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Measurements of Single-Top Production at the LHC

Measurements of Single-Top Production at the LHC. Wolfgang Wagner Bergische Universität Wuppertal on behalf of the ATLAS and CMS collaborations. Top Physics Workshop Sant Feliu de Guixols , Spain September 28, 2011. Content: Introduction t -channel measurements (ATLAS and CMS)

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Measurements of Single-Top Production at the LHC

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  1. Measurements of Single-Top Production at the LHC Wolfgang Wagner BergischeUniversität Wuppertalon behalf of the ATLAS and CMS collaborations Top Physics Workshop SantFeliu de Guixols, Spain September 28, 2011 Content: Introduction t-channel measurements (ATLAS and CMS) Wt searches (ATLAS and CMS) Status of s-channel search (ATLAS) Search for FCNC single-top production (ATLAS) Summary

  2. 1) Introduction top-quark production via the weak interaction. s-channel t-channel associated Wt production cross sections at LHC with √s = 7 TeV (mt = 173 GeV) 64.2 ± 2.6 pb 15.6 ± 1.3 pb 4.6 ± 0.2 pb cross sections at the Tevatron with √s = 1.96 TeV (mt = 173 GeV) 2.1 ± 0.1 pb 0.25 ± 0.03 pb 1.05 ± 0.05 pb Calculations by N. Kidonakis: arXiv 1103.2792, 1005.4451, 1001.5034at NLO + NNLL resummation (NNLOapprox)

  3. Test of the SM prediction. Does it exist? ✔ Establish different channels separately. Cross section  |Vtb|2Test unitarity of the CKM matrix, .e.g.Hints for existence of a 4th generation ? Test of b-quark PDF: DGLAP evolution Search for non-SM phenomena Search W’ or H+(Wtor s-chan.signature) Search for FCNC, e.g. ug t … Why look for single top-quarks? • Single top as an experimental benchmark • Object identification: lepton fake rates, QCD background estimates, b-quark jet identification, … • Redo measurements of top properties in different environment, for example, Mtop, W polarization in top decay, …

  4. Overview of performed analyses t channel Wt channel s channel lepton+jets and dilepton channel: cut-based cut-based • cut-based • neural network

  5. 2) t-channel analyses: documentation • Largest cross section of single-top processes • Improved S/B ratio (10%) compared to Tevatron (7%) Analysis history at ATLAS • CONF note (Moriond) with 35 pb-1 (1.6 s),ATLAS-CONF-2011-027 • CONF note (PLHC) with 156 pb-1 (6.2 s),ATLAS-CONF-2011-088 • CONF note (EPS) with 0.70 fb-1 (7.6 s), ATLAS-CONF-2011-101 ATLAS-CONF-2011-101

  6. t channel event selection: leptonic W decay • Data sets defined by single lepton (e / m) trigger • Charged lepton selection (electron / muon): • pT> 25 GeV, pT (m) > 20 GeV, ET(e) > 30 GeV • |h(m, e)| < 2.5 • Relative isolation • Missing transverse energy • ETmiss> 25 GeV • QCD multijet veto • MT(W) > 60 GeV – Etmiss • Select only events with leptonic W decays, to suppress QCD-multijets background. • Some acceptance due to W ® tndecays.

  7. t channel event selection: jets • Jet definition • Anti-kT algorithm R=0.4, R=0.5 (particle flow) • pT > 25 GeV • |h| < 4.5, Measurement of forward jets is crucial to t-channel analyses. • b quark jet identification • Exactly one secondary vertex tag(in 95% of all 2 jets events the b quark jet from top decay is tagged) • Number of jets • NN analysis : 2 • cut based: 2 & 3

  8. Background processes (before b-tagging) single top t-channel (~1%) top-antitoppair production (~1%) mainbackground: W+jetswithseveralcomponents: W + light jets (55 – 65%)W + charmjets (~20%)W + bottomjets (2 – 3%) QCD multijets(fake lepton) background (5 – 10%)

  9. Background estimation - strategy • Top-antitop pairs, Wt, s-channel, diboson, Z+jets • MC normalized to theoretical (or measured) cross-section • Acceptance / efficiency obtained by Monte Carlo Monte Carlo based backgrounds • instrumental backgroundreliableestimationonlyfromdata • reduceasmuchaspossibleQCD veto • fit discriminant; ATLAS: Etmiss • datadriveneventmodelformultivariate methods:jet-electronmodel QCDmultijetsbackground • Alpgen (ATLAS) LO+LL prediction • data driven scale factors W+jets

  10. Multijet background estimate ATLAS: fit Etmiss cut value • jet-electron model works also well for muons • Fraction of multijets background: 5 – 10% • Systematic uncertainty: ± 50%

  11. W + jets background estimate ATLAS Cutbased analysisCalculate scale factors kcc/bb, kWc, klightbased on event yield in 1-jet and 2-jet tagged sideband and 2-jet pretagged data set NN analysisFit NN output for single-top and backgrounds simultaneously Overall ALPGEN andMadgraphmodelsworkquitewellwithinuncertainties.

  12. Systematic uncertainties: background rates • Theoretical cross-section uncertainties *includes Berends scaling and HF uncertainty • W + jets and multijets normalization to data

  13. Uncertaintieson object and kinematic modeling • Detector simulation and object modeling • Jet energy scale, jet energy resolution Leptons: trigger, identification efficiencies, energy scale , lepton energy resolution • B-tagging / mistagscale factor uncertainty • Monte Carlo generators • ISR / FSR • t-channel (ATLAS): MCFM vs. AcerMC • ttbar(ATLAS): MC@NLO vs. Powheg • PDF: CTEQ6.6 vsMSTW08 • Q2 scale for W+jets • Pile-up modeling • Luminosity • ATLAS: 4.5%

  14. T-channel Cut-based analysis at ATLAS • Cuts are optimized including systematicsstrong reduction of jet energy scale uncertainty • Counting experiment • Uses 2 and 3-jet channels • Separation in channels lepton charge and flavor optimize statistical power • Statistical method: profile likelihood fit measured cross section: s(t-ch) = 90 ± 9 (stat.) +31-20 (syst.) pb Observed significance 7.6 s(expected: 5.4s) SM: st = 64.2 ± 2.6 pb • Dominating syst. uncertainties: • B-tagging: +18 / -13% • ISR / FSR: ± 14%

  15. Neural network analysis Idea: Combine many variables including correlations in one discriminate 13 input variables 33 nodes in hidden layer

  16. t-channel neural network analysis at ATLAS • Signal already well visible in Mlnb • 13 input variables • Training: 50% signal, 50% background. • Maximum likelihood fit to NN output distribution.simultaneous determination of background rates • Frequentist method to estimate systematic uncertainties. Observed cross section: s (t-ch.) = 105 ± 7 (stat.) +36-30 (syst.) pb SM: st = 64.2 ± 2.6 pb • Dominating syst. uncertainties: • Jet energy scale: +32 / -20% • B-tagging: ±13% • ISR / FSR: ± 13%

  17. 3) Wt analyses Two channels according to W decay modes: Dilepton channelboth W:W  enor W  mn2 charged leptons, ETmiss, 1 b-jet Lepton + jets channelW  enor W  mn+ W  qqbar 1 chargedlepton, ETmiss, 3 jets • CONF note with 35 pb-1(Moriond)ATLAS-CONF-2011-027 • CONF note with 0.70 fb-1 (EPS)ATLAS-CONF-2011-104

  18. Event selection in the dilepton channel • Lepton selection (electron / muon): • pT > 25 GeV (ATLAS), |h|< 2.5 • Relative Isolation • Exactly two leptons (ee /mm/ em) • Jets • pT> 30 GeV • ATLAS: |h| < 2.5, • Exactly one jet. : No b- tagging! • Missing transverse energy • ETmiss> 50 GeV (ATLAS) • Z-mass veto (ee/mm –channel) • |M(ll)-M(Z)| > 10 GeV, CMS: M(ll) > 20 GeV • Z → ttveto (ATLAS) • DF(l1, Etmiss) +DF(l1, Etmiss) > 2.5 • Kinematic selection at CMSem channel: HT > 160 GeV,pT(lljn=system) < 60 GeV

  19. Dilepton backgrounds – estimation strategy Monte Carlo based dibosonWW / WZ / ZZ Drell-Yan Z/g ll ABCD method W + jets(lepton fakes) matrix method (ATLAS) top-antitopdilepton channel normalization in W + 2 or more jets side bandCMS: exclusive W + 2 jets sample

  20. Top-antitop background and kinematic modeling signal region top-antitop sideband region simultaneous fit (similar technique in ATLAS) Good agreement with expected jet multiplicity distribution and kinematic distributions.

  21. Wtdilepton analyses’ results Event yield after final selection (Njet = 1): Final event yield for 2.1 fb-1 at CMS: • Observed significance: 2.7s(1.8s expected) • Observed cross section:sWt= 22 +9-7 (stat.+ syst.) pbSM: sWt= 15.6 ± 1.3 pb Observed cross section (significance 1.2s): sWt= 14.4 +5.3-5.1 (stat.) +9.7-9.4pb Observed limit @ 95% C.L. sWt< 39.1 pb Significances are determined with maximum likelihood ratio:

  22. Wt lepton + jets channel • Experimental signature: • Isolated charged lepton • Missing transverse energy • Three high-pT jets Event selection very similar to t-channel analysis, same background estimation strategy Analysis of 2010 data with 35 pb-1 • ATLAS-CONF-2011-27 (Moriond 2011) • Obtain S/B = 4 – 6% • Dilepton and lepton+jets channel were combined:observed limit at the 95% C.L.:s (Wt) < 158 pb • Multivariate analyses are in preparation.

  23. 4) Search for s-channel production • Smallest cross section of all single-top processes.(antiquarks in the initial state needed) • Signature similar to t-channel, but: • No forward jet. • Two central b-quark jets. • Jet definition uses: |h| < 2.5. • Use double tagged events. • First s-channel analysis at ATLAS using 0.70 fb-1. ATLAS-CONF-2011-118 Cut-based analysis

  24. Limit on s-channel production Statistical analysis:Profile likelihood Event yield after final selection: Observed limit @ the 95% C.L.: ss-channel < 26.5 pb SM: ss= 4.6 pb

  25. Summary • Single top t-channel production has been observed at ATLAS (7.6s @ 0.7 fb-1 • Measured t-channel cross sections are in agreement with the SM (64.2 ± 2.6 pb).s(t-ch.) = 90 ± 9 (stat.) +31-20 (syst.) pb • With 0.70 fb-1 (ATLAS) already systematically (~30%) limited (stat. unc. 10%). • First steps to measure subleading single-top processes: • s(Wt) < 39 pb @ 95% C.L. • s (s-chan.) < 26.5 pb @ 95% C.L.

  26. Thank You!

  27. Backup

  28. 5) FCNC in top-quark production • FCNC: Flavor-Changing Neutral Currents • significant in extensions of SM (e.g. SUSY) • any evidence reveals new physics SM GIM mechanism BR → 10-13 Very effective in the top sector! hep-ph/0409342 At a hadron collider more effective to look for FCNC production than decay.

  29. FCNC single top quark signature FCNC decays can be neglected since very large couplings are already excluded: Best current limits by DØ:Phys. Lett. B 693 (2010) 81 SM dominant FCNC single top-quark production can be studied assuming SM decay of the top quark. • Same event selection as t-channel analyses, but • Use only central jets: |h| < 2.5 • Exactly one jet Protos generator used for signal modeling.

  30. Expected event yield for Lint = 35 pb-1 • Uncertainties include statistical and cross-section uncertainties. • Assumed signal cross section: 1 pb • Scale factors for W + jets processes are determined in a simultaneous fit to the NN discriminant (not included in the table above).

  31. b-jet identification b hadron lifetime:   1.5 ps c  450 m  typical decay length: O(mm)  lifetime based b-taggers impact parameter based tagger secondary vertex based tagger documentation on b-tagging in ATLAS: ATLAS-CONF-2011-102

  32. Monte Carlo samples All samples with full detector simulation using GEANT4. Pileup is simulated with 50 ns bunch trains.

  33. Modeling of Single-Top Events: Example 2nd b Quark from b-quark PDF flavourconservation (in strong interaction):2nd b fromshower MC (DGLAP evolution) Problem in MC@NLO + Fortran Herwig: Solution:matching of bu® td and gu®tdbbar processes • Switch of signal generator in ATLASfrom 2010 to 2011 analyseschange in acceptance: -20% • issue fixed in Herwig++

  34. W+jetsmodeling: ALPGEN andMadgraph • ALPGEN (M. Manganoet al.) @ ATLAS, Madgraph (Maltoniet al.) @ CMS • models multi-gluonemissionby LO matrixelements + partonshower LO+LL accuracy • workwith Pythia and Herwig • overlapbetween ME and PS must beremoved  MLM matching • „hard“jetsaremodeledby ME, „softer“jetsbytheshower MC • eachprocessismodeledbymanyspecific „parton“samples, forexampleW+bbbarWbb+0p with W e, Wbb+1p with W e, … • Overlapbetween heavy-flavorsamples must beremovedbyhand. Overall ALPGEN andMadgraphmodelsworkquitewellwithinuncertainties.

  35. Z → t+t-veto • Z → ttveto (ATLAS) • DF(l1,Etmiss) +DF(l2, Etmiss) > 2.5

  36. Wt systematic uncertainties Profile maximum likelihood fit: Statistical uncertainty: +37 / -35% Dominating syst. uncertainties:

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