Transcript Rencontres de Moriond QCD and High Energy Interactions 2011 W and Z physics with the Atlas detector Nikola Makovec On behalf of the Atlas.

Rencontres de Moriond QCD and High Energy Interactions 2011
W and Z physics
with the Atlas detector
Nikola Makovec
On behalf of the Atlas collaboration
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Content of the talk:
W/Z inclusive cross-section
W charge asymmetry
W/Z production in association with jets
W and WW cross-section
Nikola Makovec
W and Z physics: motivation
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Predominant source of isolated high pT
leptons
 clean experimental signatures with high
cross sections
 Benchmark for leptons performance
(efficiency, scale, resolution,…)
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Candle probes strong and weak couplings
 Study of the electroweak interaction (e.g. TGC)
 Use W/Z bosons to probe the structure of the hard interactions
 Test of perturbative QCD
 constrain proton PDFs
 Deviations from SM predictions can be a sign of new physics
 Solid foundations on all these SM processes are essential for future
searches for New Physics.
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Mainly results in the electron and muon channels
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Wl: single high pt lepton+etmiss
e
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Zll: Two high pt leptons with mass close to MZpdg
e
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W and Z inclusive cross-section
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Whole 2010 data sample (33-36pb-1)
Measurement strategy:
A: acceptance
C: selection efficiency
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Main improvement compared to previous Atlas measurement
with 0.3pb-1 (JHEP, 12:060, 2010)
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Reduced statistical uncertainties (/10)
Thanks to the
Systematic uncertainties diminished (/3) larger data sample
Luminosity uncertainty greatly reduced (11%3.4%)
Z x-sec measurement extended to electrons up to ~4.9 (2.5 previously)
Zee channel
central-central
W channel
central-forward
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W and Z inclusive cross-section
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Muon and electron channels are
in good agreement
 combination
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NNLO predictions with FEWZ
are consistent with data
Remarkable success of pQCD
and pdf determination
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W charge asymmetry (muon channel)
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Proton-proton collisions  σW+ > σW-
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This asymmetry depends on the
momentum fraction x of the partons
 dependence on yW
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Difficult to reconstruct W rapidity,
 lepton charge asymmetry ():
A 
d W  / d   d W  / d 
x1, 2 
mW  y
e
s
CTEQ 6.6 2=8.8
HERA 1.0 2=27.2
MSTW 2088 2=16.7
d W  / d   d W  / d 
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Total or partial cancellation of
systematics in ratio (lumi, efficiency,..)
 Residual systematics limited by data
statistics in Z sample
 measurement provide constraints on pdf
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W/Z+jets
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Vector Boson+jets cross section stringent test of pQCD
 choice of scales, parton showering,…
Significant background for SM and beyond SM processes
Atlas results with 1.3pb-1
for W+jets
Similar results between
muon and electron
chanels
MCFM has QED and
soft QCD corrections
applied for comparison
with data.
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e
NLO calculations
(MCFM, 0-2 jets) are in
good agreement with
the measured crosssections
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Z+jets
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Whole 2010 data sample
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Jet selection:
 Reconstructed from 3D topological
clusters
 Infrared and collinear-safe anti kT
algorithm with size parameter D=0.4
 Pt>30GeV
 Pile-up jet veto:
 Only trk from best PV considered
JVF   PT
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trk  jet
e
 PT  0.75
all trk
Background
 Increase with jet multiplicity
 QCD background
 Small in muon channel
 Data driven estimate in electron
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channel (2% - 7%)
 EW background
 Mainly top at large jet multiplicity
(0.1% - 6%)
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Z+jets
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Unfolding detector effects
 Bin-by-Bin corrections
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 Nominal: Alpgen
 Systematics: Sherpa
Systematics uncertainties
dominated by JES, JER and JVF
veto
NLO predictions and
generators are in agreement
with measured cross
sections
 Except Pythia which does not
reproduce the data at high jet
multiplicity
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Z+jets
e
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Di-boson: W  and Z 
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Motivation:
 W measurement can probe WW 
triple gauge boson coupling (TGC)
vertex
TGC
NLO prediction (pT>10GeV):
 W: 69+/-4.6 pb
 Z: 13.8+/-0.09 pb
FSR
Signature:
 W/Z selection
 Isolated photon pt>15GeV
Results:
(pT>10GeV)
Calorimeter
isolation
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Systematics dominated by the
photon reconstruction and
identification efficiency and
photon isolation cut efficiency
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Di-boson: WW
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Motivation:
 Test non-abelian nature of ewk sector
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 Sensitive to Triple Gauge Couplings
Constrain New Physics in Anomalous
TGC
Dominant Background for H→WW
search
NLO prediction: 46±3 pb
 MCFM with MSTW2008 (including gg))
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Signature:
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Results:
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The statistical uncertainty (44%) is the
dominant uncertainty for this
measurement
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
TGC
2 Opposite-Sign Leptons (e,μ)
Large Missing Energy
Jet veto to remove top bkg
8 candidates (1 ee / 2 μμ / 5 eμ)
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Di-boson: WZ
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Z
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First step to H
More challenging than electron and
muon channels in particular due to
neutrino in the tau decay
e,h
51.7%
,h
e,
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Conclusion
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Overview of first Z and W cross-section measurements
with the Atlas detector
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W/Z inclusive cross-section
W charge asymmetry
W/Z production in association with jets
W and WW cross-section
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Remarkable detector performance demonstrated by the
data
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Good agreement with the SM predictions at this
unexplored energy scales
 Already testing pQCD and pdfs
 more differential measurements in preparation.
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More information at:
 https://twiki.cern.ch/twiki/bin/view/AtlasPublic
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Makovec
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Back Up
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A Toroidal LHC ApparatuS
Muon Spectrometer (|η|<2.7)
4T air core toroid with gas based muon chamber.
Measurement with momentum resolution <10% up to
1 TeV.
Inner Detector |η|<2.5, in solenoidal 2T field. Silicon
pixels, Silicon strips, Transition radiation detector
(straws)
σ/pt=3.8*10-4pt(GeV)0.015
Trigger/DAQ
3-level trigger
In: 40 MHz
Out: 200 Hz
ATLAS recorded in 2010
about 45 pb-1 at 7 TeV
~35pb-1 of recorded
data with good quality
for physics analysis
Calorimeters, |η|<5
EM: Pb-LAr accordion σ/E=10%E
HAD: Fe/Scintillator + Cu/W-Ar σ/E=50%E
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W/Z selection
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Wl
 Pt>20GeV, ||<2.5 (2.4 for )
 Etmiss>25GeV
 Track isolation for  channel
 Transverse mass>40GeV
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Zll
 Pt>20GeV, ||<2.5 (2.4 for )
 Opposite charge
 Track isolation for  channel
 66 <Mll< 106GeV
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Usual suspects
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Electrons
Sliding window cluster
(with 2.5 GeV et seed)
matched to track.
Identification relies
mostly on track quality,
transverse profile in EM
calorimeter and
transition radiation signal
Etmiss
Reconstructed from
topological energy
clusters in calorimeters,
corrections for muons
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Muons
Combined: Association
with ID track
combination of
parameters
Standalone:
Spectrometer only
Jets:
Reconstructed from 3D
topological clusters
Infrared and collinearsafe anti kT algorithm
with size parameter
D=0.4
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