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Data-driven Wln and Znn backgrounds Estimating the backgrounds in the control sample

This presentation discusses a data-driven approach for estimating the backgrounds in the control sample (CS) for W+lnu and Z->nunu processes. The method utilizes the kinematic acceptance, ID efficiency, and W/Z ratio to apply weights to each CS event. The benefits of this data-driven approach include reduced JES/JER systematic and no luminosity systematic. The presentation also explores the use of different control samples and uncertainties in the estimation.

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Data-driven Wln and Znn backgrounds Estimating the backgrounds in the control sample

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  1. Data-driven Wln and Znn backgroundsEstimating the backgrounds in the control sample Alex Pinder University of Oxford W/Z/Top informal meeting 29 March 2011

  2. Recap/motivation • Use Wen and Wmn control samples (CS) to get V+jets background (BG) normalisation and shape from data • To each CS event, apply a weight based on the CS kinematic acceptance, ID efficiency, and W/Z ratio • Benefits of DD approach: • Much reduced JES/JER systematic • No luminosity systematic • Measure shape as well as normalisation Weν BG Zνν BG Weν CS W/Z background (0-lepton)

  3. Backgrounds-within-backgrounds • The 1-lepton control sample is as follows: • 1 tight lepton (e/μ) as defined by SUSY obj defs • No other “medium” leptons • MET > 25 GeV • 40 GeV < MT < 80 GeV • Plus jets, Meff, Δφ etc as required by 0-lepton search • But not 100% Wlν: • Large amount of ttbar • Leptonic Wτν • Also QCD (in e channel) W/Z background (0-lepton)

  4. 2010 approach • All backgrounds in the control sample ignored, except ttbar • Subtract this using MC W/Z background (0-lepton)

  5. 2011 approach? • Have tried a simultaneous fit • Inspired by the 1-lepton SUSY analysis • Three control samples: • Wlν + Wτν (b-tag veto, tight lepton) WCS • Top (b-tag, tight lepton) TCS • QCD (medium-but-not tight electron) QCS • Control samples defined by object rather than kinematic cuts • Uncertainties from efficiencies and fake rates W/Z background (0-lepton)

  6. Method • For each signal region, count events in each control sample: • Can relate these to the true number of W/top/QCD by: • Matrix elements taken from simulation • Invert the matrix to find the result we want: W/Z background (0-lepton)

  7. Example: electron control sample W/Z background (0-lepton)

  8. Example: electron control sample W/Z background (0-lepton)

  9. Example: electron control sample W/Z background (0-lepton)

  10. Muon control sample • Muon channel has ~ 0 QCD contamination • Require only two control samples (and 2×2 matrix) W/Z background (0-lepton)

  11. Electron 2 control sample • Have another electron control sample where the electron-matched jet is not removed • Also electron isn’t “neutrinofied” • Used to estimate Weν b’ground where electron not identified • Again, QCD contamination is minimal W/Z background (0-lepton)

  12. Test on data (35 pb-1) WCSTCSQCS WCSTCS Electron (1) Muon • Reasonable results, consistent with the MC • Electron control sample (#1) – predicted W fraction in WCS: • Signal region 1: 0.94 • Signal region 2: 0.99 • Signal region 3: 0.92 • Signal region 4: 1.07  Because no events in TCS W/Z background (0-lepton)

  13. Wτν contamination • How to deal with this? • When estimating Wlν background, do not remove it • Get estimate of W  τν  lνν background for free • When estimating Zνν, remove it using MC • Fine if systematic error is huge W/Z background (0-lepton)

  14. Open questions • Are these the best control samples to use? • How best to estimate the effect of uncertainties? • B-tag efficiency / fake rate • Loose electron efficiency / fake rate • Tight electron efficiency / fake rate • Muon efficiency • Is there a simpler way? W/Z background (0-lepton)

  15. Final thoughts • Have presented a potential method to remove backgrounds from the W control samples • Uses a simultaneous fit to three sub-control samples • Seems to work rather well so far • But not clear yet how to handle systematics • People listening may have expertise here • When we estimate the W background, do we want to remove ttbar from the control sample? • If we leave it in, we estimate the ttbar  missed lepton background as well • Can subtract the W component to get an independent check on existing ttbar estimation techniques W/Z background (0-lepton)

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