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Top physics at startup

Top physics at startup. CMS France – 1 er avril 2010 Anne-Catherine Le Bihan. Outline. Top group strategy Cross-section measurement in the dilepton channel Cross-section measurement in the lepton+ jet channel Top mass. Top physics @ 7 TeV. Top group strategy.

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Top physics at startup

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  1. Top physics at startup CMS France – 1er avril 2010 Anne-Catherine Le Bihan

  2. Outline • Top group strategy • Cross-section measurement in the dilepton channel • Cross-section measurement in the lepton+ jet channel • Top mass

  3. Top physics @ 7 TeV

  4. Top group strategy https://twiki.cern.ch/twiki/pub/CMS/TWikiTopQuark/Top2010Strategy_v08.pdf • A. First signals with up to 10 pb-1 • First candidates, control distributions… (3-5 pb-1) Observation and evidence, first cross section measurements (5-10 pb-1) • B. Refined measurements with 50-500pb-1 • More precise bkg estimation, multi-variate techniques.. • Differential cross sections • Tau and fully hadronic channels • JES and b-eff. using top events • Single top observation • top mass and other properties, … • C. High precision Top Quark physics • Precise m(top), helicity, spin correlations, rare decays, … (see talk by F.P. Schilling physics plenary 18/03 )

  5. Goals for 5-10 pb-1 (ICHEP) • Observation and cross-sectionmeasurements : • di-lepton, muon+jets and electron+jets channels • highest sensitivity in dilepton channel : low bkg and data-driven techniques https://twiki.cern.ch/twiki/bien/viewauth/CMS/TOP4ICHEP • Cross-section measurements as well as ratios : • no luminosity uncertainty • (partial) lepton uncertainty cancellation • dilepton channel : Z ratio lepton+jet channel : W ratio (see talk by F.P. Schilling physics plenary 18/03)

  6. Dilepton cross-section

  7. Analysis strategy • Reference analyses @ 10 TeV : PAS TOP-09-002, AN 2009/47, AN 2009/50, AN 2009/51 • Baseline selection (MET based) : • 2 opp.ch. isolated leptons, Z mass veto, >=2 jets, MET • alternative selections with b-tagging and track jets • Backgrounds : DY : data driven W+jets : matrix method QCD : matrix method Dibosons : from simulation Single top : from simulation  ~ 14 pb

  8. Top dilepton cross-section : 5 pb-1 UCSB,UCSD,FNAL • For Njet>=2, expect : 21.4 signal events 2 bkg events (single-top, dibosons) • Prospects to increase signal yield : • Inclusion of Njet=1 bin + use projected METto reduce Z→ • Lower pT on second lepton 10<pT<20 GeV (projected MET = MET if <90)

  9. Top dilepton cross-section : 100 pb-1 • Use of pfMET or tcMet : • very efficient to get rid of Drell-Yan events with fake MET • similar performance for ttbar events • Expected S/B improvement : Cumulative efficiency of MET cut Z→ TTbar efficiency Calo MET TC MET PFlow MET MET MET

  10. Top dilepton cross-section : 100 pb-1 Strasbourg • Expect robust b-tagging at startup • PAS analysis w/o MET cut and medium b-tagging (trackcounting) : • more sensitive to Z/W + heavy flavours • but similar results to the use of pfMET • Profile likelihood ratio method implemented to extract cross-section :  = 165 ± 20 (stat+sys) pb (sys = JES,b-tagging, lepton id/iso, residual bkg)

  11. Top dilepton cross-section : 100 pb-1 Pedrame Bargassa • Topological selection w/o b-tagging MET and (MET) need to be understood • Variable selection using : S/√(S+B+sys2), so far sys = JES(±10%) → xsec measurement S/ √B → enriched ttbar sample for b-tag studies • e final state  final state • Similar performance to btag analysis 2 isolated leptons, ST=pT(e)+pT()+MET>110, Njet(pT>30)>1 S/B~5 L= 100pb-1 2 isolated leptons, (jet1,MET) vs MET, Z mass veto, Njet(pT>30)>1 S/B~5 L = 100pb-1

  12. Lepton isolation and fake rate AN-2008/085 • Use matrix method to estimate the fake isolated leptons • Define 3 sub-samples for each level of lepton isolation (loose, medium, tight) Ns = events containing 2 real leptons (signal like) NW = events containing 1 real lepton (W+jets like) NQCD = events containing 2 fake isolated leptons (QCD like) • System of 3 equations allows to solve the 3 unknowns Nsl, NWl, NQCDl: • Leading to the estimated number of signal, W+jet events, and QCD events ~20% error estimate for the W+jets background L=100pb-1

  13. Z+jets background Strasbourg • Select events with MET<30 GeV, dominated by Z+jets events → fit dilepton invariant mass Mll by a breitwigner • Select events with MET>30 GeV, containing ttbar + Z+jets events → use e channel to describe TTbar Mll shape → fit Mll by a breitwigner + landau like function From the Z/DY estimates inside the Z mass window (high and low MET), rescale the residual Z/DY contribution at high MET outside the Z mass window : Systematics ~23-30% : Z/DY statistics outside Z mass region, Z peak position shift (dibosons), Z/DY Mll shape difference @ high & low MET…

  14. Lepton + jets cross-section

  15. Analysis strategy • Reference analyses @ 10 TeV : PAS TOP-09-003, AN 2009/080, AN 2009/084 PAS TOP-09-004 and AN 2009/075, AN 2009/080 • Baseline analysis : • Simple event selection: exactly one isolated lepton (pT>20 GeV) , >= 4 jets, no MET, no btag • W+jets and QCD background estimated from data  ~ 56 pb

  16. Lepton + jet cross-section • Template fit to M3or  to extract Ntt → M3 = inv. mass of three jets with highest ΣpT •  + jet channel : Estimated stat. error at L=5 pb-1 : ~45% • e + jet channel : Estimated stat. error at L=5 pb-1 : ~40% L= 5 pb-1 L=5 pb-1

  17. QCD background •  + jet channel : Estimated using ABCD method : (IP) and  isolation are independent NA/NB = NC/ND in signal region NA=NBNC/ND • e + jet channel : Increased level of QCD background, because of photon conversions ! Different options to reduce QCD background : • conversion removal, MET cut, |(e)|<1.44 or HT(lep)>110 GeV Reliso extrapolation method : side band region fit to combined (tracker + calo) isolation variable

  18. W+jets background AN-2009/078 Jet multiplicity with prediction for events leading to charge asymmetry (ECA) • Use the W charge asymmetry • Measure the difference of lepton to anti-lepton in candidate events • Estimated number of W+jets background : (N++N-)data = R±(N+-N-)data R±= (NW+ +NW-)/(NW+ -NW-) Estimated precision is 30% in 100 pb-1, PDF systematics to be evaluated

  19. Top mass at startup

  20. Top mass in dilepton channel • 2 neutrinos escaping : 6 unknowns 5 constraints : → kinematically underconstrained • Additional constraint : MET = pT + pT Select lepton+jet combinations with softest M(tt) Gaussian fit to top mass : ~16 GeV/c2 with L=10 pb-1 • Event weight for each hypothized value of Mtop Select per event Mtop which gives the max weight Expect  ~23 GeV/c2 with L=20 pb-1 - LIP | | Brown

  21. Summary • Top group aims for following results for ICHEP : • Most data driven background estimate methods useable with ~10 pb-1 (?) • Task force to provide priority analyses

  22. Backup

  23. Single top in +jets • Analysis strategy : one isolated  and lepton veto two jets far from the  one b jet, 2nd jet must fail loose b-tag MT>50 GeV (on-shell W, anti-QCD) • 200 pb-1 @10 TeV : S/B~0.45 (S=102, ttbar =136, tW=22, QCD=12, W+x = 50 ) • Single top xsec : fit to polarization angle and/or charge asymmetry • Sensitivity ~2.7 for L=200 pb-1 @ 10 TeV Rescaling of xsec → ~4 for L= 1 fb-1 @ 7 TeV • Startup : validation time

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