Top physics at startup CMS France – 1 er 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 7 TeV √s (TeV) xsec(pb) TTbar165 (NNLO) 385 (NLO) 852 NLO) Single top (t channel NLO) Single top (tW channel NLO) Dibosons (NLO) W→l + jets (NLO) Z→ll + jets (NLO, m ll >10 GeV) ~6100 QCD (LO) (P T hat >100 GeV)

4 Top group strategy 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 pb-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-section measurements :  di-lepton, muon+jets and electron+jets channels  highest sensitivity in dilepton channel : low bkg and data-driven techniques 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 10 TeV : PAS TOP , 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 (projected MET = MET if  <90) For N jet >=2, expect : 21.4 signal events 2 bkg events (single-top, dibosons) Prospects to increase signal yield :  Inclusion of N jet =1 bin + use projected MET to reduce Z→   Lower pT on second lepton 10<pT<20 GeV

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 : Z→  TTbar Cumulative efficiency of MET cut Calo MET TC MET PFlow MET MET efficiency

10 Top dilepton cross-section : 100 pb-1 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) Strasbourg main bkgAnalysis w/o MET and medium b-tag (evts/pb) Baseline analysis w/ pfMET (evts/pb)  DY & single top signal : 0.55 Bkg : 0.17 signal : 0.5 Bkg : 0.08 eeDY & single top signal : 0.36 Bkg : 0.1 signal : 0.35 Bkg : 0.05 ee single topsignal : 1.2 Bkg : 0.06 signal : 1.2 Bkg : 0.11

11 Top dilepton cross-section : 100 pb-1 Topological selection w/o b-tagging MET and  (MET) need to be understood Variable selection using : S/√(S+B+sys 2 ), 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 Pedrame Bargassa 2 isolated leptons,  (jet1,MET) vs MET, Z mass veto, Njet(pT>30)>1 S/B~5 L = 100pb-1 2 isolated leptons, S T =pT(e)+pT(  )+MET>110, Njet(pT>30)>1 S/B~5 L = 100pb-1

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

13 Z+jets background Select events with MET<30 GeV, dominated by Z+jets events → fit dilepton invariant mass M ll by a breitwigner Select events with MET>30 GeV, containing ttbar + Z+jets events → use e  channel to describe TTbar M ll shape → fit M ll 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 M ll shape high & low MET… Strasbourg

14 Lepton + jets cross-section

15 Analysis strategy Reference 10 TeV : PAS TOP , AN 2009/080, AN 2009/084 PAS TOP 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 M3 or   to extract N tt → 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

17 QCD background   + jet channel : Estimated using ABCD method :  (IP  ) and  isolation are independent N A /N B = N C /N D in signal region N A =N B N C /N D  e + jet channel : Increased level of QCD background, because of photon conversions ! Different options to reduce QCD background : conversion removal, MET cut, |  (e)| 110 GeV Reliso extrapolation method : side band region fit to combined (tracker + calo) isolation variable

18 W+jets background 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±= (N W+ +N W- )/(N W+ -N W- ) Jet multiplicity with prediction for events leading to charge asymmetry (ECA) Estimated precision is 30% in 100 pb-1, PDF systematics to be evaluated AN-2009/078

19 Top mass at startup

20 2 neutrinos escaping : 6 unknowns 5 constraints : → kinematically underconstrained Additional constraint : MET = p T + p T 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 M top Select per event M top which gives the max weight Expect  ~23 GeV/c2 with L=20 pb-1 Top mass in dilepton channel LIP Brown - | |

21 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 Summary Channelrec. Top Events / pb-1 Dilepton~2-4 Muon+Jets~7 Electron+Jets~5

22 Backup

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26 Single top in  +jets Analysis strategy : one isolated  and lepton veto two jets far from the  one b jet, 2 nd jet must fail loose b-tag M T >50 GeV (on-shell W, anti-QCD) 200 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= TeV Rescaling of xsec → ~4  for L= 1 7 TeV Startup : validation time