SingleTop Production at CDF Rainer Wallny University of

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 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester

The Tevatron Accelerator • World’s highest energy collider (until 2008) – Proton-antiproton Synchrotron – Experiments CDF and DØ • Run I (1992 -1996) – s = 1. 8 Te. V – 100 pb-1 int. luminosity • Major upgrade to accelerator complex – Main Injector (x 5) – Pbar Recycler (x 2) • Run II (2001 -2009) – – s = 1. 96 Te. V Current peak luminosity ~28. 0 x 1031 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 Design goal • Only place in the world to produce top quarks. July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 2

Top Quark Production at the Tevatron • At the Tevatron, top quarks are primarily produced in pairs via the strong interaction: NLO = 6. 7± 0. 8 pb • The Standard Model also predicts a single top production via the electroweak interaction: ( st ~ ½ tt) s-channel NLO = 0. 88± 0. 07 pb t-channel NLO = 1. 98± 0. 21 pb July 19 th, 2007 The single top quark production mode was not yet observed – first evidence reported recently … Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 3

Why measure Single Top Production ? • Source of single ~100% polarized top quarks: Direct measurements – Test V-A structure of W-t-b vertex – Access to the top quark spin • Allows direct Measurement of CKMMatrix Element Vtb: Ratio from Bs oscillations Not precisely measured – st ~|Vtb|2 – indirect determinations: Vtb ~0. 999… – verify Vtb s-channel July 19 th, 2007 t-channel Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 4

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) Z c - s-channel signature: Heavy W boson, charged Higgs H+, Kaluza Klein excited WKK t • s-channel single top has the same final state as WH l bb => benchmark for WH search! 1. 25 t (pb) W , H+ s (pb) Tait, Yuan PRD 63, 014018(2001) July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 5

Single Top Candidate Selection Event Selection: Electron • 1 Lepton, ET >15 Ge. V, | |< 2. 0 CDF W(l ) + 2 jets Candidate Event: W Close-up View of Layer 00 Silicon Detector • Missing ET (MET) > 25 Ge. V • 2 Jets, ET > 15 Ge. V, | |< 2. 8 Jet 2 • Veto QCD, Conversions, Cosmics • At least one b-tagged jet, (secondary vertex tag) 12 mm Run: 205964, Event: 337705 Electron ET= 39. 6 Ge. V, Missing ET = 37. 1 Ge. V Jet 1: ET = 62. 8 Ge. V, Lxy = 2. 9 mm Jet 2: ET = 42. 7 Ge. V, Lxy = 3. 9 mm Number of Events / 955 pb-1 Jet Single Top Background S/B W(l ) + 2 jets 74 15500 ~1/210 ~ 0. 6 W(l ) + 2 jets + b-tag 38 550 ~1/15 ~ 1. 6 July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester S/ B 6

Background Estimate W+HF jets (Wbb/Wcc/Wc) Top/EWK (WW/WZ/Z→ττ, ttbar) • W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo, calibrated in generic multijet data • MC normalized to theoretical cross-section Non-W (QCD) b • Fit low missing ET data and extrapolate into signal region tt Di non-W Z/ • Multijet events with semileptonic b-decays or mismeasured jets Wbb W+HF jets (Wbb/Wcc/Wc) Mistags Wcc Wc • W+jets normalization from data and heavy flavor (HF) fraction from MC Mistags (W+2 jets) • Falsely tagged light quark or gluon jets • Mistag probability parameterization obtained from inclusive jet data July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 7

Signal and Background Event Yield CDF Run II Preliminary, L=955 pb-1 Event yield in W+2 jets s-channel 15. 4 ± 2. 2 t-channel 22. 4 ± 3. 6 tt 58. 4 ± 13. 5 Diboson 13. 7 ± 1. 9 Z + jets 11. 9 ± 4. 4 Wbb 170. 9 ± 50. 7 Wcc 63. 5 ± 19. 9 Wc 68. 6 ± 19. 0 Non-W 26. 2 ± 15. 9 Mistags 136. 1 ± 19. 7 Single top 37. 8 ± 5. 9 Total background 549. 3 ± 95. 2 Total prediction 587. 1 ± 96. 6 Observed July 19 th, 2007 Single top hidden behind background uncertainty! Makes counting experiment impossible! 644 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 8

Jet Flavor Separation • 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 mistags / charm …………. beauty Background Estimate Neural Network Fit • Obtain good jet-flavor separation! W+bottom 299. 0 56. 8 292. 8 26. 3 W+charm 148. 1 39. 4 171. 6 53. 8 Improves sensitivity by ~15 -20%! Mistags 140. 0 19. 8 179. 5 42. 5 Sum 587. 1 96. 6 644. 0 July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 9

Analysis Techniques Likelihood Analysis Neural Network Analysis Matrix Element Analysis July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 10

The Likelihood Function Analysis Nsig Unit Area Nbkg Background. Wbb ttbar Signal tchan schan Leading Jet ET (Ge. V) i, indexes input variable Uses 8 (5) kinematic variables for t-channel (s-channel) Likelihood Function i. e. M(Wb), HT, Qx. Eta, Neural Network flavor separator, Madgraph Matrix Elements etc. July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester Expected signal significance: 2. 0 11

Kinematic Variables HT = ET(lepton, MET, Jets) Qx. Eta(un-tagged jet) Mass(W, b) Unit area Background Wbb ttbar July 19 th, 2007 Signal tchan schan Background Wbb ttbar Signal Background tchan schan Signal tchan schan Wbb ttbar Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 12

Analysis Techniques Likelihood Analysis Neural Network Analysis Matrix Element Analysis July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 13

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 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 14

Neural Network Analysis - Separate Search t-channel Wbb • Two NN’s trained separately for s-channel and t-channel (same input variables) July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 15

Analysis Techniques Likelihood Analysis Neural Network Analysis Matrix Element Analysis July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 16

Matrix Element Method • Pioneered by top quark mass analyses • Now applied to a search! • Calculate event probability density for signal and background Integration over part of the phase space Φ 4 W(Ejet, Epart) gives the probability of measuring a jet energy Ejet when Epart was produced Leading Order matrix element (Mad. Event) E pa rto E jet n Inputs only lepton and 2 jets 4 -vectors! Parton distribution functions c July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 17

Kinematic Variables Used in Matrix Element Analysis • Input to the Matrix Element Analysis are the measured four-vectors of the Lepton, Jet 1 and Jet 2 in the W+2 jets data (>=1 b-tagged jet) Lepton July 19 th, 2007 Jet 1 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester Jet 2 18

Event Probability Discriminant (EPD) • 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): ; b = Neural Network b-tagger output Background-like Expected signal significance: 2. 5 Signal-like July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 19

Results July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 20

CDF Single Top Results Likelihood Neural Networks Matrix Element No evidence of signal s+t<2. 7 pb at 95% C. L. No evidence of signal s+t<2. 6 pb at 95% C. L. p-value = 1. 0% (2. 3 ) s+t=2. 7(+1. 5/-1. 3)pb Expected signal significance 2. 0σ Expected signal significance 2. 6σ Expected signal significance 2. 5σ July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 21

Single Top Candidate Event t-channel single top production has a kinematic peculiarity: - Distinct asymmetry in Q x distribution: Signal region EPD>0. 9 lepton charge (Q) x pseudo-rapidity =-log (tan /2) of untagged jet Jet 1 Run: 211883, Event: 1911511 Lepton Jet 2 Central Electron Candidate Charge: -1, Eta=-0. 72 MET=41. 6 Ge. V Jet 1: Et=46. 7 Ge. V Eta=-0. 6 b-tag=1 Jet 2: Et=16. 6 Ge. V Eta=-2. 9 b-tag=0 Qx. Eta = 2. 9 (t-channel signature) EPD=0. 95 July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 22

Compatibility of CDF Results • Performed common pseudo-experiments CDF Run II Preliminary – Use identical events – ME uses 4 -vectors of lepton, Jet 1/Jet 2 as inputs – LF/NN uses several sensitive variables LF (ME) – 1. 2% of these pseudo-experiments fluctuated as unlucky as the observed data SM Theory – Correlation among analyses: ~60 -70% ME NN (L F) July 19 th, 2007 (NN) • Extensive cross-checks performed • Next round of analysis will shed some light on this issue – coming soon! LF Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 23

Conclusions • Single top production – measures |Vtb| 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 ~ 2. 0 -2. 5σ – 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 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 24

BACKUP slides July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 25

Search for Heavy W Boson • Search for heavy W boson in W + 2, 3 jets • Assume Standard Model coupling strengths (Z. Sullivan, Phys. Rev. D 66, 075011, 2002) • Perform fit to MWjj distribution New Result! July 19 th, 2007 W Previous Limits: • CDF Run I: M(W R) > 566 Ge. V/c 2 at 95% C. L. • D 0 Run II: M(W R) > 630 Ge. V/c 2 at 95% C. L. Limit at 95% C. L. M(W´) > 760 Ge. V/c 2 for M(W´) > M(νR) M(W´) > 790 Ge. V/c 2 for M(W´) < M(νR) Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 26

Four Fold Ambiguity in building m(W, b) 1) Neutrino Pz: • Neutrino px, py from MET • Pz from W-mass constraint Mtop • This yields two solutions: Smaller solution is correct 67. 6% 2) Mlνb reconstruction: • Assumption that tagged jet is from top • In double tagged events, take jet with larger Qlept x ηjet Parton/Jet matching t-channel s-channel 1 tag 96. 6% 51. 1% 2 tags 61. 9% 68. 5% Mlνj 2~170 Ge. V/c 2 • Works well for t-channel only July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 27

Hypothesis Test (ME-Analysis) • • We use the CLs/CLb Method L. Read, J. Phys. G 28, 2693 (2002) T. Junk, Nucl. Instrum. Meth. A 434, 435 (1999) developed at LEP Define Likelihood ratio test statistic: CDF Run. II Preliminary, L=955 pb-1 b s+b Most sensitive bins: Median (‘expected’) p-value = 0. 6% (2. 5 ) Observed p-value = 1. 0% (2. 3 ) July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 28

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 Dilepton+2 jets Lepton+4 jets • b-tagged dilepton+2 jets data (99% ttbar) • b-tagged lepton+4 jets data (85% ttbar) July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 29

Sources of Systematic Uncertainty CDF Run. II Preliminary, L=955 pb-1 Systematic (-1 /+1 ) s-channel t-channel All Single top Shape Variations Jet Energy Scale -1. 4% / 1. 3% -2. 4% / 1. 8% -2. 0% / 1. 6% Initial State Radiation 1. 1% / -2. 0% 2. 6% / 2. 0% / 0. 3% Final State Radiation 1. 3% / 1. 4% 3. 4% / 2. 2% 2. 6% / 1. 9% Parton Dist. Function 1. 0% / -0. 6% 1. 7% / -0. 3% 1. 4% / -0. 4% 1% 2% 1. 6% 6. 1% 7. 8% 7. 4% Luminosity 6% 6% 6% Neural Net b-tagger N/A N/A Mistag Model N/A N/A Non-W Model N/A N/A Q 2 Scale in Alpgen MC N/A N/A Total Rate Uncertainty 9. 1% 11. 3% 10. 5% N/A Monte Carlo Generator Event Det. Efficiency • All rate and shape systematic uncertainties are included as nuisance parameters in the analyses! July 19 th, 2007 Rainer Wallny - Single Top Production at CDF - 2007 EPS Conference Manchester 30