Top Quark Properties and Search for Single Top Quark at the Tevatron Meenakshi Narain Boston University Presented at EPS 2005

Top Quark at the Tevatron Top quark discovered a decade ago –(in 1995)! Run I ( ) –  s = 1.8 TeV –Integrated luminosity 120 pb -1 Run II (2001-present) –  s = 1.96 TeV –3 fold increase performance since June’03 –Integrated luminosity by June ‘05: Delivered >1fb -1 On tape ~800pb -1 Analyzed up to ~350 pb -1 World’s only top factory! We are here 8.2 fb fb fb -1 Design Base

Top Quark Physics Top is very massive – It probes physics at much higher energy scale than the other fermions. Top decays before hadronizing –momentum and spin information is passed to its decay products. –No hadron spectroscopy. Top mass constrains the Higgs mass –M top, enters as a parameter in the calculation of radiative corrections to other Standard Model observables –it is also related, along with the mass of the W boson, to the that of the Higgs boson. M top (world average)=  2.9 GeV  top ~ sec

The Top Properties Tour W helicity Anomalous Couplings CP ViolationTop Mass Top Charge Top Spin Top Width Production X-Section Production Kinematics Top Spin Polarization Resonance Production Rare/non SM decaysBranching Fractions|Vtb| Y

Top Quark Decay Properties Does top quark decay 100% of the times to Wb? –B(t  Wb) Search for exotic decay modes of the top quark –t  H+b Properties of the W-t-b vertex –W Helicity –Top quark Charge

Is B(t  Wb) ~ 100%? Within the SM, assuming unitarity of the CKM matrix, B(t  Wb)~1. An observation of a B(t  Wb) significantly different than unity would be a clear indication of new physics: – non-SM top decay, non-SM background to top decay, fourth fermion generation,..

B(t  Wb) can be accessed directly in single top production. Top decays give access to B(t  Wb)/B(t  Wq): R can be measured by comparing the number of ttbar candidates with 0, 1 and 2 jets tagged. –In the 0-tag bin, a discriminant variable exploiting the differences in event kinematics between ttbar and background is used. DØ Run II Preliminary Lepton+jets (~230 pb -1 ) Lepton+jets and dilepton (~160 pb -1 ) In the SM hep-ex/ Results consistent with the SM prediction Measurement of B(t  Wb)/B(t  Wq)

Exotic Decays of the top quark Since R is about 1 –Top quark decays to a b-quark  t  X+b Is X = W + ? OR could X = H + ?. –as predicted by generic 2Higgs Doublet Models?

Search for t  H + b If M H± <m t -m b –then t  H+b competes with t  W+b –results in B(t  Wb)<1. H  decays are different than W decays – affect  (tt) measurements in different channels (dileptons, lepton+jets, lepton+tau). Perform simultaneous fit –to the observation in all channels and –determine model-dependent exclusion region in (tan , M H± ).

Are the other Properties of the Top Quark as Expected? W-t-b Vertex: W helicity Top Charge

W helicity in Top quark Decays Large top quark mass: –Are there new interactions at energy scales near EWSB? helicity of the W boson: –examines the nature of the tbW vertex –provides a stringent test of Standard Model V-A coupling W - Left-Handed fraction F - t b W +1/2 -1/2 +1 W + Right-Handed fraction F + t W b +1/ /2 t W 0 Longitudinal fraction F 0 W b +1/2 0 V-A SUPPRESSED gWtb  |Vtb| (V-A) F 0 = 0.7 F - = 0.3 F + =0

W helicity In the Standard Model (with m b =0): The P T of the lepton has information about the helicity of the W boson: –longitudinal: leptons are emitted perpendicular to the W (harder lepton P T ) –left-handed: leptons are emitted opposite to W boson (softer lepton P T ) Longitudinal Left-handed Right-handed SM: F - = 0.3 F 0 = 0.7 F + =0

W Helicity Likelihood analysis of P T spectrum Consider dilepton channels Fix F 0 =0.7, measure F + (F - =1-F 0 -F + ) Binned likelihood and estimate F + using Bayesian method Run I best (DØ 125 pb -1 ): F 0 =0.56  0.31 Likelihood analysis of cos  * Consider lepton+jets channels Fix F 0 =0.7, measure F + (F-=1-F 0 -F + ) Two analysis: topological and b-tag Run I best (CDF 109 pb -1 ): F + 95% CL Results consistent with the SM prediction: F 0 =0.7, F + =0 L=230 pb -1 hep-ex/ Lepton+jets (  1 b-tag)

Measurement of top quark Charge Is it the Standard Model top ? OR An exotic doublet of quarks (Q1, Q4) –with charges (-1/3,-4/3) and M ~ 175 GeV/c 2 –while M(top) ~ 274 GeV/c 2 W.-F. Chang et al.,hep-ph/ q = -4/3 is consistent with EW data, –new b-couplings improve the EW fit (E. Ma et al., hep-ph/ )

Top Quark Charge Measurement Goal: discriminate between – |Q top | = 2e/3 and |Q ”top” | = 4e/3 Top quark charge is given by the sum of the charge of its decay products Determine: –Charge of W (lepton) –Charge of b-jet Q jet =  q i p Ti a /  p Ti a (here, a=0.6) –Associate b-jets to correct W (charged lepton) The charge of the quark is correlated with the charge of the highest pT hadron during hadronization

Top Quark Charge We need an observable and an expectation for the ”2/3” and ”4/3” scenarios Consider only lepton+jets double-tagged events –Two top quarks in the event  measure the charge ”twice” The exotic scenario is obtained by permuting the charge of the tagged jets q b and q B are taken from the data derived jet charge templates Results coming soon... q b = b lept. side q B = b hadr. side q b = b lept. side q B = b hadr. side qBqB qBqB qbqb qBqB qbqb

Top Quark Production Properties Since top decay properties look quite consistent with SM predictions…. What about its production? –Could it be a “t-prime”? –Search for t’t’ production (t’  Wq) –Could the ttbar pair originate from the decay of a resonance? –Model independent search for narrowresonance X  tt used to exclude a leptophobic X boson: What about single top production? Run I search of X with G=1.2%M: M X >560 95% CL (DØ) and M X >480 95% CL (CDF)

Search for Single Top Quark

Search for Single Top Electroweak Production of top quark: –Measure production cross sections –Direct measurement of |Vtb| (s  |Vtb|2) –Top spin physics (~100% polarized top quark) –s- and t-channels sensitive to different New Physics –Irreducible background to associated Higgs production –Exotic Models (FCNC, Top Flavor, 4th Gen) s-channelt-channel  NLO = 0.88pb ± 8%  NLO = 1.98pb ± 11% hep-hp/ (Harris, Laenen, Phaf, Sullivan, Weinzierl) PRD 63, (2001) 3  theo  (s-channel) (pb)  (t-channel) (pb) SM

Single Top Status Cross sections: s-channel t-channel s+t –NLO calculation: 0.88pb (±8%) 1.98pb (±11%) –Run I 95% CL limits, DØ: < 17pb < 22pb CDF: < 18pb < 13pb < 14pb –Run II CDF 95% CL limits: < 14pb < 10pb < 18pb Other Standard Model production mode (Wt) negligible q  q' W t bb  s-channel  t-channel u d b t W

Signature & Backgrounds Signal for s and t channel mostly similar Lepton + Missing E T + Jets t-channel extra b tends to be forward Similar to top pair production, but with less jets Harder Signal To Find Backgrounds W/Z + jets Production Fake Leptons Top Pair Production WW, WZ, Z , etc. Anything with a lepton + jets + E T signature (t-channel) Much worse than for pair production because of lower jet multiplicity

Object kinematics –Jet p T for different jets Tagged, untagged,... Event kinematics –H (total energy) –H T (transverse energy) –M (invariant mass) – M T (transverse mass) –Summing over various objects in the event Angular variables –Jet-jet separation –Jet pseudorapidity (t- channel) –Top quark spin Discriminating Variables

Separating Signal from Backgrounds Four analysis methods Three (Cut, NN, DT) use the same structure: –Optimize separately for s-channel and t-channel Optimize separately for electron and muon channel ( same variables ) –Focus on dominant backgrounds: W+jets, tt W+jets – train on tb-Wbb and tqb-Wbb tt – train on tb – tt  l + jets and tqb – tt  l + jets –Based on same set of discriminating variables ➔ 8 separate sets of cuts/networks/trees Cut-BasedNeural Networks Decision Trees Likelihood Discriminant

1. Cut-Based Analysis Cuts on sensitive variables to isolate single top –Separate optimizations for s-channel and t-channel –Loose cuts on energy- related variables: p T (jet1 tagged ) H(alljets – jet1 tagged ) H(alljets – jet1 best ) H T (alljets) M(top tagged ) M(alljets) M(alljets – jet1 tagged )  ŝ Factor 2 improvement!

2. Neural Network Analysis Output Node: linear combination of hidden nodes Hidden Nodes: Sigmoid dependent on the input variables Input Nodes: One for each variable x i full dataset electron muon =1 b-tag  2 b-tags =1 b-tag  2 b-tags 2d histograms, Wbb vs tt filter construct networks

Result No evidence for single top signal ➔ Set 95% CL upper cross section limit –Using Bayesian approach –Combine all analysis channels (e, , =1 tag,  2 tags) –Take systematics and correlations into account  t < 12.4 / 11.3 pb  s < 9.8 / 10.6 pb Expected/Observed limit: Expected limit: set N obs to background yield Systematic uncertainty:

Neural Network Output e+   1 tag e+   1 tag e+   1 tag e+   1 tag

Result No evidence for single top signal ➔ Set 95% CL upper cross section limit –Using Bayesian approach and binned likelihood Built from 2-d histogram of Wbb NN vs tt NN Including bin-by-bin systematics and correlations  t < 5.8 / 5.0 pb  s < 4.5 / 6.4 pb Expected/Observed limit:

3. Decision Tree Analysis Sensitivity comparable to Neural Network analysis  t < 6.4 / 8.1 pb  s < 4.5 / 8.3 pb Expected/Observed limit: Replace Neural Networks by Decision Trees –single tree, ~100 nodes –Remaining analysis steps identical Same inputs Same filter configuration Binned likelihood analysis H T >212 Pass Fail p t <31.6M t <352 purity

4. Likelihood Discriminant Analysis New Analysis based on 370pb -1 dataset Different btagging algorithm and selection Likelihood Discriminant: –Input Variables:

Result …  t < 4.3 / 4.4 pb  s < 3.3 / 5.0 pb Expected/Observed limit: Best Limit !!!! Comparison of LH with NN analysis

Single Top Summary Cross sections: s-channel t-channel s+t –NLO calculation: 0.88pb (±8%) 1.98pb (±11%) –Run I 95% CL limits, DØ: < 17pb < 22pb CDF: < 18pb < 13pb < 14pb –Run II CDF 95% CL limits: < 14pb < 10pb < 18pb –RunII DØ 95% Cl Limits: (230 pb -1 ) Cut Based < 10.6pb < 11.3pb Decision Tree < 8.3pb < 8.1pb Neural Network* < 6.4pb < 5.0pb (370 pb -1 ) (new analysis) Likelihood Discriminant < 5.0pb < 4.4pb * = Accepted for publication, hep-ex/

Sensitivity to non-SM Single Top using only electron channel data using only muon channel data

Conclusion Measurements of various top quark properties are underway and will improve with larger data sets The single top cross section limits and sensitivity of the analyses are getting to a level where we can expect to observe single top quark production soon!. Stay Tuned.