Standard Model Physics in ATLAS Monika Wielers RAL.
-
date post
20-Dec-2015 -
Category
Documents
-
view
216 -
download
1
Transcript of Standard Model Physics in ATLAS Monika Wielers RAL.
Standard Model Physics in ATLAS
Monika Wielers
RAL
HEPChile, Jan 2008 M. Wielers (RAL) 2
LHC numbers
Process σ (nb) Events
(Ldt = 100 pb-1) Min bias 108 ~1013
bb 5·105 ~1012
Inclusive jetspT > 200 GeV
100 ~ 107
W → eν, 15 ~ 106
Z → ee, 1.5 ~ 105
tt 0.8 ~ 104
Typical Standard Model processes
W
Z
LHC is a W, Z, top factory Small statistical errors in precision measurements can search for rare processes large samples for studies of systematic effects
HEPChile, Jan 2008 M. Wielers (RAL) 3
Overview
Can’t cover everything, soJust focus on few topics Emphasis on first measurements
Will coverUnderlying eventZ cross-sectionsConstraints on PDF’s using W’sGauge boson pair productionJet cross section measurementTop mass and cross section
Will not cover
EW precision measurement e.g. sin2W, W mass and widthTop precision physics e.g. polarisation, single top production etc ( talk by E. Chabert)B-physicsStandard Model Higgs Searches
Good understanding needed as this is the background for searches for New Physics ( talk by H.P. Beck)
HEPChile, Jan 2008 M. Wielers (RAL) 4
Measurement of Underlying EventModelling of underlying event necessary tool for high pT physics
Important ingredient for jet and lepton isolation, energy flow, jet tagging, etc
Underlying event uncertain at LHC, depends on
multiple interactions, PDFs, gluon radiation
Look at tracks in transverse region w.r.t. jet activity
pT leading jet (GeV)
<N
chg>
No charged particles in transverse region:pT>0.5 GeV and |η|<1
model dependency
Pythia tuned
Phojet
CDF data
HEPChile, Jan 2008 M. Wielers (RAL) 5
W and Z productionLHC is W/Z factory
(Wℓ) ~ 15nb ~106 events in L L dt = 100 pb-1
(Zℓℓ) ~ 1.5nb ~105 events in LL dt = 100 pb-1
First physics measurementsMeasurement of W/Z inclusive cross section as well as W/Z+jetConstraining PDF’sMeasurement of gauge boson pair productionMeasurement of triple gauge couplings
Useful to understand detector and performanceIn situ calibration of EM calorimeter using ZeeMomentum scale from Z, ZeeAlignment via ZExtraction of trigger and offline lepton identification efficiencies from Z decays ( see talk by V. Perez-Reale)Luminosity measurement
HEPChile, Jan 2008 M. Wielers (RAL) 6
Z cross sectionATLAS
(Preliminary)Xbb
jetsjetsW
ZjetjetWbWbtt
0.1(lumi)s)0.02(th.sy(ex.sys) 008.0(stat) 004.0)/( *
XZpp
Background Processes Background Uncertainty < 0.002
SelectionTwo reconstructed opposite charged isolated muons in |η|<2.5pT1>15 GeV, pT2>25 GeV
|91.2 GeV-Mµµ|<30 GeVExperimental systematics from
Efficiency extraction, momentum scale, misalignment, magnetic field knowledge, collision point uncertainty, underlying events, (pileup)
Theoretical uncertainties arising fromPDF choice, initial state radiation, pT effects (LO to NLO)
Plus 10% uncertainty from luminosity measurementExpected Precision for L L dt = 100 pb-1
HEPChile, Jan 2008 M. Wielers (RAL) 7
Constrain PDF using W→ℓProduction
Main (LO) contribution
Dominant sea-sea parton interactions at low x
At Q2=MW2 dominated by g qq
Low x gluon has large uncertaintyStudying W and Z production can increase our knowledge of gluon SF
PDF error sensitive to W→e rapidity distribution
Exp. uncertainty (dominated by systematics) sufficiently small to distinguish between different PDF setsPDF uncertainties only slightly degraded after detector simulation and selection cuts
CTEQ61 MRST02 ZEUS02
CTEQ61 MRST02 ZEUS02
e- rapidity e+ rapidity
GeneratedGenerated
Wdu Wud
x
Q2 (
Ge
V)
HEPChile, Jan 2008 M. Wielers (RAL) 8Normalisation free independent of luminosity
Include ATLAS W Rapidity “pseudo-data” in global PDF Fits
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 to simulate exp uncertainties) in the global ZEUS PDF fit (with Det.Gen. level correction).
low-x gluon distribution determined by shape parameter λ, xg(x) ~ x –λ :
BEFORE λ = -0.199 ± 0.046AFTER λ = -0.186 ± 0.027
41% error reduction (after ~ LLdt = 100 pb-1)
ZEUS-PDF AFTER including W data
e+ CTEQ6.1 pseudo-data
PDF constraining potential of ATLAS
ZEUS-PDF BEFORE including W data
e+ CTEQ6.1 pseudo-data
dd
(n
b)
HEPChile, Jan 2008 M. Wielers (RAL) 9
Di-boson productionProbes non abelian SU(2)xU(1) structure of SMTrilinear gauge boson couplings measured directly from ZW, WW, ZZ cross sectionCompare to SM predictions for
Cross section
Boson pT(V=W, Z, )
Production angleThese variables sensitive to modification to TGC structure from BSM effectsDi-boson production for Ldt = 1 fb-1
Z
Z
(SM)=58 pb
(SM)=128 pb
(SM)=17 pbs-channel suppressed by O(10-4)
Channel # events bkgs S/BZW 75.7 ZZ4ℓ, Z+jet,
Z, DY30.1
WW 358.7 DY, Z+jet, tt, ZW, Z, ZZ, W+jet
18.9
ZZ 13 Nearly bkg free, Z, tt, Zbb
0 bkg events
HEPChile, Jan 2008 M. Wielers (RAL) 10
Measurements with jets
Will jump immediately into new territory!
Inclusive jet spectra provide
Study of high-pT tails of cross-section sensitive to New Physics (e.g. quark compositeness)
Test of QCD (running of s)
di-jet cross section and properties (ET,η1,η2) constrain parton distribution function
Good understanding of errors needed!
Tevatron
LHC
QCD jet cross section
10 events with 100 pb-1
HEPChile, Jan 2008 M. Wielers (RAL) 11
Jet Reconstruction
Experimental factorsDead material in front of caloNon-sensitive regionsDetector inefficienciesNon-compensationLongitudinal leakageLateral shower sizeRead-out granularityNon-linearitiesElectronics noisePile-up noiseMagnetic field effects
Physics related factorsInitial state radiationFinal state radiationFragmentation (flavour dependence)Underlying eventMinimum bias pile-up
Effects due to jet finding algorithm
EfficiencyJet sizeTreatment of nearby jetsJet direction calculationJet corrections…
General task: Transform calorimeter response into four-vectors representing the properties of a jet/parton
HEPChile, Jan 2008 M. Wielers (RAL) 12
Inclusive jet cross sectionCross section measurement
Test of QCD
High pT region sensitive to new physics
Statistical Errors Naïve Error Estimation N = N
1% error at pT 1TeV with Ldt= 1 fb-1 in || < 3 For 3.2 < || < 5 error up to 10%
Experimental ErrorsLuminosity measurementJet Energy ScaleJet Resolution, UE, trigger efficiency …
NLOCTEQ6.1F=R=pt/2kt algorithm
pT[GeV]
d/
dpT (
nb/
Ge
V)
Jet pT spectra for different
HEPChile, Jan 2008 M. Wielers (RAL) 13
Jet cross section Jet Energy Scale:
If known to 1-2%, experimental errors not dominantFirst estimate of uncert. at start-up from pion test beam data
At EM scale: ~3% diff between data and MCAt had scale: 4-5%
Theoretical Error
scale uncertainties (Factorisation F,
Renormalisation R)
~ 10% uncert. at 1TeV, less belowPDF uncertainties
~ 15% uncert. at 1TeV, less below
Uncertainty Error on 1% 10%5% 30%10% 70%
EM scale
Had scale
HEPChile, Jan 2008 M. Wielers (RAL) 14
tt final states (LHC, Ldt = 100pb-1)
BR (tWb) ~ 100 %Weqq
Golden channel
(early physics, precision meas.)
• Full hadronic (37 K) : 6 jets
• Semileptonic (25 K) : ℓ + + 4jets, ℓ=e,• Dileptonic (4 K) : 2ℓ + 2 + 2jets
Top Production and Decay at LHC
σtt(LHC) ~ 830 ± 100 pb
For comparison:
Cross section LHC: 100 x Tevatron
Background LHC: 10 x Tevatron10%
90%
HEPChile, Jan 2008 M. Wielers (RAL) 15
Early Top mass measurement
Event selection:no b-tag yet on day-1 (might not be well understood)
Isolated lepton : pT> 20 GeV
missing ET > 20 GeV
4 jets pT> 40 GeV
3 jets with highest ∑ pT
Mjjj (GeV)
L=100 pb-1
Signal
W+n jets (Alpgen) + combinatorial
Eve
nts Without
b-tagATLAS preliminary
HEPChile, Jan 2008 M. Wielers (RAL) 16
Early Top measurements
Top mass with Ldt=30 pb-1:mtop ~ 3.2 GeV
Sys. error dominant: FSR, b-jet energy scale those 30 pb-1 must be well understood (ie actually need more data)
Top cross sectiontt measured with ~20% precision
Excellent samples forCommission b-tagging
Jet energy scale calibration using Wjj from tbW
HEPChile, Jan 2008 M. Wielers (RAL) 17
ConclusionsAs soon as collisions at 14 TeV happen, interesting SM physics available in recorded data First SM physics studies
Underlying event and min. bias productionW, Z (+jet) production:
understand detector performance (calib/alignment, eff extraction)Improve knowledge of PDFs
Di-boson production: probe gauge couplingJet cross-sectionPhoton cross sectionsMeasure top mass and cross section
also useful for hadronic calibration and b-taggingFocus here was on first measurements, many more topics studied
EW precision measurement e.g. sin2W, W mass and width
Top precision physics e.g. polarisation, single top production etcStandard Model Higgs Searches
Very good understanding of SM processes needed for searches for New Physics
HEPChile, Jan 2008 M. Wielers (RAL) 18
Backup
HEPChile, Jan 2008 M. Wielers (RAL) 19
Jet cross section Jet Energy Scale:
If known to 1-2%, experimental errors not dominantFirst estimate of uncert. at start-up from pion test beam data
At EM scale: ~3% diff between data and MCAt had scale: 4-5%
Uncertainty Error on 1% 10%5% 30%10% 70%
EM scale
Had scale
ATLAS preliminary
ATLAS preliminary
HEPChile, Jan 2008 M. Wielers (RAL) 20
Theoretical errors
Two main sources of theoretical errors (CDF) :
scale uncertainties
Factorisation F
Renormalisation R
PDF uncertaintiesScale uncertainties:
independent variation of F and R within pT
max/2<<2·pTmax
~ 10% uncertainty at 1TeVPDF uncertainties dominant
High pT PDF errors dominated by high x-gluon
At pT 1 TeV around 15% uncertainty