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Bertrand Martin – for the LPSC group D ø France Strasbourg 11/26/07

e +. e -. p17 : tt → ee cross section p20 : W helicity prospects. Bertrand Martin – for the LPSC group D ø France Strasbourg 11/26/07. Outline. Status of the p17 ee selection success failures Around MET jet resolution Prospects for p20 W helicity

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Bertrand Martin – for the LPSC group D ø France Strasbourg 11/26/07

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  1. e+ e- p17 : tt→ ee cross section p20 : W helicity prospects Bertrand Martin – for the LPSC group Dø France Strasbourg 11/26/07

  2. Outline • Status of the p17 ee selection • success • failures • Around MET • jet resolution • Prospects for p20 W helicity • improvements in the dilepton channel

  3. e+ e- The dielectron signature • The main physics backgrounds : • Drell-Yan : Z/γ* + jets ee / ττ + jets • Diboson processes (WW , WZ , ZZ) • Instrumental backgrounds (fake leptons and fake MET) 2e top_tight pT > 15 GeV pT(1) > 30 GeV pT(2) > 20 GeV ≥ 2 high pT jets • Further cuts : • 15 < Mee< 80 GeV  MET > 40 GeV • or • Mee > 100 GeV  MET > 35 GeV • Sphericity > 0.15

  4. Status of σtt Preliminary results approved for Moriond E.W. 2007 : • 16 observed events , 9.5 expected ttbar ( for σ = 7 pb ) , 3.0 background • No optimization w.r.t. p14 analysis • Still statistics dominated. However, large systematics were also quoted : • EMID ~ 5.5 % per object • (ηdet , Φdet)  accounted for (scale factors) • ΔR(e,closest jet) , inst. lumi not corrected = systematics

  5. EMID systematics Assuming an efficiency depending on 3 variables : ε (η,Φ,ΔR) (η,Φ) and ΔR are independent variables Efficiencies from the(η,Φ) spc files areaveraged over the ΔR variation : Can’t make a 3D .spc file spc files may not be directly applied for ttbar as the “hidden” variable is integrated over the Zee spectrum…

  6. EMID cont’d Finally, after some more equations…, one gets : Average reco × ID efficiency of a Zee electron Can be used for ttbar now • Applying additional corrections on ttbar (ΔR , lumi) : • does not change much the shape of the distributions, • but rather the normalization : 2.3 % between full / (η,Φ)

  7. Post-Moriond life… Along with the final JES / SSR , came a new version of ReComputeMET , with a “right way” to propagate jet smearing to MET. See the effect for ee + ≥ 2 jets : Post-Moriond propagation Before Moriond ~ no propagation Numerous attempts to explain the MET oversmearing – in vain… “Some people” suspect that jet smearing might be too large.

  8. Jet Resolution Or “ How to absorb Mikko’s work in 1 week ? ” I don’t know… Mikko’s code developed for JCCA with special JES corrections Switch to JCCB and usual JES, to compare jet resolutions in data and MC + SSR Compute asymmetries in a dijet sample & extract jet resolution |y| < 0.4 Of course, not so simple…

  9. Jet resolution , bias & corrections Equivalent gaussian “core resolution” : Reco cut : 6 GeV pT imbalance expected at particle level ( missing ν , μ )

  10. MC + SSR from σgaussian MC + SSR from RMS Raw Jet Resolution in CC Data from σgaussian Data from RMS

  11. MC + SSR from σgaussian MC + SSR from RMS Raw Jet Resolution in CC MC – no SSR from σgaussian MC – no SSR from RMS

  12. Jet Resolution - status • This study is at its very beginning… • Can’t conclude anything yet : • low pT region driven by ksoftcorrections • – needs further checks • CC : needs punch-through tails correction at high pT • ICR : needs the double gaussian structure • EC : needs kbiascorrections • Dijet sample  access to high pT jets resolution • ( fits constrained above ~ 20 – 30 GeV ) • While dielectron + jets : rather low pT …

  13. Now, the future l+jets (1 fb-1) Dilepton (1 fb-1) Later… Now… l+jets 230 pb-1 The past… Dilepton 370 pb-1 l+jets 370 pb-1 ( Once we are done with p17 MET…) p20 prospects : get involved in the W Helicity measurement

  14. Extracting f+ with –log(L) l+jets Dilepton l+jets and dilepton results are marginally consistent (2.4σ stat) → Assumed it originates from a statistical fluctuation &proceed to combination What about adding another 1 fb-1 in each channel…

  15. p17 W helicity ν θ* t b W+ l+ … ttbar multivariate selection : likelihood discriminant … Reconstruction of the 2 W leptonic decays ( to find cosθ* ) : 2 neutrinos unknown momenta : kinematically underconstrained Algebraic resolution : needs mass assumptions ( MW , Mtop ) lepton / b– jet pairing ambiguity More powerful approaches : Decorrelated likelihood , N.N. , Boosted decision trees Also room for improvement (see next slides)

  16. Kinematics 6 final state particles (2b , 2l, 2v) 3 independent variables ( px , py , pz ) 12directly measured quantities ( 2b and 2l ) + 2 MW + 2 Mtop assumptions Last constraints in p17 : 18 unknowns → 2 unknowns left Quadratic equations give several solutions for neutrino momenta… p17 way of picking up the right solution :

  17. Alternative approach Try an approach a-la “neutrino weighting” ( top mass analysis ) Abandon the METx,y constraints • Make an assumption on the neutrinos rapidities ~ gaussian →Enough constraints now • Compute the neutrinos momenta • Then one can compare the expected METx,y with the observed MET (weight w) Should give better results than the current average… 

  18. Summary Preliminary p17 cross section measurement (1.04 fb-1) : OLD • Main analysis improvement : EMID systematics BUT • Cross section analysis “stuck” due to the MET oversmearing  Waiting for the jet resolution results • Optimization is the next step • … • Publication… • W helicity … • the end

  19. Backup slides

  20. Fake electron background • “Fake electron” : jet faking electron, or a non-isolated electron (b decay). Includes QCD, W+jets  l+jets, ttbar l+jets • Signal/fake separation : electron likelihood (~1 for signal, ~0 for background) • 2 steps : • Determination of likelihood shapes for signal-like & background-like samples • Likelihood fit to extract the background contamination in the analysis sample. Estimated from data Signal sample Zee (incl) Bkg sample (incl) • Signal sample : Zee , MET<15 • Background sample : same topology as the analysis sample, but require : 1 anti-isolatedelectron , MET < 15 Likelihood fit : determine ne and ns

  21. Fake MET background Fake rate ≥ 2 jets • Main background : Z/γ* + jets  ee + jets no real MET (no ν) but fake MET can appear due to resolution effects MET may not be accurately modeled in the MC • Fake rate measurement : samplekinematically close to Z+jets , with no real MET : γ + jets sample Estimated from data Major impact of jets on MET • Event yield calculation : from analysis sample in the low MET regime (reversed MET cut) Yield in the low MET regime

  22. tt cross section The cross section is extracted with a likelihood minimization : where The statistical error on σ arises from the likelihood minimization procedure Compared to p14 results (425 pb-1) : Preliminary p17 result … (1.04 fb-1)

  23. Z pT 0 jet Parameterization with generated Z pT Z pT 1 jet

  24. Z pT ≥ 2 jets # jets

  25. Jet 1 pT Jet 2 pT

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