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W → e n and W →e n +jets : a data-driven selection method

W → e n and W →e n +jets : a data-driven selection method. By Alessandro Tricoli. In Collaboration with M. Wielers (RAL) D. Prieur (RAL) T. Guillemin (LAPP). ATLAS RAL Physics Meeting 11 th August 2008. Overview. This work is part of the CSC notes

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W → e n and W →e n +jets : a data-driven selection method

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  1. W→en and W→en +jets:a data-driven selection method By Alessandro Tricoli In Collaboration with M. Wielers (RAL) D. Prieur (RAL) T. Guillemin (LAPP) ATLAS RAL Physics Meeting 11th August 2008

  2. Overview This work is part of the CSC notes W and Z cross section measurements and W/Z + jets cross section measurements • W/Z incl. cross section measurement: • Study for early (1031 lumi) and later (1033 lumi) running. • Standard Cut-based selection • Data-driven selection method • Application to W+jets cross section measurement • Conclusions Alessandro Tricoli, RAL

  3. PART IInclusive W→encross section measurement Alessandro Tricoli, RAL

  4. Data Samples • Reconstruction ATHENA v12.0.6 • Offline Analysis: AOD & customAAN-tuples • use panda to submit analysis jobs • Signal: Pythia PythiaWenu DS no. 5104 No Evt. 187650 • Backgrounds: Pythia PythiaZee DS no. 5144 No Evt. 109900 PythiaWtaunu DS no. 5106 No Evt. 158350 JF17 DS no. 5802 No Evt. 4890386 Alessandro Tricoli, RAL

  5. Cut-based Selection • On-line/off-line electron ID: • early data (50 pb-1): • trigger e20 (1031 menu) [Monika W.’s code to extract 1031 decision from AOD] • ele-ID: medium isem • higher lumi (1 fb-1) • trigger e25i (1033 menu) • ele-ID: tight isem • W selection: • acceptance and cracks removal cuts: h=1.37-1.52 and |h|<2.4 • Electron ET>25 GeV • Missing-ET >25 GeV • Jet Veto: Jets ET<30 GeV (Tower CONE DR=0.7, ele-jet overlap removed) • no recoil PT cut applied: no improvement on B/S only exception QCD, due to poor stat.: no trigger sel., but corrected normalization Alessandro Tricoli, RAL

  6. Cut-based Event Selection Early data: e20, medium isem, lumi 50 pb-1 Higher Lumi: e25i, tight isem, lumi 1 fb-1 50 pb-1 Bkg contamination can be rather small after cuts, but large uncertainties on QCD background (both theoretical and experimental) This Cut-Based selection completely relies on Monte Carlo Alessandro Tricoli, RAL

  7. Data-driven W Event Selection • We want to • Minimise our dependence on MCs • Measure backgrounds from data • Use simple and reliable method • Problems with Cut-Based: • difficult estimation of QCD background after all cuts: • not enough MC stat. to trust MC (plenty of data) • amount and shape under W peak difficult to estimate: • ETmiss and jet Veto remove most of backgrounds, but leave ‘some’ under W peak => can only rely on MC estimate Alessandro Tricoli, RAL

  8. Data-driven W Event Selection • Proposed W Selection: • Electron Trigger selection (e20 or e25i) • Electron Identification (medium or tight) • acceptance and cracks removal cuts: h=1.37-1.52 and |h|<2.4 • Electron ET>25 GeV • Missing-ET >25 GeV • Jet Veto: jet ET<30 GeV Etmis and jet veto remove most of backgrounds, loose knowledge of shapes • Z->ee Background Removal: • cut on Mee invariant mass • QCD Background Removal: • fit ETmiss spectrum in high purity QCD sample • subtract QCD spectrum away Replaced by: Alessandro Tricoli, RAL

  9. R(had/EM) in HEC/EMEC R(had/EM) in FCal MT Before Z->ee Removal MT After Z->ee Removal W -> en W -> en QCD QCD Z -> e-e+ Z -> e-e+ W -> tn W -> tn Z->ee Removal QCD Jet TruthEle Matching Jet TruthEle Matching Jet QCD Jet Z→ee contamination reduced from B/S=24.5% to 3.0% Negligible effect on W→en, W→tn, QCD and their shapes Alessandro Tricoli, RAL

  10. Data-driven QCD subtraction Method • QCD Subtraction Method: Fit QCD background from a pure QCD sample to access distr. tails under W peak • Find a pure QCD Control Sample: • fake-photons: photon trigger (g20) and look at photon container • W→ensignal swamped by QCD events (98% purely QCD) • Fit ETmissfrom QCD Control Sample in range ETmiss>10 GeV • ETmiss shape for photon and electron samples must be similar • Normalise Fit to electron-sample in side band (10 GeV< ETmiss <22.5 GeV) • Subtract normalised fit under W peak:ETmiss>22.5 GeV Alessandro Tricoli, RAL

  11. Photon/Electron Shape Study W -> en QCD Z -> e-e+ W -> tn Data-Driven QCD SubtractionPure QCD Control Sample • QCD Control Sample: • g20 trigger, medium photon isem, ET>25 GeV, acceptance and crack cuts • 98% purely QCD • for ETmiss>30 GeV small jacobian peak from W→en : ~19% of QCD • for ETmiss>40 GeV small jacobian peak from W→en : ~42% of QCD • next slide explains hot to get rid of this contamination • ETmiss shape of QCD in Control Sample same as in Signal Sample QCD only High purity QCD sample Similar shape in electron and photon samples for ETmiss>10 GeV Alessandro Tricoli, RAL

  12. etcone/ETphot etcone/ETphot >0.15 W -> en High purity QCD sample W -> en QCD QCD Z -> e-e+ W -> tn Data-Driven QCD SubtractionW→en contamination in QCD Control Sample (ETmiss>30 GeV) Anti-Isolation cut can remove most of this W→en contamination: Tried etcone (Calo Isolation in DR<0.45 Cone) etcone/ETphot [GeV] • W→en contamination reduction: • ETmiss>30 GeV: from ~19% to ~5% • ETmiss>40 GeV: from ~42% to ~6.4% • Improvement with data: • make etcone cut tighter • add track anti-isol cut No visible distortions On ETmiss shape of QCD by Anti-Isolation cut W→en contamination can be also tested with data by an iterative signal+bkg fit (fit signal in electron sample and bkd in photon sample) Alessandro Tricoli, RAL

  13. Signal region Side-band For norm. Normalised Fit with error band compared to QCD fake-electron data points Data-driven QCD subtractionFit and Normalization • Fit ETmiss in QCD Control Sample • exp(ax)*(1+bx2) best fit • fit stable with different parametrisations and fit ranges Fit compared to QCD Control Sample data points (photon sample) • Normalisation of Fit to electron sample • normalisation in side band ETmiss=10-22.5 GeV • dominated by MC stat. uncertainty Alessandro Tricoli, RAL

  14. W -> en Z -> e-e+ W -> tn Data-driven QCD subtractionQCD subtractionresults ETmiss spectrum after QCD subtraction compared to W→en, W→tn, Z→ee (no QCD) • QCD subtraction accuracy 0.1%-1.2% • uncertainty dominated by MC statistics (~4%) Early Data: e20, medium isem, lumi 50 pb-1 Higher Lumi: e25i, tight isem, lumi 1 fb-1 Alessandro Tricoli, RAL

  15. Cross section measurement • Background contaminations and Acceptance after data-driven selection • Global Trigger and Electron Eff. from Tag&Probe (Ellie D., Mike F., Maria F., Guillaume K.) • Lumi uncertainty assumed: 10% at low lumi, 5% at higher lumi • Theory (NNLO, CTEQ6.1M) 20.5 nb: good agreement with our results!! Alessandro Tricoli, RAL

  16. PART IIW→en + Jetscross section measurement Alessandro Tricoli, RAL

  17. Data Samples • Reconstruction ATHENA v12.0.6 • Offline Analysis: AOD & customAAN-tuples • use panda to submit analysis jobs • W+jets Analysis Signal: Alpgen+Jimmy • Generator Filter: N jets≥ 1 with PT>20 GeV AlpgenJimmyWenuNp0 DS no. 6101 No Evt. 30000 AlpgenJimmyWenuNp1 DS no. 6102 No Evt. 39100 AlpgenJimmyWenuNp2 DS no. 6103 No Evt. 48670 AlpgenJimmyWenuNp3 DS no. 6104 No Evt. 48400 AlpgenJimmyWenuNp4 DS no. 6105 No Evt. 31700 AlpgenJimmyWenuNp5 DS no. 6106 No Evt. 9700 • Backgrounds: Pythia PythiaZee DS no. 5144 No Evt. 109900 PythiaWtaunu DS no. 5106 No Evt. 158350 ttbar_Pythia DS no. 5568 No Evt. 324800 JF17 DS no. 5802 No Evt. 4890386 Alessandro Tricoli, RAL

  18. W +JetsCut-based Selection • W Selection: • Trigger menu e25i applied: • one isolated e±, tuned for efficiently select e± with ET> 25 GeV • only exception QCD, due to poor stat.: no trigger sel. • Electron Identification: Medium • cracks removal h=1.37-1.52 and |h|<2.4 • Electron ET>25 GeV • Trask Isolation (to reduce QCD): • tracks in DR<0.2 cone around electron: n.track≤4 and SpT≤4GeV • Missing-ET >25 GeV • Jets: • Jet Algorithm: Tower CONE DR=0.4 • ET>20 GeV • ele-jet overlap removal: DR=0.4 Alessandro Tricoli, RAL

  19. W -> en Z -> e-e+ W -> tn QCD ttbar W -> en + Jetscut-based selection Lumi = 1 fb-1 Cumulative Jet Multiplicity Jet Multiplicity W Selection: e25i Trig+ isEM medium+ ETele >25GeV + Track Isol. + Miss ET >25GeV Jet Selection: Tower CONE 04 Jet Et > 20 GeV Ele-jet overlap removal: DR=0.4 • QCD dominates at lower Jet Mult. • ttbar dominates at larger Jet Multi. Alessandro Tricoli, RAL

  20. W -> en Z -> e-e+ W -> tn QCD ttbar W -> en and backgrounds Lumi = 1 fb-1 after e25i + isEM + ETele >25GeV + Track Isol+ Miss ET >25GeV • ETmiss cut removes most of Zee bkg and large part of QCD • QCD tails under the W MT peak • QCD tails difficult to estimate (limited MC stat, large uncertainty on x-sect etc.) Data-driven method necessary Alessandro Tricoli, RAL

  21. W -> en Z -> e-e+ W -> tn QCD ttbar Data-driven After Zee Removal Before Zee removal Lumi = 1 fb-1 • Follow same procedure as in inclusive measurement: • Z→ee Removal event-by-event • QCD subtraction for each jet multiplicity • x-sect as function of jet mult. with QCD subtracted away • In addition data-driven selection for ttbar background: • Work in progress with student, Maria Fiascaris (Oxford) Alessandro Tricoli, RAL

  22. Conclusions • Standard Cut-based W Selection (a la TDR): • large uncertainties on amount and shape of QCD background • Alternative Data-Driven W selection: • replace ETmiss and jet veto cuts with • explicit Z->ee removal by cutting on e-e pair invariant mass • good rejection of Z->ee background is possible from 25% to ~3% • fit of QCD ETmiss distribution • QCD background subtraction: accuracy ≤1% (±4% MC stat uncertainty) • accurate inclusive W cross section meas. both with early data and higher lumi. • Apply similar Data-Driven selection on W+jets • also ttbar data-driven background removal (work in progress with oxford student, Maria) • Can we apply similar technique on electron trigger? • work in progress with Ellie Davies (Bath-RAL student) and Monika Wielers (RAL) Alessandro Tricoli, RAL

  23. EXTRAS Alessandro Tricoli, RAL

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