1

S. Li1,2

Supervised by: Z. Zhao1 Y. Liu1 E. Monnier2

Center of Particle Physics and Technology, University of Science and Technology of China1

& Centre de Physique des Particules de Marseille, CNRS/IN2P32

Seminar@MPHY,USTC09/09/2011

Measurement of the Standard Model WW→lνlν Production Cross Section at

√s=7TeV in ATLAS experiment

Support Note & Conf. Note

2

1.02 fb-1 Support Note and Conf. Note available on CDS:ATL-COM-PHYS-2011-864: http://cdsweb.cern.ch/record/1366384/ATLAS-COM-CONF-2011-125:http://cdsweb.cern.ch/record/1366687/

Outline• Introduction

• Event Selection

• Background Estimations

• Systematic uncertainties

• 1.02fb-1 Results (full EPS datasets)

• Conclusion

3

Introduction

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Motivation:Irreducible background for H→W+W- searchPossible approach to new physics through aTGCs

WW→lνlν characteristics Dileptonic decay channels allow signal extraction from large BG Isolated high pT di-lepton final states are considered: ee, eµ, µµ Cascaded W→τ+X → e/µ+X also included in addition to the promt W(e or µ) decays Experiment signature in WW:

two OS isolated high pT leptons plus large Etmiss

Backgrounds:

Top (both single and pair production): real leptons and MET use data-driven normalisation

W+jets: 1 real + 1 fake lepton, real MET use data-driven shape and normalisation

Drell-Yan Z/g*: real leptons + fake MET use data-driven systematic uncertainty

Other Dibosons(WZ, ZZ, Wɣ/Zɣ) based on MC

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2010 results 8 WW observed (1 ee, 2 µµ, 5 eµ) with 34pb-1

6.85±0.07±0.66 signal Vs 1.68±0.37±0.42 bgd 3σ evidence for WW processesDocuments: Conf. Note, INT Note, PRL draft (accepted)

With now 1.02fb-1 more precise measurements expected !

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Object Selection• Selection strategy similar to 2010 analysis: 2010, 2011• Optimization driven by increased Luminosity, worse pileup effects and better S/B

Muon definition: STACO Combined muon, pT>20 GeV, |η|<2.4, z0, d0 significance, other MCP

recommended cuts Isolation: pT(cone20)/pT< 0.1

Electron definition: Tight, ET>20GeV (leading electron ET>25 GeV for ee and eμ), |η|<2.47 w/o

crack region, z0, d0 significance, OTX cleaning cut (acceptance loss weighted in MC)

Isolation: Etcone30_corrected<4GeV (electron energy leakage and pileup corrections inside the isolation cone applied)

Jet definition (antiKt4topo jet): EM+JES pT>30GeV and |η|<4.5)

MET definition: MET_LocHadTopo with |η|<4.5, lepton energy smearing/rescaling corrections as well as Mutag Muon correction are propogated

Latest MCP/Egamma Energy Rescaling/smearings/eff SFs are applied.

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Event Selection• General pre-selection: Combine Muons and Egamma streams with duplication removed Official GRL: WZjets all channels, period D-H Object overlap removal(e/e, e/µ, e/jet) MET cleaning (reject larError events, reject SumET<0 events & events with jets in LAr Hole in

2011) PV (at least 3 associated tracks for the first vertex, Pileup Reweighting applied in MC using

official package PileupReweighting-00-00-13 accounting both in-time and out-time pileup in 2011)

Trigger: EF_e20_medium (ee), EF_mu18_MG||EF_mu40_MSonly_barrel (μμ), OR of both triggers (eμ), trigger matching applied

• Channel-specific selection: Exactly two prompt, isolated, opposite charge leptons with pT>20GeV Standard offline physics object –trigger objects matching cone size (0.15 for electrons and 0.1

for muons)

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Event Selection Remove Drell-Yan contribution: |Mll-MZ|>15GeV for ee and μμ Mll>15GeV for ee and μμ, and Mll> 10 GeV for eμ

Further remove Drell-Yan and QCD multi-jet contributions: METRel > 45,40 GeV for µµ and ee; > 25 GeV for eµ

Remove top contribution: Jet veto: no jets of ET > 30 GeV within |η| < 4.5

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10

ee: METrel>40GeV reject DY

µµ: METrel>45GeVreject DY

eµ: METrel>25GeVreject DY

Jet Veto:reject Top

Cut flow with 1.02fb-1 data

414 candidates observed compared with 8 candidates last year

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MC signal/bgd expectation(1.02fb-1)

S+B 59.5 87.4 233.0 380.0

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All backgrounds are estimated

using MC simulation in this

table

Jet multiplicity after Z-veto and METrel

13

ee

eµ

µµ

Combined

WW signal region

DATA/MC comparison after Jet Veto

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pT(l+l-)

MT(l+l-, Etmiss)

Δφ(l+l-)

PT(l+l-, Etmiss)

Data-driven Drell-Yan background estimation

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Data-driven Drell-Yan background estimation• DY background: lepton or jet energy not well measured • Data-driven method: (ATLAS note: ATL-COM-PHYS-2010-176)

Assume the fraction of DY events after the METrel cut is the same inside or outside of the Z mass window

The non-DY backgrounds can be estimated either using MC simulation or eμ events

e/μ acceptance and efficiency differences accounted using Z→ee/μμ events

• MC closure test performed: good agreement between the input and the estimated non-DY background has been observed

• Drell-Yan background determined fromAlpgen MC prediction and syst. uncertaintiesdetermined by comparing METrel distribution in DATA/MC within Z peak

Estimated DY yields:

Data-driven results:15.8±1.55±1.7 16.1±1.39±2.7 13.5±2.34±1.9

16

METrel in Z control region

ee µµ

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Data-driven W+jets background estimation

Data-driven W+jets background treatment• Wjets backgrounds estimated by scaling the number of events in the W+jet control sample,

Nlepton ID + Jet-Rich ID with a measured fake factor:

• Fake factors(for both e and µ) measured from dijet sample driven from data:

• Way-side jet pT requirement assigned to different sub-samples and lepton fake factors are calculated respectively with corresponding syst. addressed

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e fake factor

e fake factor

µ fake factor

W+jets background estimation & Same Sign region validation

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• Final estimation(total contribution: 50.5±4.8(stat)±14.7(syst)):

• Systematics: trigger bias, way-side jet pT sub-sample deviation, sample dependence (W+jet vs dijet), real lepton contaminations, etc.

• 30% systematics assigned for the fake factor for both electron and muon• Validated in Same Sign Control Region:

• Xchecked by Matrix method with good agreement (see backup)

• Fake factors are different between Wjets and dijets sample, which is the major syst. Uncertainty of the method. Take the deviation as the corresponding syst.(40%)

• Fake factors measured from HF samples(bbcc->e) look a bit surprising. Haven’t assigned yet. Should not be a big deal because of smaller cross section.

20

Syst. study: Sample Dependence

Zoom in

• Using JF17 pythia MC sample by removing signal component at truth level• Not dramatic systematic eff observed.• Statistics in numerator sample are still not that much• Assign overall deviation~10% conservatively as trigger bias systematic uncertainty

21

Syst. study: Trigger Bias

No Trigger EF_e20_etcut

Fake Rate 0.018 0.021

• Almost negligible : take 8% conservatively

Syst. study: Electron Fake period dependence

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Syst.Source

Trigger bias Period dependence

SampleDependence

Xsection sum

uncertainty 10% 8% 40% / 42%

ee channel: 5.26±0.39±2.21, eµ channel: 7.08±0.57±2.97 MC extraction effect is dramatic in Fake Rate estimation but not that much in Wjet

background yields correction(so neither pseudo nor pure data-driven) Compatible with John’s results for periodD-H(1fb-1) those syst. need to be further studied:

Fake rate w/o MC subtraction

Fake rate w/ MCsubtraction

Electron Fake rate 0.0371±0.0002±0.0156 0.0257±0.0001±0.0108

Channel ee eµ(e-fake only)

ee eµ(e-fake only)

Wjets backgroundw/o MC correction

8.30±0.56±3.49

13.35±0.71±5.61

5.75±0.39±2.42

9.22±0.49±3.87

Wjets backgroundw/ MC correction

7.60±0.57±3.19

10.25±0.83±4.30

5.26±0.39±2.21

7.08±0.57±2.97

Wjets background estimation: ee and eµ channel

23

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Data-driven top background estimation

Data-driven top background estimation• Full jet veto suppress top background(single top and ttbar)• A semi-data-driven method:

Njet≥2: control sample Assume Events fraction with Njet= 0 and Njet≥2 similar in data and MC (residual SM

backgrounds for Njet≥2 in data removed using MC simulation)

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Njet= 0 top events: 58.6±2.1(stat)±22.3(syst)JES (37%) dominant

MC estimation: 56.7 events

Cross checked with b-tagged top control sample (see backup)

WW signal region

Other diboson backgrounds and final results Purely MC prediction (normalized to 1 fb−1). Zγ excluded from total contribution due to overlap with Z+jet backgrounds.

Final results:

S+B 63.9 98.8 239.6 402.27/19/2011 S. Li (USTC/CPPM) 26

Systematics• Syst. Sources accounted: (see backup for detailed summary)• Lepton Systematic:

Lepton reconstruction and identification efficiencies Lepton isolation efficiency Lepton energy/momentum scaling and smearing

• Jet Veto• MET syst.(in-time and out-time pileup included)• PDF uncertainty.• Dedicated syst. uncertainty from Data-driven background estimation• The luminosity uncertainty (3.7%, listed separately)

• The systematic uncertainty of the total cross section measurement is 13.4%, which includes the signal acceptance uncertainty ( ) of 6.8% and uncertainty of the background estimation ( ) of 11.5%. The systematic error is calculated using propagation:

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• Fiducialcross section measured in the following phase space:(Aww and Cww denotation and detail treatment see backup)

Lepton pT>20 GeV, |η|<2.4 for µ and |η|<1.37 or 1.52<|η|<2.47 for electron Jet pT>30 GeV, |η|<4.5 and ΔR(e, jet) >0.3 ee channel: MET>40 GeV, mee>15 GeVand |m-mZ|>15 GeV μμ channel: MET>45 GeV, mμμ>15 GeVand |m-mZ|>15 GeV eμ channel: MET>25 GeV, meμ>10 GeV

WW fiducial cross section

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Expectation: 46±3 pb and 5.84±0.37 pb

Conclusion WW cross section measurement in dileptonic channel using 1.02fb-1 data 414 candidates observed in 2011 compared to 8 in 2010 Data-driven methods used for almost all the backgrounds (Drell-Yan, top

and W+jets) Detailed studies done on systematic uncertainties for both signal and

backgrounds 13.4% overall systematic uncertainty and 3.7% for Luminosity accounted

separately Measured xsection 48.2±4.0(stat.)±6.4(syst.)±1.8(lumi.) is consistent with

the theoretical prediction of 46±3 pb. Both inclusive and fiducial cross sections measured for three channels

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Ongoing work….Wrapping up baseline selection that has been updated

and frozen recently post EPS-HEP conference:Cut optimization aiming for a better s/b:

Further suppress top backgrounds:B-jet veto with b-tagging technique to suppress the top

backgroundsLower the jet-pt threshold

CP recommendation updates(eff SFs, lepton smearing)Pending studies after 1fb-1 publication:

DY background treatment:Combine the track and calo based MET to further suppress

the DY background(CMS recommendation)Pt(ll) and dφ(ll) possible approach

Full dataset update with 2011 DATA and new MC production

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Backup

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DATA & MC samples• PeriodD –H with L=1.02 fb-1 (Period B not

used), 3.7% Lumi. Uncertianty

• Official GRL: Wzjets all channels

• Unprescaled single lepton triggers:EF_e20_medium, EF_mu18_MG||EF_mu40_MSonly_barrel

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MC samples with p591 tags:Full list

Event Selection in 2011(changes Highlighted)

• Event selection strategy is similar to 2010 analysis: 2010, 2011• Optimization is driven by increased Luminosity and worse pileup effects as well as aiming for a

better S/B ratio.

• Object selection: Muon definition: Combined muon, STACO, pT>20 GeV, |η|<2.4, z0, d0 significance, other MCP recommended cuts

(pT_MS and pT_ID fractional difference used in 2010 analysis, adding more ID requirements in 2011)

Isolation: pT(cone20)/pT< 0.1

Electron definition: Tight, ET>20GeV (leading electron ET>25 GeV for ee and eμ), |η|<2.47 w/o crack region, z0, d0

significance, OTX cleaning cut (acceptance loss weighted in MC) Isolation: Etcone30_corrected < 4 GeV(electron energy leakage and pileup corrections inside the

isolation cone applied) (Etcone30 < 6 GeV used in 2010 analysis)

Jet definition (antiKt4topo jet): EM+JES pT>30GeV and |η|<4.5) (EM+JES pT>20 GeV and |η|<3 used in 2010 analysis)

MET definition: MET_LocHadTopo with |η|<4.5, lepton energy smearing/rescaling corrections as well as Mutag Muon correction are propogated

• Latest MCP/Egamma Energy Rescaling/smearings/eff SFs are applied.7/19/2011 S. Li (USTC/CPPM) 33

Event Selection in 2011(changes Highlighted)

• General preselection: Combine Muons and Egamma streams with duplication removed DQ Object overlap removal(e/e, e/µ, e/jet) MET cleaning (reject larError events, reject SumET<0 events & events with jets in LAr Hole in

2011) PV (at least 3 associated tracks for the first vertex, Pileup Reweighting applied in MC using

official package PileupReweighting-00-00-13 accounting both in-time and out-time pileup in 2011)

• Channel-specific selection: Trigger: EF_e20_medium (ee), EF_mu18_MG||EF_mu40_MSonly_barrel (μμ), OR of both

triggers (eμ), trigger matching applied correspondingly Standard offline physics object –trigger objects matching cone size (0.15 for electrons and 0.1

for muons) Exactly two prompt, isolated, opposite charge leptons with pT>20 GeV

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Event Selection in 2011(changes Highlighted)

Remove Drell-Yan contribution: |Mll-MZ|>15GeV for ee and μμ (|Mll-MZ|>10GeV in 2010 analysis) Mll>15GeV for ee and μμ, and Mll> 10 GeV for eμ (no cut for eμ in 2010)

Further remove Drell-Yan and QCD multi-jet contributions:• METRel > 45,40 GeV for µµ and ee (40 and 40 in 2011); • > 25 GeV for eµ (20 in 2011)

Remove top contribution:• Jet veto: no jets of ET > 30 GeV within |η| < 4.5

(changes of jet definiton w.r.t. 2010)7/19/2011 S. Li (USTC/CPPM) 35

Wjets Estimation: background yields

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Wjets Estimation: Same Sign yields

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WW cross section measurement strategy• The W+W− fiducial cross section and total cross section are determined from the three

dilepton channels (WW→eνeν, μνμν and eνμν) by maximizing log-likelihood functions shown in the following equations:

• the coefficients AWW andCWW are definedas follows: AWW denotes the acceptance for the W+W− decays under consideration, defined as the

fraction of decays satisfying the geometrical and kinematical constraints at the generator level (fiducial acceptance). This quantity can only be determined from Monte-Carlo simulations. It is defined here after the decay leptons emit photons via QED final state radiation; photons within a DR < 0.1 cone are added back to the deca leptons (“dressed” leptons).

CWW denotes the ratios between the total number of generated events which pass the final selection requirements after reconstruction and the total number of generated events within the fiducial acceptance. This corrections factor includes the efficiencies for triggering, reconstructing, and identifying the W+W− decays falling within the acceptance.

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Systematic uncertainties on WW signal acceptance

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W+jet estimation using Matrix Method

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Top background estimation from top control sample using b-tagging

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