Nov 11, 2007Jim Alexanderp1 /13 MET Significance New Physics Neutralinos (SUSY) Gravitinos (Xtra dim) Heavy photon (Littlest Higgs with T-parity) Unparticles, hidden valley stuff, etc Something else… Old Physics and Detector Issues Jet Mismeasurement (resolution) Underlying event/ unclustered energy Neutrinos in b, c jets Z , W l Muons Pile up (at hi lumi) Hot/dead cells, cracks, dead regions E T has many possible sources We want to identify New Physics and suppress the rest
Nov 11, 2007Jim Alexanderp2 /13 D0 idea: Use Likelihood Method (B. Knuteson et al, D0 Note 3629) Probable MismeasurementPossible MET candidate Example: distinguish measurement error from real MET MET Fluctuations can cause this Fluctuations are unlikely to cause this L ( E T event ) Can we construct a likelihood function to distinguish these cases evt by evt?
Nov 11, 2007Jim Alexanderp3 /13 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 improves bkg suppression x2 Simple MET cut MET Significance cut Background Efficiency Signal Efficiency Example
Nov 11, 2007Jim Alexanderp4 /13 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 l –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 l Today’s talk
Nov 11, 2007Jim Alexanderp5 /13 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( x, y ), which can be used to plot likelihood contours in the x, y plane.
Nov 11, 2007Jim Alexanderp6 /13 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 2 11 2 If not gaussians, linear combos of gaussians is just as easy to use.
Nov 11, 2007Jim Alexanderp7 /13 Definition of the Significance Based on jet measurement uncertainties (black ellipses), calculate the uncertainty of the E T vector… red ellipses Define S = ln In this illustration, 3.5 S = 6.1 Contours shown: -2lnL = -2lnL min + {1,4,9,…} L(E T =observed) L(E T =0) ) (
Nov 11, 2007Jim Alexanderp8 /13 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. MET
Nov 11, 2007Jim Alexanderp9 /13 Toy Monte Carlo Studies Contents of the toy Monte Carlo: –2-5 “jets” per event, poisson distributed –Jet “E T ” is flat between 10 and 100 –“Measured” Jet E T vector is smeared by (E T )/ E T = 144%/Sqrt(E T ) ( ) = 2 O –~100 lines of code: VERY SIMPLE Generated E T can be set to 0 or to any desired value. We will call E T =0 samples “QCD”, and E T =40 samples, “W l ” 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.
Nov 11, 2007Jim Alexanderp10 /13 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”
Nov 11, 2007Jim Alexanderp11 /13 Compare “QCD” and “W l ” events in the Toy Monte Carlo MET Significance “QCD” events: E T (true)=0 “W l ” events: E T (true)=40 Significance Generate 10k evts with E T =0 (QCD) and 10k evts with E T =40 (W l )
Nov 11, 2007Jim Alexanderp12 /13 “Signal” Eff and “Background” Eff in 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.
Nov 11, 2007Jim Alexanderp13 /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