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Backgrounds to the Diphoton + MET Analysis

Backgrounds to the Diphoton + MET Analysis. Bruce Schumm, channeling Ben Auerbach (Argonne) and Osamu Jinnouchi (Tokyo Tech) UC Santa Cruz / SCIPP 04 June 2013 SUSY Background Forum Meeting. Three background sources: QCD (gamma-gamma, gamma-jet, jet-jet; no primary MET)

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Backgrounds to the Diphoton + MET Analysis

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  1. Backgrounds to the Diphoton + MET Analysis Bruce Schumm, channeling Ben Auerbach (Argonne) and Osamu Jinnouchi (Tokyo Tech) UC Santa Cruz / SCIPP 04 June 2013 SUSY Background Forum Meeting

  2. Three background sources: • QCD (gamma-gamma, gamma-jet, jet-jet; no primary MET) • Electroweak (W-gamma, ttbar; usually with e fake) • Irreducible (W, Z; with neutrinos in decay) • Start with QCD background • Recall: 2x2x2=8 considered control samples, defined by data with one pseudophoton, with • Zero (QCDg) or one (QCDtg) tight isolated photon • 50 or 75 GeV ET cut on pseudophoton • pseudophoton isolated or not • Nominal MET is standard MetRefFinal, but also look at LocHadTopo as cross-check • Which of all of these are useful?

  3. First: MC performance on tight-tight sample  LocHadTopo has slightly larger tails than MetRefFinal

  4. Statistics are limited: must make use of 50 GeV ET cut, no-iso if possible • QCDtg+Iso close to tight-tight (signal) distribution  proxy for high MET • QCDtg provides good representation of tight-tight MET distribution

  5. For LocHadTopo, both QCDtg and QCDg seen to provide a good representation of the tight-tight MET distribution (again, using QCDtg-Iso as a proxy at high MET)

  6. Next: Signal Regions • We define five signal regions, for: • Strong production, high and low bino mass (SP1,SP2) • Weak production, high and low bino mass (WP1,WP2) • Choose MET cut to suppress backgrounds (MIS)

  7. Direct Background Estimate Methodology e.g. for signal region WP2 Estimate = C*(A/B) Nominal control sample is QCDtg_50_noIso This is METRefFinal; can also look at QCDtg and QCDg of LocHadTopo A B C MetRefFinal MetRefFinal

  8. Direct Background Results and Systematics LocHadTopo QCDtg and QCDg provide good systematic check MetRefFinal QCDg expected to be a bit high; ignore SP1, SP2 statistics a bit poor  complement with extrapolation technique

  9. SP1 Meff Extrapolations

  10. SP2 Meff Extrapolations

  11. Combining all the above information yields the following overall result for QCD background (See Note for justifications…)

  12. Electroweak Backgrounds (W, ttbar, etc.) • ~75% involve e fake; much of remainder incorporated • in QCD backgrounds • Reconstruct e sample; scale by measured e fake rate • +/- 25% uncertainty from non e fake processes • +/- 10% uncertainty from fake rate measurement

  13. e Fake Rate Results

  14. e Sample Statistics and EW Background Estimates

  15. Irreducible Backgrounds • Z; Z • Small contribution • NLO K-factor 2.0 +/- 0.3 (well understood) •  Estimate directly from MC • W; Wl • Larger contribution • NLO K-factor 3.0 +/- 3.0 • Dominant background systematic •  Constrain with new data-driven study

  16. W K Factor LO “radiation zero” eliminated at NLO Grows with hardness of radiation  Rapidly-varying function of W system recoil http://arxiv.org/pdf/1103.4613v1.pdf

  17. Can we constrain the W K factor with an lgg (l = e,) sample? Ben Auerbach

  18. Choose study region to be • 50 < MET < 250 (leave MIS signal region blind) • PT(l) > 100 Nexpected = 7.4 (6.5 W) Nobserved = 7.0  K factor of 3.0 +\- 1.2 Ben Auerbach

  19. Irreducible background results And then putting it all together…

  20. Wrap-Up • Preliminary estimates of background completed • Updating the Note with new background studies (nearly done) • Addressing comments from prior review (before p1328/p1181 MET changes that threw us back) • When end is in sight, will request Ed Board meeting to discuss unblinding • In the mean time, are developing limit-setting approach, and beginning to evaluate signal systematics

  21. MET Issues • Are latest (“post-Moriond”?) object definitions included in p3128 EGamma10NoTauLoose MET? • We will need in any case to assemble our own “fluctuated” EGamma10NoTauLoose in order to do systematic studies • But for now, background estimates largely insensitive to MET systematics (data-driven), so could use intrinsic p1328 variable if “approved” • Will definitely need to be able to assemble EGamma10NoTauLoose from scratch soon though.

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