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 Feynman diagrams of single-top quark production processes t-channel (64.6pb) Wt-channel (15.7pb) s-channel (4.6pb)

ATLAS and LHC status 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 High pileup machine <mu> was increased to ~15 after August technical stop

ATLAS Single top status t-channel cross section measurement s-channel cross section measurement Wt-channel discovery Search for new physics: FCNC, W’, b’… Rel 16 papers: Cut-based and MVA analysis!! 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

Contribution T-channl lepton+jets analysis Wt-channel dilepton analysis Event generation and validation Background estimation Multivariable analysis Systematic uncertainties estimation

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

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)

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

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

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)

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

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)

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.