Evidence for Single Top Quark Production at CDF Bernd Stelzer University of California, Los Angeles HEP Seminar, University of Pennsylvania September, 18th 2007 2 Outline 1.Introduction to Top Quarks 2.Motivation for Single Top Search 3.The Experimental Challenge 4.Analysis Techniques Likelihood Function Discriminant (1.51fb -1 ) Matrix Element Analysis (1.51fb -1 ) 5.Measurement of |V tb | 6.More Tevatron Results 7.Summary / Conclusions / Outlook 3 The Tevatron Collider Tevatron is worlds highest energy Collider (until 2008) Proton Anti-proton Collisions at E CM =1.96 TeV 4 Top Production at the Tevatron Once every 10,000,000,000 inelastic collision.. 5 Top Production at the Tevatron At the Tevatron, top quarks are primarily produced in pairs via the strong interaction: Single top quark production is also predicted by the Standard Model through the electroweak interaction: ( st ~ tt ) NLO = 6.70.8 pb m t =175GeV/c 2 s-channel NLO = 0.880.07 pb t-channel NLO = 1.980.21 pb Discovered 1995! Cross-sections at m t =175GeV/c 2, B.W. Harris et al., Phys.Rev. D70 (2004) , Z. Sullivan hep-ph/ 6 Top Quark in the Standard Model >10 orders of magnitude! Top Quark is heaviest particle to date m t =170.9 1.8 GeV/c 2 March 2007 Close to the scale of electroweak symmetry breaking Special role in the Standard Model? Top Quark decays within ~ s -No time to hadronize -We can study a bare quark 7 Why measure Single Top Production ? V tb Direct measurements Ratio from Bs oscillations Not precisely measured s-channel t-channel Ceccucci, Ligeti, Sakai PDG Review 2006 Precision EW rules out simple fourth generation extensions, but see J. Alwall et. al., Is |V tb |~1? Eur. Phys. J. C (2007). Source of single ~100% polarized top quarks: Short lifetime, information passed to decay products Test V-A structure of W-t-b vertex Allows direct Measurement of CKM- Matrix Element V tb : single top ~|V tb | 2 indirect determinations of V tb enforce 3x3 unitarity 8 Sensitivity to New Physics and Benchmark for WH Single top rate can be altered due to the presence of New Physics: - t-channel signature: Flavor changing neutral currents (t-Z//g-c couplings) - s-channel signature: Heavy W boson, charged Higgs H +, Kaluza Klein excited W KK Tait, Yuan PRD63, (2001) Z c t W,H+W,H+ s (pb) 1.25 t (pb) s-channel single top has the same final state as WH l bb => benchmark for WH! ( WH ~ 1/10 s-channe )) CMSSM Study: Buchmuller, Cavanaugh, deRoeck, S.H., Isidori, Paradisi, Ronga, Weber, G. Weiglein07] 9 Experimental Challenge 10 Top Pair Production with decay Into Lepton + 4 Jets final state are very striking signatures! Jet4 Jet3 Event Signatures Jet1 Jet2 Electron MET Single top Production with decay Into Lepton + 2 Jets final state Is less distinct! 11 = 1.0 = 2.8 = 2.0 CDF II Detector (Cartoon) Silicon tracking detectors Central drift chambers (COT) Solenoid Coil EM calorimeter Hadronic calorimeter Muon scintillator counters Muon drift chambers Steel shielding Single top analysis needs full detector! Thanks to great work of detector experts and shift crew! 12 CDF II Detector Silicon detector Central muon Central calorimeters Endplug calorimeters Drift chamber tracker 13 Data Collected at CDF This analysis uses 1.51 fb -1 (All detector components ON) CDF is getting faster, too! 6 weeks turnaround time to calibrate, validate and process raw data Tevatron people are doing a fantastic job! 3fb -1 party coming up! Design goal Delivered : 3.0 fb -1 Collected : 2.7 fb -1 14 Single Top Selection Event Selection: 1 Lepton, E T >20 GeV, | e( ) |< 2.0 (1.0) Missing E T, (MET) > 25 GeV 2 Jets, E T > 20 GeV, | |< 2.8 Veto Fake W, Z, Dileptons, Conversions, Cosmics At least one b-tagged jet, (displaced secondary vertex tag) CDF W+2jet Candidate Event: Close-up View of Layer 00 Silicon Detector Jet2 Jet1 Electron 12mm Number of Events / 1.51 fb -1 Single TopBackground S/B W(l ) + 2 jets ~1/210 W(l ) + 2 jets + b-tag ~1/17 Run: , Event: Electron E T = 39.6 GeV, Missing E T = 37.1 GeV Jet 1: E T = 62.8 GeV, L xy = 2.9mm Jet 2: E T = 42.7 GeV, L xy = 3.9mm 15 B-quark Tagging and Jet Flavor Separation Separate tagged b-jets from charm/light jets using a Neural Network trained with tracking information L xy, vertex mass, track multiplicity, impact parameter, semilepton decay information, etc... Used in all single top analyses Neural Network Jet-Flavor Separator NN Output Charm tagging rate ~10% Mistag rate ~ 0.5% Exploit long lifetime of B hadrons (c ~450 m)+boost L xy ~3mmB hadrons travel L xy ~3mm before decay with large track multiplicity 16 Mistags (W+2jets) Falsely tagged light quark or gluon jets Mistag probability parameterization obtained from inclusive jet data Background Estimate W+HF jets (Wbb/Wcc/Wc) W+jets normalization from data and heavy flavor (HF) fraction from MC Top/EWK (WW/WZ/Z , ttbar) MC normalized to theoretical cross-section Non-W (QCD) Multijet events with semileptonic b-decays or mismeasured jets Fit low MET data and extrapolate into signal region Wbb Wcc Wc non-W Z/Dib Mistags tt W+HF jets (Wbb/Wcc/Wc) W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo 17 Non-W Estimate Build non-W model from anti-electron selection Require at least two non-kinematic lepton ID variables to fail: EM Shower Profile 2, shower maximum matching (dX and dZ), E had /E em, Data is superposition of non-W and W+jets contribution -> Likelihood Fit Signal Region Before b-tagging:After b-tagging: Signal Region 18 W + Heavy Flavor Estimate Method inherited from CDF Run I (G. Unal et. al.) Measure fraction of W+jets events with heavy flavor (b,c) in Monte Carlo Normalize fractions to W+jets events found in data Correct data for non W+jets events Heavy flavor fractions and b-tagging efficiencies from LO ALPGEN Monte Carlo Calibrate ALPGEN heavy flavor Fractions by comparing W + 1jet Data with ALPGEN jet Monte Carlo Note: Similar for W+charm background Large uncertainties from Monte Carlo estimate and heavy flavor calibration (36%) K HF =1.4 0.4 19 Signal and Background Event Yield CDF RunII Preliminary, L=1.51 fb Predicted Event Yield in W+2jets CDF RunII Preliminary, L=1.51 fb -1 Predicted Event Yield in W+2jets Single top swamped by background and hidden behind background uncertainty. Makes counting experiment impossible! s-channel23.96.1 t-channel37.05.4 Single top60.911.5 tt85.317.8 Diboson40.74.0 Z + jets13.82.0 W + bottom319.6112.3 W + charm324.2115.8 W + light214.627.3 Non-W44.517.8 Total background1042.8218.2 Total prediction1103.7230.9 Observed1078 20 Analysis Flow Chart Analysis Event Selection CDF Data Monte Carlo Signal/Background Monte Carlo Signal/Background Apply MC Corrections Apply MC Corrections Analysis Technique Result Template Fit to Data Template Fit to Data Discriminant Signal Background Cross Section 21 Analysis Techniques 22 The Likelihood Function Analysis N sig N bkg i, index input variable Bkgr Signal Unit Area tchan schan Wbb ttbar Leading Jet E T (GeV) Uses 7 (8) kinematic variables for t-channel (s-channel) Likelihood Function e.g. M(Wb) or kin. Solver 2, H T, QxEta, NN flavor separator, Madgraph Matrix Elements, M(jj) Discriminant 23 Kinematic Variables BackgroundSignalBackgroundSignal Wbb ttbar Wbb ttbar tchan schan tchan schan H T = E T (lepton,MET,Jets) Wbb ttbar tchan schan 24 Analysis Techniques 25 Matrix Element Approach No single golden kinematic variable! Attempt to include all available kinematic information by using Matrix Element approach Start from Fermis Golden rule: Cross-sections ~ |Matrix Element| 2 Phase space Calculate an event-by-event probability (based on fully differential cross-section calculation) for signal and background hypothesis 26 Matrix Element Method Parton distribution function (CTEQ5) Leading Order matrix element (MadEvent) W(E jet,E part ) is the probability of measuring a jet energy E jet when E part was produced Integration over part of the phase space 4 Event probability for signal and background hypothesis: Input only lepton and 2 jets 4-vectors! c 27 Transfer Functions E parton E jet Full simulation vs parton energy: E parto n E jet Double Gaussian parameterization: where: E = (E parton E jet ) Double Gaussian parameterization: 28 Event Probability Discriminant (EPD) ;b = Neural Network b-tagger output We compute probabilities for signal and background hypothesis per event Use full kinematic correlation between signal and background events Define ratio of probabilities as event probability discriminant (EPD): SignalBackground 29 Event Probabilty Discriminant S/B~1/1 In most sensitive bin! S/B~1/17 over full range Likelihood fit will pin down background in low score region 30 Cross-Checks 31 Cross-Checks in Data Control Samples Validate method in various data control samples W+2 jets data (veto b-jets, selection orthogonal to the candidate sample) Similar kinematics, with very little contribution from top (0.90 EPD>0.95 Look for signal features in high score output 49 QxEta Distributions in Signal Region EPD>0.9 3)4) EPD>0.95 50 m(W,b) Distributions in Signal Region EPD>0.9EPD>0.95 51 Unconstrained Likelihood Fit Remove all background normalization constraints and perform a five parameter likelihood fit (all template shapes float freely) Best fit for signal almost unchanged. Uncertainty increased by about 20% 52 Direct |V tb | Measurement Using the Matrix Element cross Section PDF we measure |V tb | Assume Standard Model V-A coupling and |V tb | >> |V ts |, |V td | |V tb |= 1.02 0.18 (experiment) 0.07 (theory) t-channel Z. Sullivan, Phys.Rev. D70 (2004) Flat prior 0 < |V tb | 2 < 1 |V tb |>0.55 at 95% C.L. s-channel 53 Single Top Results from D 54 D0 Results First direct limit on V tb : 0.68 760 GeV/c 2 for M(W) > M( R ) M(W) > 790 GeV/c 2 for M(W) < M( R ) W Search for heavy W boson in W + 2, 3 jets Assume Standard Model coupling strengths (Z. Sullivan, Phys. Rev. D 66, , 2002) Perform fit to M Wjj distribution Previous Limits: CDF Run I: M(W R ) > 566 GeV/c 2 at 95% C.L. D0 Run II: M(W R ) > 630 GeV/c 2 at 95% C.L. 59 LHC is the Future Large Hadron Collider 60 LHC is the Future LHC will be a top quark factory tt ~ 800 pb t-channel ~ 243 pb (153 pb for top and 90 pb for antitop production) s-channel ~ 11 pb (6.6 pb for top and 4.8 pb for antitop production) Wt ~ pb (negligible at the Tevatron) First precision t-channel measurement (10%) expected after 1 st year of running (10 fb -1 /year) s-channel measurement harder because of small cross section and large backgrounds (sounds familiar!) The associated Wt production is tough because of large top-pair background (W+3jets signature) Wt- production Additional single top process at the LHC! (negligible at the Tevatron) 61 Backup Slides Backup