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Soft Electron Tagging

Soft Electron Tagging. John Paul Chou DOE Review 2008 Thursday, August 14, 2008. Overview. Soft Electron b-tagging (SLT e ) Algorithm Simulation Applying the Tagger Top Pair Cross Section W+Charm Cross Section Top Charge. Soft Electron b-Tagging.

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Soft Electron Tagging

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  1. Soft Electron Tagging John Paul Chou DOE Review 2008 Thursday, August 14, 2008

  2. Overview • Soft Electron b-tagging (SLTe) • Algorithm • Simulation • Applying the Tagger • Top Pair Cross Section • W+Charm Cross Section • Top Charge DOE Review 2008 -- JPC

  3. Soft Electron b-Tagging • Look for soft electrons from heavy flavor decay • BF(b→eνX) ~ 10% • BF(b→c→eνX) ~ 10% • ~35% of top events have a soft electron from HF • High PT Electrons • Charged track • Associated with electromagnetic shower • Little to no hadronic energy • No other tracks nearby (i.e. isolated) • Question: How do we ID electrons when the electron is embedded in a jet? • Specifications: • b-jets from top decay are high PT and dense, but the tagger must work over a couple decades in scale • To be useful, we need ~100:1 e:π/k/p separation DOE Review 2008 -- JPC

  4. Soft Electron Algorithm • Begin by using calorimeter quantities: • E/P: Electromagnetic energy on par with track P • Had/Em: Cluster dominated by EM component • Not finely segmented: very sensitive to local environment • Main engine of the tagger: CES • Wire and strip chambers located at approximately shower maximum within the EM calorimeter • Measures transverse EM shower profile in two orthogonal directions • Advantage: less environmental dependence • Finely segmented: ~2-3 mm resolution • Measure position and shape of shower (not amplitude) • Disadvantage: not well modeled in MC • Combine CES elements into a likelihood and cut DOE Review 2008 -- JPC

  5. Conversions • Conversion electrons are also a significant background • Conversions dominate signal at low PT • ~3 times as many candidate tracks in top are from conversion photons than from HF • Two techniques for removal • Locate partner electron track geometrically • But partner track may not be reconstructed • Low PT threshold for candidate electrons • Asymmetric energy sharing • Use material interaction behavior to identify conversions • Extrapolate track through silicon detector • Use double-sided silicon layers • Reject tracks with more than three layers expecting hits on each side but having none • 70% efficiency for low PT conversion electrons in jets • 7% over-efficiency to misidentify prompts as conversions DOE Review 2008 -- JPC

  6. Simulation B enriched dijets • Calorimeter variables are well-modeled, but CES variables are not • Parameterize data and apply it to MC • Consider the effects of • Kinematics (PT) • geometry (η) • and local environment (track isolation) • Cross check as many places as we can • (Z’s, b-jets, etc.) • Tag Matrix – predicts the tagging rate of electrons • Use conversion electrons as a template • Correct tag matrix for jet environment since most conversion electrons are not in jets • Fake Matrix – predicts the tagging rate of non-electrons (a.k.a. fakes) • Use tracks from generic jets (20/50/70/100) as a template • Correct Fake matrix for real electron contamination (HF, conversions, Dalitz, etc.) • Use conversion (over-)efficiency Scale Factors to adjust MC • Measured in generic jet data DOE Review 2008 -- JPC

  7. Top Production Cross Section • The cross section is a cross check of the tagger itself • We extrapolated the SLTe tagger into a High PT, dense environment • Second order effects could come into play: should check that it works • Lepton+Jets event selection • 1 isolated, high PT lepton (electron or muon) with PT/ET > 20 GeV • ≥ 3 jets (Corrected ET > 20 GeV, |η| < 2.0) • Missing ET > 30 GeV • Scalar sum of transverse energy, HT > 250 GeV • ≥1 soft electron tag • Missing ET, HT, and SLTe likelihood cut optimized for total uncertainty Background method similar to secvtx xs Luminosity = 1.7 fb-1 Acceptance*Efficiency DOE Review 2008 -- JPC

  8. Cross Section First measurement of cross section with soft electron tags in run II Moving towards PRD DOE Review 2008 -- JPC

  9. Kinematics DOE Review 2008 -- JPC

  10. W+Charm • Measure W+Charm production cross section • Important background in W+1,2 jet sample (Higgs, etc.) • Charges of W lepton and soft lepton are anti-correlated • Count Opposite Sign (OS) minus Same Sign (SS) events • Use 1, 2 jet bin of top cross section measurement as control region • Plan: precision measurement combining soft electron and soft muon channels with 3.0 fb-1 DOE Review 2008 -- JPC

  11. Top Charge (I) • Theoretically, exotic top model with mass near 175 GeV/c2 could have -4/3 charge • Decays into W- and b, instead of W+ and b • PRD59(091503) • Measurement techniques • Measure associated photon production cross section • Not practical at Tevatron, but possible at LHC • Use jet charge algorithm to determine b-jet charge • Kinematic fitter determines which jets are “leptonic b” and “hadronic b” • Assign top charge based on b-jet charge and lepton charge • high efficiency, low purity • Exotic quark model excluded at 87% confidence level • Use soft lepton tagging instead of jet charge • Want to maximize εD2 (Dilution, D≡2P-1) • Lower efficiency, higher purity: competitive, orthogonal measurement DOE Review 2008 -- JPC

  12. Top Charge (II) • Increase purity by selecting high PT soft lepton tags • Optimizing on separate channels triples measurement significance: • SecVtx + SLT (different jets) • Use SLT tag in kinematic fitter to enhance fitter purity • SecVtx + SLT (same jet) • Low purity high efficiency channel • 2 SecVtx + SLT (in same a SecVtx jet) • Best εD2 • Electrons are better than muons! • Can tune S/B with different operating points • Less efficiency but higher fake rejection at high PT ~80% purity at PT>8 GeV/c DOE Review 2008 -- JPC

  13. Summary • We have implemented a soft electron tagger at CDF • Tagger used to measure top cross section • Godparent committee has been chosen • W+charm measurement is in the works • Top charge measurement with soft lepton tags shows promise DOE Review 2008 -- JPC

  14. Backup Slides

  15. Figures of Merit • Environment overstates HF electron tagging efficiency • In top events, tagging efficiency for HF electrons ~40% per track (L1) • Electron contamination overstates non-electron tagging rate • In top events, per track non-electron tag rate ~0.5% per track (L1) Per track tagging efficiency for conversion electrons and tracks in generic jets DOE Review 2008 -- JPC

  16. Schematic Data Tag/Fake Matrix Depend on choice of Likelihood cut MC Both DOE Review 2008 -- JPC

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