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ZZ Production Cross-Section Measurement with ll +met final state

ZZ Production Cross-Section Measurement with ll +met final state. Lailin Xu On Behalf of the ZZ->llvv analysis working group. 8/21/2014. 1. Analysis Documentation. Support Note and Paper Draft in CDS: ATL-COM-PHYS-2012-147 (submitted on Feb. 11)

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ZZ Production Cross-Section Measurement with ll +met final state

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  1. ZZ Production Cross-Section Measurement with ll+met final state Lailin Xu On Behalf of the ZZ->llvv analysis working group 8/21/2014 1

  2. Analysis Documentation Support Note and Paper Draft in CDS: ATL-COM-PHYS-2012-147(submitted on Feb. 11) ATLAS-COM-CONF-2012-025(submitted on Feb. 18)

  3. EB members: Carolina Deluca Silberberg Masaya Ishino Gideon Bella Daniel Fournier Review comments: Editor Board and Reviews 18 discussions, all comments are answered Thanks for EB's careful reading and valuable suggestions!

  4. ZZ Production and Decays • ZZ production • SM ZZ (non-resonance) total cross section: 6.5+0.3-0.2 pb • -using MSTW2008 PDF (60 GeV Mz < 116 GeV) • -gg contribution included qq ZZ gg  ZZ • ZZ decays Study of ZZ signal from ee/mm + MET final state Br (Z nn) =20%, Br(Z ll) = 0.0336 (l = e, or m) Br(ZZ llnn)=00336*0.2*4=0.0268 8/21/2014 4

  5. Experimental signature for ZZ  llnn: two isolated high PT leptons(same flavor, opposite charge) and large MET Major backgrounds: WW, WZ, Top, Z+jets, W+jets, Wg(g*) ZZ cross section measurement Analysis Overview Total: Fiducial:

  6. Data: Period D-M: 4.7fb-1 GRL: WZjets_allchannels_DtoM.xml trigger: MC modeling: qqZZ: MC@NLO (scale up by 6% to include gg contribution) WW: MC@NLO WZ: Herwig tt: MC@NLO; Wt,single top: AcerMC W/Z+jets: Alpgen Wg: MadGraph+Pythia; Wg*: Alpgen Data and MCSamples 8/21/2014 6

  7. From common Electroweak definitions: Objects Selection Two isolated high pT leptons+MET with jet-veto • Muons: • Pt>20GeV • Staco Combined • |eta|<2.4 • MCP tracking cuts • Isolation: etcone30/pt<0.14, ptcone30/pt < 0.15 • Impact parameters: |Z0|<1, |d0/sig(d0)|<3 • Electrons: • Pt>20GeV, leading Pt>25GeV • Medium++ • |eta|<2.47 • Isolation: etcone30/et<0.14 • ptcone30/et<0.13 • Impact parameter: |Z0|<1 • Jets: • AntiKt4TopoEM jets • Pt>25GeV • |eta|<4.5 • |JVF|>0.75 • MET: • RefFinal Official corrections for physics objects are applied.

  8. ZZ llnnEvent Selection (1) Mll>15GeV and |Mll-Mz|<15GeV ee mm To select the Z  ll events.

  9. ZZ llnnEvent Selection (2) Axial-MET > 80GeV Axial-MET: MET projection along the Z direction ee mm To remove Drell-Yan events.

  10. ZZ llnnEvent Selection (3) Jet Veto ee mm To remove Top events.

  11. ZZ llnnEvent Selection (4) |MET-Pt(ll)|/Pt(ll) < 0.6 ee mm To further remove Drell-Yan events.

  12. Final ZZ llnn Candidates Combined ee+mm channels: Mll MET Candidates: 33 in ee channel, 45 in mm channel

  13. Final ZZ eenn Candidates Mee Pt(ee) axial-MET MET

  14. Final ZZ mmnn Candidates Mmm Pt(mm) axial-MET MET

  15. Estimated Background Contributions Data-driven methods used to estimate the WW+Top, W/Z+jets background

  16. em WW/Top/ttBackground Estimation • Use the em data to estimate the the tt/WW/Wt/tt backgrounds: • tt/WW/Wt/tt  ee:mm:em= 1:1:2 • In the Z control region, em events are dominanted by tt/WW/Wt/tt Formular: MC subtraction:

  17. Z+jets Background Estimation Using g+jets events • Z+jets background has large uncertainties in signal region, due to MET resolution and poor MC statistics • Data driven method: Reweighting kinematic distributions of g+jet data to model Z+jet 8/21/2014 17

  18. W+jets Background Estimation Matrix method Data efficiency:e fake rate:e fake rate:m efficiency:m 8/21/2014 18

  19. Signal Acceptance (AZZ) Using MC@NLO sample to study acceptance • Fiducial cuts (to determine AZZ): to electron • Azz systematic uncertainties (Include gg AZZ uncertainty) % % % % % % % % % % 8/21/2014 19

  20. AZZ From Different MC Generators All the MC results are consistent within ~3% except PowhegBox, which gives the Azz value lower than that from MC@NLO by 13.7%. WW group also sees a large difference of Aww between MC@NLO and Alpgen-- this needs to further understanding. We include this difference in the systematic uncertainties in Azz for now.

  21. Czz Systematic Uncertainties Muon Electron MET Jet Veto Trigger 8/21/2014 21

  22. Azz and Czz and Associated Uncertainties Summary of Observation and Predictions 8/21/2014 22

  23. Maximum Likelihood Fitting to Determine Cross Section • The profile likelihood function L: For fiducial cross section: For total cross section, append to the likelihood function. Number of signal events:

  24. Measured Fiducial and Total Cross-sections Fitting results: Consistent with the SM prediction of total cross-section of

  25. Summary • Observed total 78 ZZ llnn candidate events with an observation significance of 4.3s • The first LHC measurement of SM ZZ production with ZZ->llvv final state • MC agree well with data • Background estimation using DD or MC with detailed uncertainty analysis • Signal acceptance are corrected using control samples, and the uncertainties are studied with MC simulations • Measured total cross-section is in good agreement with the SM NLO cross-section prediction 8/21/2014 25

  26. mm ee Pt(ll) in Mz Window

  27. MET in Mz Window ee mm 8/21/2014 27

  28. Acceptance curve after all the cuts except axial-MET cut

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