nsvers
e <
Nch
g >
PYTHIA6.214 - tuned
PHOJET1.12
x 3
LHC
x1.5
Extrapolation of UE to LHC is unknownDepends on• Multiple interactions• Radiation• PDFs• Striing formation
High PT scatter
Beam remnants
ISR
• Lepton isolation • Top• Jet energy• VBF
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Ra
tio
<N
Tra
ckR
eco>
/<N
Tra
ckM
C>
Leading jet ET (GeV)
Reconstructing the underlying event
CDF Run 1 underlying event analysisPhys. Rev. D, 65 092002 (2002)
Njets > 1, |ηjet| < 2.5, ETjet >10 GeV,
|ηtrack | < 2.5, pTtrack > 1.0 GeV/c
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Underlying event for different processes
• The underlying event for electroweak processes needs to be studied– Critical for Higgs search in
VBF
Charged Particle Density: dN/dd
0.1
1.0
10.0
0 30 60 90 120 150 180 210 240 270 300 330 360
(degrees)C
ha
rge
d P
art
icle
De
ns
ity
Leading Jet
Leading Photon
Z-boson
RDF PreliminaryPYTHIA Tune A
Charged Particles (||<1.0, PT>0.5 GeV/c)
"Transverse" Region
Jet#1PhotonZ-boson
Jet #1 Direction
“Toward”
“Transverse” “Transverse”
“Away”
Photon #1 Direction
“Toward”
“Transverse” “Transverse”
“Away”
Z-boson Direction
“Toward”
“Transverse” “Transverse”
“Away”
R.Field
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Resolving hard and soft components
Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
“Away”
Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
Jet #2 Direction
“Away”
“Leading Jet”
“Back-to-Back”
•TransMAX and transMIN sensitive to radiation and soft UE respectively •Back-to-back sample suppresses radiation• difference between tranMAX region and transMIN in leading jet and b-2-b jet sample
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Simulation of underlying event
• MC tools for simulation of underlying event– PYTHIA (UE+min bias)
– Herwig + Jimmy (UE only, pt-cut)
– PHOJET (Min bias and UE)
• All give a reasonable description of Tevatron data with tuning
• Energy extrapolation is essentially a free parameter and uncertain data required
• SHERPA also has simulation of underlying event but has been studied less
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PDFs
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Impact of PDF uncertainty on new physics
Similarly PDF uncertainties limits the sensitivity in inclusive xsect to BSM physics
Extra-dimensions affect the di-jet cross section through the running of s. Parameterised by number of extra dimensions D and compactification scale Mc.
PDF uncertainties (mainly due to high-x gluon) reduce sensitivity to compactification scale from ~5 TeV to 2 TeV
2XD
4XD
6XD
SM
Mc= 2 TeV
uncertainties
Mc= 2 TeV Mc= 6 TeV
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Measure high-x gluon pdfs from inclusive jet cross-section
• Measure inclusive xsect to get high-x gluons
• Measure in different rapidity bins– New physics vs pdf
• Theoretical uncertainties in QCD calculation– Scale dependence
• Experimental errors– Jet energy scale
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Uncertainty due to PDF
High pT PDF errors dominated by the high x-gluon. Estimates below from CTEQ6.1 error sets 29 and 30 compared to best fit (NLOJET).
At 1TeV in central region error is 10-15%
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Theoretical Errors•NLO QCD cross-sections can be calculated to compare with the experimental results.
•There are errors on the theoretical prediction due to PDFs and the finite order of the calculation (renormalisation and factorisation scales).
5%-10% scale error.
From changing scale µr=µf from 0.5pT jet to 2.0pT jet
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Experimental Errors
For a jet with pT=1TeV:
1% error on jet energy -> 6% on 5% error on jet energy -> 30% on
10% error on jet energy -> 70% on
Used NLOJET.
10% JES
5% JES
1% JES
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Analysis – Constraining High x-Gluon
•Effect of adding simulated ATLAS collider data to gluon uncertainty in a global PDF fit (NLOGRID) Fits by Claire Gwenlan:
x x
Glu
on u
ncer
tain
ty
Reducing JES from 3% to 1%
•A very good control (1%) of the Jet Energy Scale is needed in order to constrain PDFs using collider data.
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At the LHC we will have dominantly sea-sea parton interactions at low-x And at Q2~M2
W/Z the sea is driven by the gluon by the flavour blind g ->qq gluon is far less precisely determined for all x values
_
The uncertainties on the W and Z rapidity distributions are dominated by the uncertainties along gluon PDF dominated eigenvectors
Measurement of W and Z rapidity distributions can increase our knowledge of the gluon PDF key to using W,Z as luminosity monitor
Improving low-x gluon using rapidity distribution in W-decay
2.5
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At y=0 the total W PDF uncertainty is ~ ±5.2% from ZEUS-S~ ±3.6% from MRST01E~ ±8.7% from CTEQ6.1MZEUS to MRST01 central value difference ~5%ZEUS to CTEQ6.1 central value difference ~3.5% (From LHAPDF eigenvectors)
W and Z Rapidity Distributions for different PDFs
CTEQ6.1M MRST02 ZEUS-S
Analytic calculations: Error bands are the full PDF Uncertainties
GOAL: syst. exp. error ~3-5%
CTEQ6.1M
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PDF constraining potential of ATLASEffect of including the ATLAS W Rapidity “pseudo-data” in global PDF Fits:
how much can we reduce the PDF errors when LHC is up and running?Simulate real experimental conditions:
Generate 1M “data” sample with CTEQ6.1 PDF through ATLFAST detector simulation and then include this pseudo-data (with imposed 4% error) in the global ZEUS PDF fit (with Det.->Gen. level correction).Central value of ZEUS-PDF prediction shifts and uncertainty is reduced:
ZEUS-PDF BEFORE including W data
e+ CTEQ6.1 pseudo-data
low-x gluon shape parameter λ, xg(x) ~ x –λ :BEFORE λ = -0.199 ± 0.046
AFTER λ = -0.181 ± 0.030 35% error reduction
NB: in ZEUS-PDF fit the e± Normalisation is left free => no assumption on Luminosity measurement
ZEUS-PDF AFTER including W data
e+ CTEQ6.1 pseudo-data
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Multijets
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Azimuthal dijet decorrelation
Early measurement to benchmark generators particularly parton showers/higher orders
2 dijet
dijet 2
dijet=
dijet~ 2
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Reconstructed di-jet azimuthal decorrelations
Selecting di-jet events:
300 < ETMAX < 600 GeV
Cone jet algorithm (R=0.7)Njets = 2, |ηjet| < 0.5, ET
jet #2 > 80 GeV,
Two analysis regions:
600 < ETMAX < 1200 GeV
J5J5
J6J6
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Multijets in top events
• MC@NLO and ALPGEN agree for hardest jet
• HERWIG fails at high pt • Significant number of events
have 3 additional jets there is a discrepency between MC@NLO and HERWIG vs ALPGEN
• Key that such multijet spectra are measured
• Possible with early high energy running
Spectra include tt
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Effect of multijets on inclusive SUSY studies
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Summary and conclusions
• QCD benchmarks (inc low-pt processes)– Minimum bias and underlying event
• Related, need to understand for many analyses
• Baseline for HI, minijets evolution from low pt
• Do this before high lumi
– PDFs• New regime in PDFs
• Need to measure for precision SM and high-pt physics
– Multijets• Measure azimuthal decorrelations to validate simulations
• Many more jets in events tt6J+nJ
• Need to understand multiplicities
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Tracking in MB events
• Acceptance limited in rapidity and pt
• Rapidity coverage– Tracking covers ||<2.5
• pT problem
– Need to extrapolate by ~x2 Need to understand low pt
charge track reconstruction
1000 events1000 events
dNdNchch/d/d
dNdNchch/dp/dpTT
Black = Generated (Pythia6.2)
Blue = TrkTrack: iPatRec
Red = TrkTrack: xKalman
Reconstruct tracks with:Reconstruct tracks with: 1) pT>500MeV1) pT>500MeV 2) |d2) |d00| < 1mm| < 1mm 3) # B-layer hits >= 13) # B-layer hits >= 1 4) # precision hits >= 84) # precision hits >= 8
pT (MeV)
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Minimum bias studies: Charged particle density at = 0
LHC?
Large uncertainties in predicted particle density in minimum bias events ~x2
Measurement with central tracker at level of ~10% with ~10k events – first data
Why? soft physics, pile-up at higher luminosities, calibration of experiment
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Compare abrupt and smooth pt-cut-off:Abrupt cut-off generates a Poisson distribution with too few multi-parton interactions in a single event
Compare matter distributions:uniform, gaussian, double gaussian Use double gaussian
number of interactionsn
zn
Use MB multiplicity distributions to tune fluctuations in number of events
abrupt
smooth
Uniform
gaussian
d-gaussian
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Herwig+Jimmy
• Jimmy is multi-parton interaction model similar to PYTHIA
• Main parameter is pt-min• Only for hard UE cannot
model low-pt ie MB difficult to get energy dependence
• Matter distribution is determined from em form factor
• Gives a good description of CDF data with increased pt-min2.53.25GeV
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Herwig+Jimmy
• Extrapolation to LHC predicts much larger UE c.f. underlying event
• No energy dependent pt-min
• Which is correct?T
ran
sver
se <
Nch
g >
Pt (leading jet in GeV)TevatronTevatron
LHCLHC
x3
x5
x4
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VBF Signal (HWWll)
•forward tagging jets
•correlated isolated leptons
• low hadronic activity in central region
•central Higgs production
Tagging jet
Tagging jet
H
W
WZ/WZ/W
Important discovery channelFor Higgs in mass range120-200GeV
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Model Parameter
Simple MSTP(82)=1
PARP(81)=1.9
Complex MSTP(82)=4
PARP(82)=1.9
Tuned MSTP(82)=4
PARP(82)=1.8
PARP(84)=0.5
Model CJV (eff) LEPACC (eff) All Vetoes (eff)
Simple 0.943 ± 0.003 0.610 ±0.001 0.084 ±0.001
Complex 0.885 ± 0.003 0.575 ±0.001 0.076 ±0.001
Tuned 0.915 ± 0.003 0.589 ±0.001 0.080 ±0.001
Uncertainty at the level of ~6% on CJV Pt>20GeV and ~3% on leptonGiving a total uncertaintly in the range ~8%
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• Effect of UE on lepton efficiency• Vary pt-min by 3• Determine from data
• Good muons– Barrel <1.1 Pt>7GeV– Endcap 1.1<<2.4 P>9GeV
• Isolation Pt for charged tracks excluding s
with Pt>0.8GeV and R<0.3 around in - space
S.Abdullin et al (CMS) Les Houches 05
Lepton isolation in H->4
Process Event eff Default Event eff –3 Event eff +3
H4 MH=150GeV 0.775 ± 0.004 0.707 ± 0.005 0.812 ± 0.004
ZZ background 0.780 ± 0.004 0.721 ± 0.005 0.838 ± 0.004
4 random Z-inclusive 0.762 ± 0.007 0.706 ± 0.007 0.821 ±0.006
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Extract effect of UE from data
• Use inclusive Z-sample, high statistics• Similar dependence to ZZ sample but small systematic shift
random
UE