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Update on Diphoton + MET Analysis

This document provides an update on the background sources and methods used in the Diphoton + MET analysis. It includes discussions on QCD, Electroweak, and Irreducible backgrounds, as well as results and conclusions from various studies.

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Update on Diphoton + MET Analysis

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  1. Update on Backgrounds to the Diphoton + MET Analysis Bruce Schumm, channeling Ben Auerbach (Argonne) and Osamu Jinnouchi (Tokyo Tech) UC Santa Cruz / SCIPP 16 July 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: 2x2=4 considered control samples, defined by data with one pseudophoton, with • Zero (QCDg) or one (QCDtg) tight isolated photon • 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. QCDtg+Iso close to tight-tight (signal) distribution  proxy for high MET • QCDtg provides good representation of tight-tight MET distribution • QCDg+Iso also looks good but statistics are low.

  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. Nominal QCD Background Control Region Study

  9. DPHI_JET_MET SYSTEMATIC We have performed the WP2 QCD background estimate without the \dphij cut, finding that the expected background rises from $0.90 \pm 0.35$ to $1.7 \pm 0.5$. It should be noted that the background would be expected to rise with the removal of the \dphij cut; if the `gg' distribution of Fig.~\ref{fig:dphij} is the correct distribution of the WP2 QCD background, this increase would be about 20\%, to 1.1 events. The observed value of 1.7 events is approximately 50% higher than this, which we interpret as an additional 50% systematic uncertainty on both the WP2 and MIS QCD background estimates.

  10. QCDg+Iso Comparison LocHadTopo Comparison

  11. SP1 Meff Extrapolations

  12. SP2 Meff Extrapolations

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

  14. 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

  15. e Fake Rate Results

  16. e Sample Statistics and EW Background Estimates

  17. 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

  18. 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

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

  20. 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

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

  22. Wrap-Up • Preliminary estimates of background completed • Note updated with new background studies (nearly done) ATL-COM-PHYS-2013-109 • Addressing comments from prior review (before p1328/p1181 MET changes that threw us back) • Starting to build toward request for unblinding • In the mean time, are developing limit-setting approach, and beginning to evaluate signal systematics

  23. 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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