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MET Significance

MET Significance. New Physics Neutralinos (SUSY) Gravitinos (Xtra dim) Heavy photon (Littlest Higgs with T-parity) Unparticles, hidden valley stuff, etc Something else…. E T has many possible sources. Old Physics and Detector Issues Jet Mismeasurement (resolution)

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MET Significance

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  1. MET Significance New Physics • Neutralinos (SUSY) • Gravitinos (Xtra dim) • Heavy photon (Littlest Higgs with T-parity) • Unparticles, hidden valley stuff, etc • Something else… ET has many possible sources Old Physics and Detector Issues • Jet Mismeasurement (resolution) • Underlying event/ unclustered energy • Neutrinos in b, c jets • Znn, W nl • Muons • Pile up (at hi lumi) • Hot/dead cells, cracks, dead regions We want to identify New Physics and suppress the rest Jim Alexander

  2. L( ET event) D0 idea: Use Likelihood Method(B. Knuteson et al, D0 Note 3629) Example: distinguish measurement error from real MET Probable Mismeasurement Possible MET candidate MET Fluctuations can cause this MET Fluctuations are unlikely to cause this Can we construct a likelihood function to distinguish these cases evt by evt? Jim Alexander

  3. D0 Study(A. Schwartzman, D0 Note ????) • Signal: W+jets Bkg: QCD ≥ 2jets. • For 50% signal eff (example) bkg eff is 7.5% with ordinary MET cut, • Bkg eff is 3.8% with MET signif cut. D0 study MET Significance cut Signal Efficiency Simple MET cut Example MET Significance improves bkg suppression x2 Background Efficiency Jim Alexander

  4. MET Significance for CMS… • MET Significance is on this group’s official to-do list • First steps taken by Koji Terashi (now gone to ATLAS…) • Our plan: • Start with one issue: jet mismeasurement. (Save others for later…) • Construct toy Monte Carlo • explore general features of the problem in a simple environment • develop structure, code, math • Move to CSA07 / Spring07 Monte Carlo samples: QCD, W nl • Add other features one by one: • b, c jets • underlying event • detector flaws • etc • Develop general tool for use by all • Apply to a first physics measurement: W nl Today’s talk Jim Alexander

  5. Jet Mismeasurement • Likelihood functions for combining mis-measured vectors. • If you add two vectors (think of these as jet vectors, with resolution smearing) • where is the resultant likely to end up? • This is a 2d likelihood function, L(ex,ey), which can be used to plot likelihood contours in the ex,ey plane. Jim Alexander

  6. Gaussian case is elegant • If the primordial likelihood functions are gaussian, then everything is analytical. • Likelihood shape is simply characterized: V • Gaussians convolve to give more gaussians f1 -f2 2 If not gaussians, linear combos of gaussians is just as easy to use. Jim Alexander

  7. Definition of the Significance • Based on jet measurement uncertainties (black ellipses), calculate the uncertainty of the ET vector…  red ellipses • Define S = ln • In this illustration, 3.5s, S = 6.1 4 3.5 3 2 Contours shown: -2lnL = -2lnLmin+ {1,4,9,…} 1 ( ) L(ET=observed) L(ET=0) Jim Alexander

  8. Angular effects are automatic • In the example from p2, what makes the MET on the right look anomalous is largely its direction • Error ellipses automatically take this into account. > 4s MET < 1s MET Jim Alexander

  9. Toy Monte Carlo Studies • Contents of the toy Monte Carlo: • 2-5 “jets” per event, poisson distributed <2.5> • Jet “ET” is flat between 10 and 100 • “Measured” Jet ET vector is smeared by • s(ET)/ ET = 144%/Sqrt(ET) • s(f) = 2O • ~100 lines of code: VERY SIMPLE • Generated ET can be set to 0 or to any desired value. We will call ET=0 samples “QCD”, and ET=40 samples, “W nl ” Warning: Despite the nomenclature,there is no physics in this! This is totally made up…. don’t over-interpret! Purpose is to explore general features and methods. Jim Alexander

  10. Example from Toy Monte Carlo • Blue arrows are the “true” underlying jet vectors. • Dots are results of 10k smearings of the same true underlying event • Contours are calculated values of -2lnL • In this case,true MET was fixed at 20. (Green arrow) “Event Display” Jim Alexander

  11. Compare “QCD” and “W nl” events in the Toy Monte Carlo MET Significance Generate 10k evts with ET=0 (QCD) and 10k evts with ET=40 (W nl) “QCD” events: ET(true)=0 “Wnl” events: ET(true)=40 Significance Jim Alexander

  12. “Signal” Eff and “Background” Effin Toy Monte Carlo “Signal” = “W” events “Background” = “QCD” events Compare cut on MET significance with simple cut on MET itself. MET Significance cut Simple MET cut D0 Plot, from p3. Jim Alexander

  13. Summary & Plans • So far: • Toy Monte Carlo only • Jet mismeasurement is the only effect considered (resolution smearing) • General structure and approach clarified by this exercise • Next: • QCD and EWK CMSSW files: expose method to “real” physics • Add more components to the likelihood: semileptonics in b, c jets, underlying event, etc etc. • Eventually: apply to W, W+jets • Lots to do! • Participants… • Xin Shi * • Kyle Story * • Deb Mohapatra ** * grad student * post doc Jim Alexander

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