Single-Top Production at CDF Rainer Wallny University of California, Los Angeles On behalf of the CDF Collaboration 2007 Europhysics Conference on High Energy Physics

July 19th, 20072Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Design goal The Tevatron Accelerator World’s highest energy collider (until 2008) –Proton-antiproton Synchrotron –Experiments CDF and DØ Run I ( ) –  s = 1.8 TeV –100 pb -1 int. luminosity Major upgrade to accelerator complex –Main Injector (x5) –Pbar Recycler (x2) Run II ( ) –  s = 1.96 TeV –Current peak luminosity ~28.0 x cm -2 s -1 –Both experiments have now > 2.5 fb -1 on tape. –Aim for 4-9 fb -1 int. luminosity in Run II Only place in the world to produce top quarks.

July 19th, 20073Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Top Quark Production at the Tevatron At the Tevatron, top quarks are primarily produced in pairs via the strong interaction: The Standard Model also predicts a single top production via the electroweak interaction: (  st ~ ½  tt )  NLO = 6.7± 0.8 pb s-channel  NLO = 0.88±0.07 pb t-channel  NLO = 1.98±0.21 pb The single top quark production mode was not yet observed – first evidence reported recently …

July 19th, 20074Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Why measure Single Top Production ? Source of single ~100% polarized top quarks: – Test V-A structure of W-t-b vertex – Access to the top quark spin Allows direct Measurement of CKM- Matrix Element V tb : –  st ~|V tb | 2 – indirect determinations: V tb ~0.999 … –verify V tb Direct measurements Ratio from Bs oscillations Not precisely measured s-channel t-channel

July 19th, 20075Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Sensitivity to New Physics and WH Single top rate can be altered due to the presence of New Physics: - t-channel signature: Flavor changing neutral currents (t-Z/γ/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 search!

July 19th, 20076Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Single Top Candidate Selection Event Selection: 1 Lepton, E T >15 GeV, |  |< 2.0 Missing E T (MET) > 25 GeV 2 Jets, E T > 15 GeV, |  |< 2.8 Veto QCD, Conversions, Cosmics At least one b-tagged jet, (secondary vertex tag) CDF W + 2 jets Candidate Event: CDF W(l ) + 2 jets Candidate Event: Close-up View of Layer 00 Silicon Detector Jet2 Jet1 Electron 12mm Number of Events / 955 pb -1 Single TopBackground S/B S/  B W(l ) + 2 jets ~1/210~ 0.6 W(l ) + 2 jets + b-tag 38550~1/15~ 1.6 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

July 19th, 20077Rainer Wallny - Single Top Production at CDF EPS Conference Manchester 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→ττ, ttbar) MC normalized to theoretical cross-section Non-W (QCD) Multijet events with semileptonic b-decays or mismeasured jets Fit low missing E T 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, calibrated in generic multijet data

July 19th, 20078Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Signal and Background Event Yield CDF Run II Preliminary, L=955 pb Event yield in W+2jets CDF Run II Preliminary, L=955 pb -1 Event yield in W+2jets Single top hidden behind background uncertainty!  Makes counting experiment impossible! s-channel15.4 ± 2.2 t-channel22.4 ± 3.6 tt58.4 ±13.5 Diboson13.7 ± 1.9 Z + jets11.9 ± 4.4 Wbb170.9 ± 50.7 Wcc63.5 ± 19.9 Wc68.6 ± 19.0 Non-W26.2 ± 15.9 Mistags136.1 ± 19.7 Single top37.8 ± 5.9 Total background549.3 ± 95.2 Total prediction587.1 ± 96.6 Observed644

July 19th, 20079Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Jet Flavor Separation Background Estimate Neural Network Fit W+bottom   26.3 W+charm   53.8 Mistags   42.5 Sum  mistags / charm ……….…………. beauty Even with a reconstructed secondary vertex required, about 50% of the background in the W + 2 jets sample do NOT contain b quarks Train Neural Network with secondary vertex tracking information (25 input variables) to distinguish b/c/light quark jets Replaces Yes-No decision by a continuous variable - used in all analyses Obtain good jet-flavor separation! Improves sensitivity by ~15-20%!

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Analysis Techniques

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester The Likelihood Function Analysis N sig N bkg i, indexes input variable BackgroundSignal Unit Area tchan schan Wbb ttbar Expected signal significance: 2.0  Leading Jet E T (GeV) Uses 8 (5) kinematic variables for t-channel (s-channel) Likelihood Function i.e. M(Wb), H T, QxEta, Neural Network flavor separator, Madgraph Matrix Elements etc.

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Kinematic Variables Background Signal BackgroundSignal Unit area Wbb ttbar Wbb ttbar Wbb ttbar tchan schan tchan schan tchan schan Mass(W,b) H T =  E T (lepton,MET,Jets) QxEta(un-tagged jet)

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Analysis Techniques

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Neural Network Analysis      combine 18 input variables into one powerful discriminant (i.e b-tagging flavor separator, M(W,b), M(j 1,j 2 ), HT, number of soft jets, angular variables, etc..) Continuous output between -1 (bkg like) and +1 (sig like) Three networks: s-channel, t-channel and combined s+t channel Expected signal significance: 2.6 

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Neural Network Analysis - Separate Search Two NN’s trained separately for s-channel and t-channel (same input variables) t-channel Wbb

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Analysis Techniques

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Matrix Element Method Parton distribution functions Leading Order matrix element (MadEvent) W(E jet,E part ) gives the probability of measuring a jet energy E jet when E part was produced Integration over part of the phase space Φ 4 Pioneered by top quark mass analyses Now applied to a search! Calculate event probability density for signal and background Inputs only lepton and 2 jets 4-vectors! c E parton E jet

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Kinematic Variables Used in Matrix Element Analysis Jet1Jet2 Input to the Matrix Element Analysis are the measured four-vectors of the Lepton, Jet1 and Jet2 in the W+2jets data (>=1 b-tagged jet) Lepton

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester 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): Expected signal significance: 2.5  Signal-like Background-like

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Results

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester CDF Single Top Results Likelihood Neural Networks No evidence of signal  s+t <2.7pb at 95% C.L. Expected signal significance 2.0σ No evidence of signal  s+t <2.6pb at 95% C.L. Expected signal significance 2.6σ p-value = 1.0% (2.3  )  s+t =2.7(+1.5/-1.3)pb Expected signal significance 2.5σ Matrix Element

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Jet1 Jet2 Lepton Central Electron Candidate Charge: -1, Eta=-0.72 MET=41.6 GeV Jet1: Et=46.7 GeV Eta=-0.6 b-tag=1 Jet2: Et=16.6 GeV Eta=-2.9 b-tag=0 QxEta = 2.9 (t-channel signature) EPD=0.95 Single Top Candidate Event t-channel single top production has a kinematic peculiarity: - Distinct asymmetry in Q x  distribution: lepton charge (Q) x pseudo-rapidity  =-log (tan  /2) of untagged jet Run: , Event: Signal region EPD>0.9

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Compatibility of CDF Results Performed common pseudo-experiments –Use identical events –ME uses 4-vectors of lepton, Jet1/Jet2 as inputs –LF/NN uses several sensitive variables –Correlation among analyses: ~60-70% –1.2% of these pseudo-experiments fluctuated as unlucky as the observed data Extensive cross-checks performed Next round of analysis will shed some light on this issue – coming soon! NN LF NN ME LF (NN) (LF) (ME) SM Theory CDF Run II Preliminary

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Conclusions Single top production –measures |V tb | directly –probes physics beyond the Standard Model – serves as a benchmark for Higgs boson searches at the Tevatron Presented three multi-variate analysis techniques to improve Signal/Background separation –Individual sensitivity ~ σ –Combined sensitivity 3σ using all three discriminants Two of the three analyses show deficit in the signal region, one analysis observes a 2.3σ hint of the signal at almost the Standard Model rate –Compatibility of the analyses is ~1% –We have ~ twice the data available, we are improving our analyses and we expect the results to converge for the next update => Expect news on CDF single top analyses soon !

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester BACKUP slides

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Search for Heavy W Boson Limit at 95% C.L. M(W´) > 760 GeV/c 2 for M(W´) > M( ν R ) M(W´) > 790 GeV/c 2 for M(W´) < M( ν 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. New Result!

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Four Fold Ambiguity in building m(W,b) Parton/Jet matching t-channels-channel 1 tag96.6%51.1% 2 tags61.9%68.5% 2) M lνb reconstruction: Assumption that tagged jet is from top In double tagged events, take jet with larger Q lept x η jet Works well for t-channel only 1) Neutrino Pz: Neutrino p x, p y from MET P z from W-mass constraint This yields two solutions:  Smaller solution is correct 67.6% M lνj2 ~170 GeV/c 2 M top

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Hypothesis Test (ME-Analysis) Most sensitive bins: We use the CLs/CLb Method developed at LEP Define Likelihood ratio test statistic: CDF RunII Preliminary, L=955pb -1 Median ( ‘ expected ’ ) p-value = 0.6% (2.5  ) Observed p-value = 1.0% (2.3  ) b s+b L. Read, J. Phys. G 28, 2693 (2002) T. Junk, Nucl. Instrum. Meth. A 434, 435 (1999)

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Cross-Checks in Data Control Regions (ME-Analysis) Validate method using data without looking at single top candidates Compare the Monte Carlo prediction of the shape of the discriminant to various control samples in data W+2 jets data (require no b-tagged jet) CDF Run II Preliminary b-tagged dilepton+2jets data (99% ttbar) b-tagged lepton+4jets data (85% ttbar) Dilepton+2jets Lepton+4jets

July 19th, Rainer Wallny - Single Top Production at CDF EPS Conference Manchester Sources of Systematic Uncertainty Systematic (-1  /+1  ) s-channelt-channelAll Single topShape Variations Jet Energy Scale-1.4% / 1.3%-2.4% / 1.8%-2.0% / 1.6%  Initial State Radiation1.1% / -2.0%2.6% / 2.0%2.0% / 0.3%  Final State Radiation1.3% / 1.4%3.4% / 2.2%2.6% / 1.9%  Parton Dist. Function1.0% / -0.6%1.7% / -0.3%1.4% / -0.4%  Monte Carlo Generator1%2%1.6% Event Det. Efficiency6.1%7.8%7.4% Luminosity6% Neural Net b-taggerN/A  Mistag ModelN/A  Non-W ModelN/A  Q 2 Scale in Alpgen MCN/A  Total Rate Uncertainty9.1%11.3%10.5%N/A CDF RunII Preliminary, L=955pb -1 All rate and shape systematic uncertainties are included as nuisance parameters in the analyses!