Single top quark processes are interesting as direct probes of the $Wtb$
vertex, and are also an important background in searches of the Higgs boson and
beyond the standard model physics. Both ATLAS and CMS have performed studies
with simulated data to estimate the expected uncertainty on the production
cross section measurements of the three single top processes ($t$-channel,
s-channel, $tW$ production) in the first years of LHC operations. Results in
the different channels and for the two experiments are reported.Comment: 8 pages, 4 figures, presented at TOP2008: International Workshop on
  Top-Quark Physics, La Biodola, Isola d'Elba, Italy, 18-24 May 200

Cristinziani, Markus

Petrucciani, Giovanni

English

arXiv

Prospects for single top physics at the LHC in the first years are reviewed. ATLAS CSC and CMS physics TDR results are highlighted

Cristinziani, M

Petrucciani, G

CERN Document Server

Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 1Single top: prospects at LHCMarkus Cristinziani (Uni Bonn) for ATLAS,Giovanni Petrucciani (SNS & INFN Pisa) for CMSTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 2Outline• What changes between Tevatron and LHC• Overview of single top quark at LHC• Searches at CMS, mostly O(10fb−1) and early searches at ATLAS, mostly O(1fb−1) • ConclusionsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 3LHC is good for searches• High luminosity: gathering O(10fb−1) should be easy, once we start.• High energy: larger signal cross sections (and not so larger background ones)245±17 pb1.98±0.22 pbsingle top, t‐ch.~7500 pb~1200 pbWjj (*) 60±10 pb0.15±0.04 pbtW production~5x105 pb~2.4x105 pbbb+jets (*) 10±1 pb0.88±0.12 pbsingle top, s‐ch.825±150 pb6.70+0.71‐0.88 pbttbar pairsσ(Tevatron) σ(LHC)(x120)(x10)(x120)(x400)(x6)(x2)(*) hep‐ph/9806332: after selection cuts to mimic top signalsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 4LHC is bad for systematics• Rejection of backgrounds depends on observables not easy to control at startup:– jet counting: uncertainties on JES knowledge, extra jets from radiation, pile up or detector noise– b-tagging: knowledge of its performance with a misaligned detector– MET: controlling the detector resolution for a small true missing energy (~40GeV) in multi-jet events(none of these comes for free from Z->µµ/ee)• Because of this, single top can be easy to see but very hard to measure accuratelyTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 5• Only decays with at least one e/µ in the final state will be usable at the beginningOverview at LHC• The cross section hierarchy is different at LHCs-channel σ = 10pbt-channel σ = 240pbtWtW production σ = 60pbTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 6Overview at LHC• Taking into account the BR, not summing on flavours the cross sections become– t-ch (qt → qbW → qbℓν) σ·BR = 26pb (x2) – tW/1ℓ (tW → bWW → bqq'ℓν) σ·BR = 4pb (x2) – tW/2ℓ (tW → bWW → bℓνℓ'ν') σ·BR = 0.74pb (x4) – s-ch (bt → bbW → bbℓν) σ·BR = 1.1pb (x2) Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 7Studies at CMS• All the four possible final states have been studied for the Physics TDR [CERN/LHCC-2006-021]• The basic assumptions were:– 10fb−1 integrated luminosity, with the “2·1033” pileup– Ideal alignment, calibrations with 10fb−1 of data– Keeping the analysis simple: extract only σ, as a counting experiment, no multivariate methods– Generators: SingleTop and TopRex for signal, TopRex, Alpgen and Pythia for the backgrounds.– Use of full GEANT4 simulation when possible, or the fast but fairly accurate FAMOS simulationTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 8Studies at ATLAS• CSC notes will be public ~July, all results shown here are thus preliminary• The three channels are considered in the final states with exactly one lepton (electron or muon)• Studies based on:– 1fb−1 integrated luminosity, with no pileup– Realistic detector and misalignment– Cut-and-count analysis as a baseline; multivariate methods in addition for better background rejection– Generators: AcerMC for signal, MC@NLO, AlpGen and Pythia for backgrounds– Use of full GEANT4 simulationATLAS common preselection• Similar features in the three channels → common preselection to reduce backgrounds (ttbar, W+jets and QCD)– Exactly one isolated high pT lepton– 2-4 jets, one of which is tagged as a b-jet– MET > 20 GeV• Single-top efficiency – 9-10% (electrons) – 10-12% (muons)• Rejection of W+jets O(104), ttbar O(20)Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 9ATLAS: t‐channel• Cut-and-count analysis with simple kinematic cuts– pt (b-jet) > 50 GeV (against W+jets)– Hardest light jet |η| > 2.5 (against ttbar) Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 10number of jets after selectionb‐tagged jet ptATLASPreliminaryATLASPreliminaryTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 11ATLAS: t‐channel sensitivity• Results for sequential cut analysis• Significant reduction wrt to ATLAS TDR (1999)• Difference understood in terms of– Pythia new partonshower algorithm– ME: Pythia→AcerMC– W+jets: Herwig→Alpgen– tt dileptonic andtt with τ were neglectedProcess Efficiency N (1 fb-1)t-channel (µ or e) 1.8% 1460tt (l+jets) 0.6% 1560tt with τ 0.4% 740tt (dilepton) 1.3% 520W+jets 0.0017% 870Wbb+jets 0.4% 70S/B 0.37ATLAS: t‐channel BDT• MV analysis to suppress ttbar background• Boosted Decision Trees (BDT) applied after selection (except η cut)• 40 object/event level variables considered• Reduce to sets that are less sensitive to JES,e.g.– pT and cos(θ*) of leading jet– pT and η of leading non-b jets– centrality(j1 ,j2 ) , HT (j1 ,j2 ,MET,ℓ), MT (W)– ∆R(j1 ,j2 ), ∆R(j1 ,lep), ∆R(j1 non-b,ℓ)– η (max), #jetsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 12ATLAS: t‐channel BDT resultTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 13• BDT cut optimised for cross-section uncertainty including systematic effects– S/B = 1.3 (542 events)– 5.7% (stat), 22.4% (total uncert.) Reco’d top mass after D cutATLASPreliminaryATLASPreliminaryATLAS: t‐channel systematics• Experimental: b-tagging, jet energy scale• Theoretical/MC: ISR/FSR, PDF, MC modelTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 14source CutsΔσ/σBDTΔσ/σsource CutsΔσ/σBDTΔσ/σMC stat 6.5% 7.9% Bckgnd normal. 22.9% 8.2%lumi 5% 18.3% 8.8% PDF 12.3% 2.6%b-tag 5% 18.1% 6.6% Lepton ID 1.5% 0.7%JES 5% 21.6% 9.9% MC model 4.2% 4.2%ISR/FSR 9.8% 9.4% Total systematic 44.7% 22.4%Data stat 5.0% 5.7%Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 15CMS, t‐channel• Analysis performed only in the WÆµv channel• 1 muon, 1 b-tagged jet, 1 forward jet, ETmiss• Cuts on MT (W), M(top), |ΣT | = ET (µ+b+j+ETmiss)Process N/10fb−1Signal 2389t tbar 1189Wbb+jet 195W+jet 102S/B ~ 1.4Expected events[CMS NOTE 2006-084; CMS Physics TDR II, sect. 8.4.2]Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 16CMS: t‐channel, resultsUncertainties on S and B for 10fb−1, and their impact on the cross section measurementsignal ttbar Wbbj Wjj ∆σ/σStatistics 2.0% 2.9% 7.2% 4.9% 2.7%Theory 4.0% 5.0% 17.0% 5.0% 5.0%JES (5-2.5%) 3.0% 6.1% 3.1% <1% 4.3%B-tagging 4.0% 4.0% 4.0% 4.0% 4.5%Luminosity 5.0% 5.0% 5.0% 5.0% 8.7%("theory" includes PDFs, mt , mb , ΛQCD , σbackground)∆σ/σ = 2.7%(stat) + 8%(syst) + 8.7%(lumi)Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 17tW production• The final state is very similar to ttbar production, except for one less b-jet: jet counting is critical– CMS: Jets from calorimeter noise were vetoed by using information from tracks and calo tower distribution– ATLAS: b-tag veto, analysis adapted according to #jets• Can't achieve a good S/B, so background normalization from data important to avoid large systematic uncertainties– Background-like sample dominated by ttbar selected with cuts very similar to the ones for signal, to cancel out systematics on background subtractionTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 18CMS: tW dileptonic• In the e+µ channel, to avoid Z background• Select events with 1 or 2 jets, classify by the PT of the second jet (if any) and the number of b-tags.• Signal selected as 1 b-jet, background control as 2 b-jet(note: first bin in all three plots are single jet events)"signal" region"background" regionzero b-jets one b-jet two b-jetsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 19CMS: tW semi‐leptonic• Events are selected requiring exactly one lepton (e,µ), 1 b-jet and two light quark jets, and some MET (to control QCD background)• (W,b) pairing from a Fisher discriminant using PT (b+W), ∆R(W,b) and q(b)·q(W) from jet charge■ tW/1ℓ● tt/1ℓ● tt/2ℓ■ tW/1ℓ● tt/1ℓ● tt/2ℓ■ tW/1ℓ● tt/1ℓ● tt/2ℓTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 20CMS: tW summaryExpected events [10fb−1]• Semi-leptonic (S/B~0.2) signal: 1700 ttbar: 7624 W+jets: 759 t-ch top: 351• Di-leptonic (S/B~0.37) signal: 562 ttbar: 1433 WW+jets: 551L 2LStatistics 7.5% 8.8%Luminosity 7.8% 5.4%Jet E.S. 9.4% 20%b-tagging 3.6% 8.7%PDF 1.6% 6.0%Pileup 10% 6.1%TOTAL 19% 25%∆σ/σ expected 10fb −1[CMS NOTE 2006-086; CMS Physics TDR II, sect. 8.4.2]ATLAS: tW cut‐and‐count• Only semi-leptonic final state considered• Analysis divided according to #jets in final state• 1 b-tagged jet with pT >50GeV, veto second b-jet to optimally reject ttbar (main concern)• W window cut for events with >3 jetsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 21Events in 1 fb-1 1b+1jet 1b+2jets 1b+3jetstW channel 435 164 40other single top 1260 99 58ttbar 1980 770 274W(bb)+jets 3075 220 44S/B 6.8% 15% 10.6%ATLAS: tW BDT• for each background – ttbar 1l, 2l, W(bb)+jets, t-channel• and each jet multiplicity a boosted decision tree function is defined Æ12 BDTs (e and µ together)• pool of 25 discriminating variables identified– Opening angles(6), pT (3), η(2), cos∆φ– Invariant (transverse) masses (6), f(MET), HT (2)– pz (neutrino)– sphericity, aplanarity, centrality• minimize uncertainty on σ including syst. uncert.Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 22ATLAS: tW resultsTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 23Source 1 fb-1 10 fb-1Var Δσ/σ Var Δσ/σMC stat 15.6%Lumi 5% 20% 3% 7.9%b-tagging 5% 16% 3% 6.6%JES 5% 11% 1% 1.5%lepton ID 1% 2.6% 1% 2.6%Bckgnd σ 10% 23.4% 3% 9.6%ISR/FSR 9% 24.0% 3% 7.8%PDF 2% 5.2% 2% 5.2%b-fragm. 3.6% 9.4% 3.6% 9.4%data stat. 20.6% 6.6%Total uncert. 52% 20.5%• S/B ratios in the 3 classes– 35%,45%,16% (86ev. sel.)• 3σ evidence with few fb-1• 20% uncertainty with 10 fb-1ATLASPreliminaryATLASPreliminaryTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 24CMS: s‐channel• Much harder at LHC than at Tevatron as the relative cross section is much smaller.• Selection requirements:– one isolated lepton (e,µ)– exactly two jets, both b-tagged– missing ET– cuts on MT (W), M(t), PT (t), ΣT , HT[CMS NOTE 2006-084; CMS Physics TDR II, sect. 8.4.4]reco. mtop [GeV]Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 25CMS: s‐channel• Two control samples selected to constraint ttbar background (semi-leptonic and di-leptonic)• Expected events [10fb−1]  (S/B ~ 0.13)signal: 273 ttbar: 1260 t-channel: 630 Wbb: 155• Uncertainty on σ [10fb−1]∆σ/σ = 18%(stat) + 31%(syst) + 19%(lumi) dominated by the systematics on the ttbar semi- leptonic background normalization from JES[CMS NOTE 2006-084; CMS Physics TDR II, sect. 8.4.4]ATLAS: s‐channel• Require exactly two b jets, veto any further jet• Pure cut-and-count analysis not possible– S/B 10%, 25 selected events• Define 5 likelihood functions for background categories (3 ttbar, W+jets, t-channel)• Discriminating variables– see tW channel– ∆η , pT (top)– choose only most significantTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 26ATLAS: s‐channel likelihood• S/B improves to 19% (15ev)• ISR/FSR radiation andb-tagging criticalTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 27Source 1 fb-1 10 fb-1Var Δσ/σ Var Δσ/σMC stat 29%Lumi 5% 31% 3% 18%b-tagging 5% 44% 3% 25%JES 5% 25% 1% 5%lepton ID 1% 6% 1% 6%Bckgnd σ 10% 47% 3% 16%ISR/FSR 9% 52% 3% 17%PDF 2% 16% 2% 16%b-fragm. 3.6% 19% 3.6% 19%data stat. 64% 20%Total 115% 52%ATLASPreliminaryATLASPreliminaryATLAS: QCD rejection• Observation (e.g. D0): fake MET aligned with lepton Æ triangular cut in ∆φ vs. MET plane• Fake lepton rate will be determined from data– e.g. mT (W) < 50GeV extrapolated to signal region– assume QCD background fully under controlTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 28ATLASPreliminaryATLASPreliminarytWno cuts triangular cutTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 29CMS: QCD background estimation• Due to the huge cross section, the background from QCD multi-jet could only be estimated indirectly, using the cut factorization method:– The first steps of the selection were grouped into some sets of approximately independent cut sets, for which the efficiency was extracted from QCD simulations– The combined efficiency was taken as product of the efficiencies of all the cut sets– An upper limit to the efficiency of the later steps of the selection on QCD was taken using the signal efficiency• The estimated background is very small except for tW semi-leptonic analysis, for which BQCD /S ~ 30%.Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 30Conclusions• All channels with leptons have been studied• t-channel– CMS: PhysTDR study gives >5σ observation for 10fb−1 (naïve rescaling of statistical and systematic uncertainties hints that even 1fb−1 might be ok)– ATLAS: two studies shown for 1fb−1• tW channel should be visible with O(10fb−1)• s-channel might be visible with O(10fb−1), but it will be hard due to poor S/B ratioBackupTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 31ATLAS: Effect of pile‐up• With a luminosity of 1032 cm-2s-1 we estimate the relative efficiency for signal and bckgnd• Pile-up modeling will be tuned with data• Uncertainty is expected to become negligible wrt to other sources• No systematic uncertainty considered hereTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 32Channel rel. ε(signal) rel. ε(ttbar)t 75% 66%tW (2jets) 82% 84%tW (3jets) 53% 61%tW (4jets) 74% 80%s 91% 85%ATLAS: Results summaryAnalysis Stat 1fb-1 Syst 1fb-1 Stat 10fb-1 Syst 10fb-1t-channel C&C 5.0% 44.4% 1.6% 22.3%t-channel BDT 5.7% 21.7% 1.8% 9.8%tW-channel BDT 20.6% 48% 6.6% 19.4%s-channel LH 64% 95% 20% 48%Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 33ATLAS: triggering on single top• Inclusive isolated electron and muon triggers• Overall top quark efficiency 84%• Preselection requires 30 GeV leptons, well on the trigger efficiency plateauTop2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 34CMS: triggering on single top• Single lepton triggers used.• Full L1 and HLT simulation included in the analysis.• Combined L1*HLT efficiency ~50% for channels with a single lepton ~70% for tW di-leptonic (note: WÆ tau nu are included)Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 36CMS, t‐channel: selection• Selection cuts (WÆμν only): – 1 muon, PT > 19 GeV, |η| < 2.1– 1 b-jet, PT > 35 GeV, |η| < 2.5, b-discr > 2.4 (b-tag cut giving εb ~ 50%, εuds ~ 0.3%)– 1 forward jet (PT > GeV, |η| < 2.5)– ETmiss > 40 GeV– |ΣT | < 43.5 GeV (                                                  )– mT (W) within [50, 120] GeV – m(t) within [50, 120] GeV Top2008 M. Cristinziani (Bonn), G. Petrucciani (Pisa) 37CMS: t‐channel, plotsProcess Nexpected /10fb−1Signal 2389t tbar 1189Wbb+jet 195W+jet 102

Single top: prospects at LHC

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