Standard Model Physics in ATLAS Monika Wielers RAL.

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Standard Model Physics in ATLAS Monika Wielers RAL
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Transcript of Standard Model Physics in ATLAS Monika Wielers RAL.

Page 1: Standard Model Physics in ATLAS Monika Wielers RAL.

Standard Model Physics in ATLAS

Monika Wielers

RAL

Page 2: 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

Page 3: Standard Model Physics in ATLAS Monika Wielers RAL.

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)

Page 4: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 5: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 6: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 7: Standard Model Physics in ATLAS Monika Wielers RAL.

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)

Page 8: Standard Model Physics in ATLAS Monika Wielers RAL.

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)

Page 9: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 10: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 11: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 12: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 13: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 14: Standard Model Physics in ATLAS Monika Wielers RAL.

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%

Page 15: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 16: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 17: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 18: Standard Model Physics in ATLAS Monika Wielers RAL.

HEPChile, Jan 2008 M. Wielers (RAL) 18

Backup

Page 19: Standard Model Physics in ATLAS Monika Wielers RAL.

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

Page 20: Standard Model Physics in ATLAS Monika Wielers RAL.

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