1. Single Top Production Search at CDF Frontiers in Contemporary Physics III May 23-28 Vanderbilt University, Tennessee Julien Donini University of Padova and INFN On behalf of the CDF collaboration

2. 2 Search for Single Top Top has been discovered at Tevatron in tt pairs Single top production predicted by theory: Wtb coupling s-channel  Tev = 0.88 ± 0.11 pb t-channel  Tev = 1.98 ± 0.25 pb Wt-associated  Tev < 0.1 pb negligible at Tevatron Z. Sullivan hep-ph/0408049, T. Tait hep-ph/9909352 Observation of single top:  cross section is about 40% of ttbar  more difficult to observe due to large background - important W+jets background (for M top = 175 GeV)

3. 3 Single Top Physics Observation of single top allows direct access to V tb CKM matrix element  cross section ~ |V tb | 2  study top-polarization and EWK top interaction Probe b-quark PDF (t-channel) Test non-SM phenomena  heavy W’ boson  anomalous Wtb couplings  FCNC couplings like t  Z/  c  4 th generation Potentially useful for Higgs searches  single top has same final state as Higgs+W (associated) production Why look for single top ?

4. 4 Run I Results Tevatron Run I results at  s = 1.8 TeV Theoretical cross-sections were 30% smaller   t = 1.40 pb   s = 0.76 pb Single top has not been observed: both CDF and DØ set 95% limits  t-channel:22 pb (DØ)13 pb (CDF)  s-channel:17 pb (DØ)18 pb (CDF)  combined:14 pb (CDF) Run II  higher beam energy (1.96 TeV)  higher rate  more luminosity: CDF/DØ have more than 800 pb -1 on tape  better detector acceptance

5. 5 Signal Topology General strategy:  Separated search of signal of s and t-channels single top production in 1-tag and 2-tag samples  Combined search of s+t signal Event selection: we look for W+2 jets channel  one high p T isolated lepton (from W): E t > 20 GeV, |  | < 1  veto Z, dilepton, convertion events  missing E t ( from W): MET > 20 GeV  exactly two jets w/ E t > 15 GeV, |  | < 2.8  ask for at least 1 b-tag Note: s and t-channel both produce 2 b-jets but bbar from t- channel usually lost into beam pipe Additional cuts  cut on m t mass window: 140 < M l b < 210 GeV  for 1-tag sample, leading jet E t > 30 GeV

6. 6 Invariant Mass M l b Calculation b choosing  in 1-tag events, choose b-tag jet as b from top  in 2-tags evt, choose tight jet with larger Q.  as the b from top  Recipe is 97% successful for t-channel 53% (75%) successful for s-channel 1-tag sample (2-tags) Choosing correct neutrino solution  E t ( ) = MET  p z ( ) is obtained from the constraint M l = M W :  Two solutions: pick the one with lower |p z ( )| i.e most central solution. Successful matching rate: 74% t-channel, 71% s-channel

7. 7 Three methods are currently used and well tested in high p t physics  Soft lepton tagging Use semi-leptonic decay of b-quarks: b  l c (BR ~ 20%), b  c  l s (BR ~ 20%). Leptons with softer p t spectrum than W/Z, less isolated.  Secondary vertex identification Iterative fit on tracks with significant impact parameter Efficiency is 45-50% for central top b-jets Mistag rates are kept typically at 4-5%  Jet-probability Probability P that tracks associated with a jet come from primary vertex Heavy flavors: P tends to 0. b-tagging at CDF

8. 8 Signal and Background Modeling Signal modeling  Pythia (or Herwig) do not reproduce correct NLO p t distribution for the b jets in the t-channel  Use MadEvent generator  generate b+q  t+q’ and g+q  t+b+q’ separately and merge them to reproduce the b p t spectrum from NLO calculations (ZTOP) Background modeling ttbar and non-top background  Top pairs: Pythia,  (tt) = 6.7 pb.  Non-top:  W + heavy flavor (62%): Alpgen  mistags (25%) and non-W (10%): estimated from data  diboson (WW, WZ, ZZ) production (3%): Pythia

9. 9 Signal and Background Modeling Expected number of signal and background events compared with observation for L = 162 pb -1  Combined and 1-tag signal mostly dominated by t-channel (65-70%)  2-tags signal: only s-channel  Total background is dominated by non-top events (89%)  Observations are in good agreement with predictions Published CDF results: Phys. Rev. D 71, 012005 (2005)

10. 10 Separate Search for Single Top Maximum likelihood technique to extract the signal content from the data Perform search to separate s-channel and t-channel events: use variables that help discriminate the two channels. For t-channel use kinematical boost  In proton-antiproton collisions:  top production: light quark jet goes in p direction  anti-top: light quark jet goes in p-bar direction  Correlation between  pseudorapidity of untagged jet   lepton charge Q  Q.  distribution asymmetric for t-channel s-channel  Count double b-tags g u d b W+W+ t z+ direction

11. 11 Q.  Distributions for Separate Search Data and stacked MC templates weighted by the expected number of events in the 1-tag sample.

12. 12 Likelihood Function Joint likelihood function for Q.  distribution in the 1-tag sample and number of events in 2-tag samples: Poisson bg gauss. constraintsSyst. Uncert.  4 processes: t-channel (j=1), s-channel (j=2), ttpair and non top (j=3, 4)   k : mean number of events in bin k of Q.  distribution   d : mean number of events in the 2-tags sample  n k, n d : observed number in data  the background is allowed to float but is constrained to SM predictions  7 sources of systematic uncertainties t or s-channel k: bin index j: process index

13. 13 Systematic Uncertainties Systematic acceptance uncertainties for t-channel, s-channel and combined signal Main systematics: b-tagging (7%) luminosity (6%) top mass (4%) JES (4%)

14. 14 Expected Sensitivity for Separate Search Perform N MC experiments to estimate expected sensitivity:  For each exp. we integrate out all nuisance parameters (all variables except  sig ) from L sig, and calculate the upper limit at 95% C.L.  Calculate the median of N individual upper limits t-channel s-channel

15. 15 Posterior probability densities for data t-channel s-channel The maxima of the probability densities give the most probables values for the cross sections: Since all these results are compatible with zero, we set upper-limits (at 95% CL):  t-ch = 0.0 +2.4 -0.0 pb  s-ch = 4.6 ± 3.8 pb  t-ch < 10.1 pb  s-ch < 13.6 pb

16. 16 Combined Search for Single Top Measurement of the combined t-channel plus s-channel signal in the data  use kinematic variable that discriminates s+t channels from the background  total transverse energy: H T distribution (scalar sum of MET, E t of lepton and jets)  same H T distribution for s and t-channels  different distributions for ttbar and non-top background Make a likelihood function similar to that in separate search Use smoothed H T distributions

17. 17 H T Distributions for Combined Search Data Distributions versus SM Expectation for combined sample

18. 18 Limits for Combined Search Expected sensitivityProbability density from data  s+t (MPV) = 7.7 +5.1 -4.9 pb  s+t (95% CL) < 17.8 pb  s+t (a-priori) < 13.6 pb at 95% CL

19. 19 Summary  No significant evidence for single top quark production  set first limits on single top cross section  compared with Run I improvement of upper limits of 28% (t-channel) and 20% (s-channel)  Technical improvements of the analysis  improved MC modeling for single top events  full Bayesian treatment of systematic uncertainties 95% CL limits and most probable x-sections values (pb), with 162 ± 10 pb -1

20. 20 Multivariate Analysis New analysis improvements  Resolution of M l b :  Factors contributing to poor resolution: Jet energy, b-jet choosing, neutrino choosing, lepton energy.  use a kinematic fitter to find the neutrino solution most consistent with measured values of the b-jet and MET  Multivariate likelihoods  use a combination of variables to optimize signal and background separation  variables include Q* , dijet invariant mass, cos  lepton – other jet, H T, E T  likelihood function computed from distributions of each variable  compute sensitivities (i.e how much luminosity for discovery ?)

21. 21 Conclusion CDF accumulated more events than in entire Run I Single top search at the Tevatron is more challenging than anticipated CDF prepares advanced analysis techniques  multivariate (neural-network) analysis CDF expects 3  signal evidence with 1.5 fb -1 Many improvement are needed to observe single top in the next few years but it is feasible in Run II !

22. 22 Backup slides

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