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ATLAS 实验 Single top 产生截面测量

ATLAS 实验 Single top 产生截面测量. 冯 存 峰. Single Top Physics. Provides a direct probe of the Wtb coupling Sensitive to many models of new physics (FCNC,W’, b’) Determines the quark mixing matrix element Vtb without assumptions about the number of quark generations.

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ATLAS 实验 Single top 产生截面测量

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  1. ATLAS实验Single top产生截面测量 冯 存 峰

  2. Single Top Physics • Provides a direct probe of the Wtb coupling • Sensitive to many models of new physics (FCNC,W’, b’) • Determines the quark mixing matrix element Vtb without assumptions about the number of quark generations Feynman diagrams of single-top quark production processes t-channel (64.6pb) Wt-channel (15.7pb) s-channel (4.6pb)

  3. ATLAS and LHC status High pileup machine <mu> was increased to ~15 after August technical stop • 2011 LHC proton-proton collisions with √s=7TeV have finished • ATLAS detector has collected about 5 fb-1 pp data • High-quality physics results will based on the (full) 2011 data • New software release  17.0.X • Understanding of the performance with this release now

  4. ATLAS Single top status Cut-based and MVA analysis!! • t-channel cross section measurement • s-channel cross section measurement • Wt-channel discovery • Search for new physics: FCNC, W’, b’… • Rel 16 papers: t-channel: Cut-based: ATL-COM-PHYS-2011-1402 (approved) NN: ATL-COM-PHYS-2011-1404 (approved) BDT: ATL-COM-PHYS-2011-1405 (conditional) Wt di-lepton: ATL-COM-PHYS-2011-1383 W’ : ATL-COM-PHYS-2011-1395 (conditional) FCNC: ATL-COM-PHYS-2011-693 (approved) Combination: ATL-COM-PHYS-2011-1407

  5. Contribution • T-channllepton+jets analysis • Wt-channel dilepton analysis • Event generation and validation • Background estimation • Multivariable analysis • Systematic uncertainties estimation

  6. Event generation and validation • Generate and validate the POWHEG generator for single top Wt-channel at parton level POWHEG Wt cross section validation https://indico.cern.ch/getFile.py/access?contribId=10&resId=0&materialId=slides&confId=115019

  7. Fake lepton background estimation • Measure the electron/muon real/fake rate for single-top group • Measure the real rate with Z->ll control samples • Measure the fake rate with QCD rich region with iteration method • Estimate the QCD background on t-channel analysis • Matrix method (2x2 matrix) • Estimate the QCD and W+jets background on Wt-channel • Matrix method (4x4 matrix, single fake and double fake) t-channel fakes Wt-channel fakes (~1% level)

  8. Multivariable analysis on t-channel • Uses 14 discriminating variables both on 2 and 3 jets bin • BDT response and overtraining check BDT training parameters ATL-COM-PHYS-2011-1405

  9. Multivariable analysis on Wt-channel • Training on 1 jet bin • Backgrounds include ttbar, Diboson, Zee, Zmm, Ztautau (no fakes, 1% level) • Normalization factors for Zee and Zmm are applied prior to training • Uses 22 discriminating variables BDT performance in 1-jet events

  10. Systematic uncertainties estimation • Estimate the systematic uncertainties on Wt dilepton analysis for EPS conference and Wt discovery paper • Lepton scale factors uncertainties • PDF uncertainties • Pileup uncertainties (ATLAS-CONF-2011-104)

  11. Look at Wt at parton level with MadGraph • Ttbar is the most difficult background to be rejected in Wt analysis • Have a fraction about 90% in Wt signal region • Large systematic uncertainties affect the Wt measurement • Look for powerful discriminating variables on parton level with MadGraph

  12. Track Jet study for low pt Jet • Ttbar pass Wt dilepton event selection in 1 jet bin for a un-found low pt jet • Try to find the low pt b-jet with jets reconstructed from tracks • Ttbar background could be reduced in Wt dilepton channel • Signifience 1.2 -> 1.5 (based on EPS results)

  13. Next year • T channel measurement almost ready now. • More work needed for Wt channel, but should be ready within next year. • New physics topic need to define.

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