Measurement of the top quark B(t → Wb) and electric charge with the D  detector Christophe Clément (CERN) CERN EP Seminar October 16, 2006

October 16, 2006 C.Clément CERN EP seminar 2/39 Top quark t Kinematics in  -lepton data in Mark I Discovery of the Υ at FNAL GIM mechanism  b-quark in isodoublet!  t-quark must exist What Mass for top? [b/c/s = 4.5/1.5/0.5  M top = 15 GeV?] Discovered at ~175 GeV (!) in 1995 by CDF and D0 Is there anything else special about the top quark? Does it have predicted properties, standard model quantum numbers?

October 16, 2006 C.Clément CERN EP seminar 3/39 Is there anything special about the top quark? High mass  potentially important role for top quark Yukawa coupling to Higgs: New physics in EW symmetry breaking sector could be reflected in top quark properties. Top decays in ~0.5× s, before hadronizes  Decays as a free quark: spin correlation, W-helicity in top decays... Heavy enough to decay to exotic particles (H +, W’…) Same experimental signature as some exotic particles, background to supersymmetry 171 GeV 0.98

October 16, 2006 C.Clément CERN EP seminar 4/39 Top at Tevatron Tevatron p-pbar √s=1.96 TeV, L~10 32 cm-2s-1 Run I (√s=1.8 TeV L~10 30 cm -2 s -1 ) 1995 CDF and D  discover top quark Full Run I sample ~120pb -1 / experiment A few dozens of events (l + jets), dilepton channels ≤10 events Mass known ~3% Cross section ~25% B(t->Wb) ~30% Run II (√s=1.96 TeV L~10 32 cm -2 s -1 ) Already >1 fb -1 /experiment on tape Mass known to 1.2% (hep-ex/060832) Cross section ~10%  -final states observed Precise tests of production and decay mechanisms Does it have SM quantum numbers?

 So far only observed in pairs produced via QCD  Cross section derived from pQCD (from m top and √s) Strong top quark production √s = 1.96 TeV σ tt =6.8±0.4pb (theory) for √s = 1.96 TeV October 16, 2006 C.Clément CERN EP seminar 5/39 Strong production  pairs Electroweak production  single top √s = 14TeV LHC: p-p √s = 14TeV dominated by gg processes σ tt ~ 833 pb (theory) Phys. Rev. D68, (2003)

s-channel, ”tb” σ s ~0.9 pb t-channel, ”tqb” σ t ~2.0 pb Allows to measure directly |V tb |, ∝ |V tb | 2Allows to measure directly |V tb |, σ s, σ t ∝ |V tb | 2 Experimentally challengingExperimentally challenging because not too different from W+2 jet background  Use events with W → ev or W →  v  Signature: 1 isolated lepton, MET, 2 or more jets  s-channel: 1,2 b-tagged jets t-channel: 1 b-tagged jet + 1 light jet  Major backgrounds: W+jets,, fake leptons Not yet observed!Not yet observed! Electroweak Top Production October 16, 2006 C.Clément CERN EP seminar 6/39

Top Quark Decay October 16, 2006 C.Clément CERN EP seminar 7/39  3 quark generations + direct measurements of V ub and V cb predict V tb,~1  B(t → Wb)~1  SM predicts FCNC decays are tiny, t → Wq is dominant  In SM top decays via V-A charged current ⇒ Mostly left handed b-quarks in the decay

 How top quark was discovered!  Used for most measured top quark properties so far t t W W Dilepton b b l l v v t t W W Lepton+jets b b l q v q’ t t W W All hadronic b b q q’ q Top Pair Final States October 16, 2006 C.Clément CERN EP seminar 8/39 44% ee+e  +  ~5% ”golden channel” e+jets &  +jets ~32%

B(t → Wb) B(t → Wb)=1 usually assumed by CDF and D  analyses B(t → Wb) might deviate from unity: –Additional quark singlets or doublets –”Pollution” of top sample by non-top process! –Non-SM processes in the production –Non-SM in the decay (H +,...) Experimentally B(t → Wb) affects number of b-jets  need to experimentally discriminate b/w t → Wb and t → Wq light V tb unconstrained without 3x3 unitarity constraint October 16, 2006 C.Clément CERN EP seminar 9/39

Select a top-enriched sample –e+jets and  +jets channel –larges statistics, good S/B # events with 0, 1 and ≥2 b-jets –B(t → Wb) –b-tagging efficiency –Jet identification efficiency –Probability to tag background Deriving B(t → Wb) experimentally.... October 16, 2006 C.Clément CERN EP seminar 10/39 t t W W b-jets? light jets? Missing transverse energy One high p T isolated lepton light, c-jets

. W+jets Z+jets WW, WZ, ZZ single top multijet Lepton + jets sample composition October 16, 2006 C.Clément CERN EP seminar 11/39 True isolated lepton processes N true fake isolated lepton processes N fake Fake isolated electron Jets with leading  / π o, convertions, γ with random tracks,... Fake isolated   inside jets from heavy flavor or in flight decays Determine N true, N fake on a statistical basis -Two lepton ID criteria -loose ⊃ tight lepton -P(tight | loose) for fake and true leptons

. W+jets Z+jets WW, WZ, ZZ single top multijet Lepton + jets sample composition October 16, 2006 C.Clément CERN EP seminar 12/39 N true N fake Small derived from MC Lepton, jet efficiencies calibrated on data σ from data or NLO N other N before tag = N tt + N Wj + N fake + N other N n-tags = P n tt (B(t → Wb)) N n tt + P n Wj N n wj + N’ n fake + P n other N n other n-tags = 0, 1, 2 Fit B(t → Wb), N tt, N wj to the N n-tags Tagging probability

Explicitely reconstruct displaced secondary vertices: Secondary Vertex Tagger 1. Taggable jets are: Calorimeter jets with p T >15 GeV, |  |<2.5 ΔR(calo jet, track jet)=0.5, ≥ 2 tracks in ΔR=0.5, Δz<2cm ≥ 1 hit in the innermost tracking detector, p T >0.5 GeV ≥ 1 track with p T >1 GeV  Decouple b-tagging from experimental issues 2. Tagged jets: Are taggable jets Contain a SV, χ 2 L xy > n σ Lxy Similar algorithm used by CDF How to identify b-quark jets... October 16, 2006 C.Clément CERN EP seminar 13/39 L xy = Taggability, tagger independent B-tagging efficiency

b-tagging efficiency From di-jet data: extract b-tagging efficiency for muonic b-jets We need the b-tagging efficiency for ”all kinds of b-jets” P b tag (E T,  ) =  b,MC  b,MC  b → , data Taggability  C taggability (b)   b → , data  Taggability  C taggability (b)  b → ,MC  b → ,MC  b → , data bbbb Transform semi-muonic b-tag efficiency into inclusive one October 16, 2006 C.Clément CERN EP seminar 14/39 See for example Phys. Rev. D71, (2005)

P n tt versus B(t → Wb) Probability to see n-tags in events (P n tt ) depends on the number of b-jets P n tt = R 2 P n tag (tt → bb) + 2 R(1-R)P n tag (tt → bq l ) + (1-R) 2 P n tag (tt → q l q l ) October 16, 2006 C.Clément CERN EP seminar 15/39 ttbar → l+jets Take into account ”contamination” by → ll

Fit B(t→Wb) and N tt simultaneously to 12 bins: e/  + 3/≥4 jets, 0/1/≥2 tags B(t→Wb) is constrained by relative 0/1/≥2 tags populations Very poor S/B in 0-tag sample, N ttbar ~√N obs October 16, 2006 C.Clément CERN EP seminar 16/39 Prediction 7pb & B(t → Wb)=1

Single Tag l+jets Double Tag l+jets Jet multiplicity October 16, 2006 C.Clément CERN EP seminar 17/39 R=1.0, σ =7pb R=0.5, σ =7pb Illustration...

The 0-tag sample Ntt ~ √N(0-tag) Without further information on the 0-tag events low B(t → Wb) + large N tt (σ tt ) can still be consistent with data Use topological properties of events in 0-tag sample for addititional constraint of N tt in the 0-tag sample. Cross Section R Preliminary result from summer 2004 (170pb -1 ) No 0-tag sample used October 16, 2006 C.Clément CERN EP seminar 18/39 Low B(t → Wb) Large σ tt

1. Sphericity S = 3(λ 2 +λ 3 )/2, λ’s smallest eigenvalues of momentum tensor M (ttbar S~1) 2. K’ Tmin K’ Tmin = R min jj /E W T with E W T = E l T + MET 3. Centrality C =H T /H, H T is scalar sum of jets E T and H is the sum of the jet energies. H’ T2 H’ T2 = H T2 /H z, H T2 : p T sum of all jets but leading jet, H z is the scalar sum of all jets |E z | plus |E z | of the neutrino (W-assumption) Likelihood discriminant in 0-tag sample l + 4 jets before tagging October 16, 2006 C.Clément CERN EP seminar 19/39

Likelihood discriminant in l+4jets 0-tag sample Data and prediction for 7pb and R=1 Data October 16, 2006 C.Clément CERN EP seminar 20/39 Likelihood discriminant output Nevents

Systematics on template shapes Some systematic uncertainties can affect the template shapes... Systematics on template shapes on ttbar → l+jet JES JetID Jet energy resolution W-modeling (W+jets) Taggability Tagging probabilities for b, c and light jets October 16, 2006 C.Clément CERN EP seminar 21/39

Results (230 pb -1 ) October 16, 2006 C.Clément CERN EP seminar 22/39 3 jets ≥ 4 jets ≥ 4 jets, 0 tag

October 16, 2006 C.Clément CERN EP seminar 23/39 Phys. Lett. B 639 (2006) No 0-tag B(t → Wb) = Confidence contour plots in R, Ntt B(t → Wb)

Lower limit on B(t→Wb) and |V tb | Prior π( B(t → Wb) )=0 outside [0,1] Monte Carlo integration over 191 nuisance parameters associated to systematic errors Provides a 2D p.d.f. for B(t → Wb) and N tt Limit on |V tb | can be derived using |V tb |=√B(t → Wb) (SM) 68% CL : B(t → Wb) >0.78 |V tb |> % CL : B(t → Wb) >0.61 |V tb |>0.78 October 16, 2006 C.Clément CERN EP seminar 24/39

OR ? Top Quark Charge October 16, 2006 C.Clément CERN EP seminar 25/39 Lift ambiguity present in all top analyses! t →W + b or ” t” → W - b Test exotic models... t b Q1Q1 Q4Q4 +2/3 -1/3 -4/3 mixing M(Q 4 )~175GeV M top ~270GeV Phys.Rev. D65 (2002)

OR ? kinematic fit + = Q top Ingredients What is the expected shape of Q top for SM top ”top” with 4e/3 charge? October 16, 2006 C.Clément CERN EP seminar 26/39

Analysis strategy Discriminate between |Q top | = 2e/3 and |Q ”top” |=4e/3 Two |Q top | per event Use the pure sample -- l epton+4≥jets events with 2 SVT Compute the jet charge of the 2 b-tagged jets Associate the b-jets to correct W boson (charged lepton) Combine the 2 jet charges and the lepton charge to derive the 2 |Q top | Compare the observed |Q top | with expected SM and exotic distributions Double tagged events October 16, 2006 C.Clément CERN EP seminar 27/39 S/B~10

Jet Charge Algorithm Compute jet charge only for b-tagged jets - (2 per events) Jet charge = Optimizaton on MC gives a=0.6 Sum over tracks with p T > 0.5GeV, Δ R(track, jet) <0.5 of the jet axis Algorithm: October 16, 2006 C.Clément CERN EP seminar 28/39 Derive expected shape of Q jet from data with minimal input from simulation p Ti,q i

Why does it work? The charge of the quark is correlated with the charge of the highest p T hadron resulting of the hadronization Simple study carried out with Pythia: Generated QCD 2→2 process, p T >15GeV And look at hight p T b quarks produced in the process. Usually large number of hadrons Produced, most quite modest p T Thís is then smeared by detector effects... The original b-quark not always In highest p T hadron Charge of b-quark Charge of highest p T hadron October 16, 2006 C.Clément CERN EP seminar 29/39

Jet Charge Performance in Data Tag and probe method in ”pure” events In reality: Is it pure ? ? flavor excitation? g→ ? B →  B → D →  →  B → light hadrons →  Charge misidentification Tight di-jet sample  >3.0 October 16, 2006 C.Clément CERN EP seminar 30/39 _ bb Ideal case: sign of q  = sign of q b _ bb _ cc _ bb ´ _ B o → B o Charge flipping processes

Data Calibration Corrections Discriminant Power Tag and probe Method: Z→bb October 16, 2006 C.Clément CERN EP seminar 31/39 Tag and probe Method: data MC truth on tag side

Is the triple tag sample pure ”bb”? The fraction of c-jets in the triple tag sample is determined by pTrel fit of the order of a few percents, Flavor excitation/ splitting?  >3.0 2 b-jets back to back dominate Phys. Rev. D 65, (2002) - October 16, 2006 C.Clément CERN EP seminar 32/39

P  + (Q jet ) = (1-x c ) (1-x flip )P b (Q jet )+ (1-x c )x flip P b + x c P c - - Fraction of derived from p T rel spectrum of  ( %) _ cc Fraction of charge flipping processes 30 ± 1% from MC, Cross checked on data Similar equation for P  - (Q jet ) 4 Unknown p.d.f’s P b, P b, P c, P c Tight di-jet sample + p.d.f of Q jet in probe jet - - p.d.f.’s Q b, Q b, Q c, Q c from data October 16, 2006 C.Clément CERN EP seminar 33/39

p.d.f.’s Q b, Q b, Q c, Q c from data October 16, 2006 C.Clément CERN EP seminar 34/39 P  + (Q jet ) = 0.69 P b (Q jet ) P b P c P  - (Q jet ) = 0.30 P b (Q jet ) P b P c P´  + (Q jet ) = P b (Q jet ) P b P c P´  - (Q jet ) = P b (Q jet ) P b P c Tight di-jet sample loose di-jet sample -Correct for different t→Wb and bb kinematics

SM Top Charge Observables We need an observable and an expectation for the ”2e/3” and ”4e/3” scenarios Consider only lepton+jets channel (e/µ + 4 jets) double-tagged events Two top quarks in the event  measure the charge ”twice” Use kinematic fit to assign b-jets to correct W-bosons in MC q b and q B are taken from the data derived jet charge templates q b = b lept. side q B = b hadr. side qBqB qbqb qBqB qbqb October 16, 2006 C.Clément CERN EP seminar 35/39

Exotic Top Charge Observables q b and q B are taken from the data derived jet charge templates The exotic scenario is obtained by permuting the charge of the SVT tagged jets q b = b lept. side q B = b hadr. side qBqB qBqB qbqb qbqb October 16, 2006 C.Clément CERN EP seminar 36/39

Admixture of Q 4 and t-quark - not excluded by - f Q4 <0.52 at 68% C.L. σ tt Result on ~0.4 fb -1 of D  data 21 l + jets double tagged events 16 events with converged kinematic fit  32 measured top charges Perform likelihood ratio test b/w 4e/3 and 2e/3 hypothesis Exclude 4e/3 at 92%C.L. (91 % expected) October 16, 2006 C.Clément CERN EP seminar 37/39 hep-ex/060844

Systematic uncertainties October 16, 2006 C.Clément CERN EP seminar 38/39 Kinematic fit Jet charge specific

Conclusions Acknowledge collaborators: J. Strandberg and P. Hansson Large data sets from CDF and D  allow new fundamental measurements of the top quark properties Development of analysis techniques: often analyses outperform expectation First ”measurement of the top electric charge”... October 16, 2006 C.Clément CERN EP seminar 39/39

Backup slides

Top pair final states

Ensemble tests Fitted versus true B(t → Wb) Coverage for B(t → Wb)

Expected statistical errors Statistical error on R vs R Statistical error on σ vs σ

b-tagging efficiency from data     ”n-sample”:  1 jet with a  Not  -tagged n  -tagged n  Not  -tagged, SVT tagged n SVT  -tagged, SVT tagged n ,SVT   ”p-sample”: 2 b-2-b jets 1 jet with a  Not  -tagged p  -tagged p  Not  -tagged, SVT tagged p SVT  -tagged, SVT tagged p ,SVT 