Measurement of Top Quark Properties at the TeVatron

1. Measurement of Top Quark Properties at the TeVatron Jessica Levêque University of Arizona on behalf of the CDF and DØ collaborations

2. Top Quark Physics Overview Since its discovery in 1995 at Fermilab, the top quark has been intensively studied but most of its properties are still poorly understood due to statistically limited samples. Direct measurements of the top quark properties are necessary : the constraints already set by precision electroweak measurements leave plenty of room for exotic behaviors regarding the charge, mass, branching fractions, decay and production modes. Top quark properties covered in this talk :

3. Top Quark at TeVatron 85% 15% The top quark properties are measured in events where at least one of the W bosons decays into leptons (e, m, t→ e,m) • Large branching fraction ~ 40% • Limited physics background Most of the analyses presented here use b-jets identification to increase the fraction of signal in the selected data samples : • cut on the decay length of the secondary vertex found in jets At the TeVatron, the top quark is mainly produced in pairs via strong interaction. In the following, we will assume : • s = 6.8 ± 0.8 pb[*] • B(t→Wb) = 100% • mtop =175 GeV/c2 [*] Phys. Rev. D 68, 114014

4. Top Quark Lifetime at CDF Top quark lifetime constrained in Standard Model (SM) to be less than 10-24 s (ct < 3.10-10mm) in absence of a 4th quark generation. First direct measurement of the top quark lifetime: - Confirm the identity of the “top” candidates - Detect unexpected production modes through new long-lived particles Method : measure the distance between the initial collision point and the W boson decay vertex via the lepton impact parameter d0 Signal templates generated for top quark lifetimes ct between 0 and 500 mm, combined with background distributions and corrected with the detector resolution to model the impact parameter distribution in data. Detector resolution measured with Z/g*→ e +e-, m+m- Impact parameter of the lepton measured by fitting the data with templates using a maximum likelihood method

5. Top Quark Lifetime at CDF Top quark selection : • 1 lepton (e or m), 3 or more jets • 1 b-jet tagged with secondary vertex • Missing transverse energy • g / Ks / L conversions removed 97 electron tracks and 60 muon tracks selected Maximum likelihood attained with SM template ct = 0 mm Limit : ct < 52.5 mm @ 95% C. L. For comparison : • b-hadrons ct ~ 450 mm • detector resolution ~ 40 mm

6. Top Quark Charge at DØ L = 370 pb-1 Top quark selection : • 1 lepton (e or m), at least 4 jets • 2 b-jets tagged with secondary vertex • Kinematic fit used to reconstruct the top quarks and find the correct (lepton, b-jet) pairing Top (anti-top) charge defined as the jet charge plus (minus) the lepton charge b-jet charge measured with a jet-charge algorithm using tracks with PT > 0.5 GeV/c Jet-charge algorithm calibrated with data using semi-leptonic b-jet decays to determine the charge (corrected for c-jets contamination and B mixing) - 17 top pair candidates selected in data - 2 measurements per event The SM predicts a top quark charge of +2/3e Exotic model with a 4th generation of quarks and a Higgs triplet predict the existence of an additional heavy quark with a charge -4/3e [*] [*] Phys. Rev. D 59, 091503 / 61, 037301 / 65 053002

7. Top Quark Charge at DØ To test the Standard Model, a likelihood ratio is computed from data : psm(q i) = probability to observe the quark charge qi in the SM pex(qi) = probability to observe the quark charge qi in the exotic scenario The value Ldata = 11.5 is then compared to the expected distributions Lsm and Lex derived from Monte-Carlo pseudo-experiments. The number of background events is allowed to fluctuate according to a Binomial distribution. Exotic quark with charge 4/3e excluded @ 94% C. L.

8. Search for tt resonances Test an exotic model [*]predicting the top quark pair production through the decay of a new heavy gauge bosons. The measured top quark pair production cross-section would be higher than expected in the SM and resonances would appear in the tt invariant mass distribution. X ? - The search is model dependant. We consider values for the resonance mass MX in the range [350 - 1000] GeV/c2 and assume a width Gx = 0.012 Mx Monte Carlo templates [*] Phys. Rev. D 92 221801 / Phys. Rev. D 85 2065

9. Search for tt resonances at CDF Top quark selection : • 1 lepton (e or m), at least 4 jets • Large missing transverse energy Top quarks reconstructed with matrix element information and “transfer functions” describing the correlation between partons and jet energies. Background and top quark pair production normalized to SM expectations. The QCD contribution is estimated from data. No excess observed Exclude the production of a leptophobic Z’ with mass Mx < 725 GeV/c2 @ 95% C. L.

10. Search for tt resonances at DØ Top quark selection : • 1 lepton (e or m), at least 4 jets • At least 1 b-jet tagged with secondary vertex • Missing transverse energy Use a constrained kinematic fit to reconstruct the top quarks 4-vectors. Top quark and background processes normalized to SM predictions. The QCD multijets contribution is estimated from data. No excess observed Exclude the production of a leptophobic Z’ with mass Mx < 680 GeV/c2 @ 95% C. L.

11. Search for heavy top quark at CDF Looking for a 4th generation of heavy quarks. Assumptions : produced in pairs, larger mass than the top quark, decaying promptly into W+b[*] Event selection : 1 isolated lepton, at least 4 jets, large missing transverse energy t’ reconstructed mass determined with the mass template method. Signal Monte-Carlo was generated with Mt’ varying in the range [175 - 400] GeV/c2 To discriminate the new physics signal from the SM processes, 2 variables are used : • Reconstructed quark mass Mreco • Total transverse energy in the event HT [*] “beautiful mirror model” hep-ph/0109097

12. Search for heavy top quark at CDF The contribution of the t’ signal is measured with a binned likelihood fit to the 2D function (HT, M t’ ) The tt contribution is normalized to the SM cross-section. The W+jets production rate is a free parameter of the fit. 6.8 events from SM are expected in the tails 7 events are selected in data - No evidence for t’ signal observed Mt’ < 258 GeV/c2 excluded @ 95% C. L.

13. W boson helicity SM prediction : Longitudinal f0 ~ 0.70 Right-handed f+ ~ 3.6 10-4 Left-handed f- ~ 0.30 Assuming a massless bottom quark, the V-A structure of the weak interaction in the SM strongly suppressthe fraction of right-handed W boson produced in t→Wb decay A measured value of f+ ≠ 0 would be a unambiguous signature of new physics Experimentally, the W boson helicity is measured from the cos q* distribution : q* = angle between the charged lepton and the top quark boost direction measured in the W rest frame. SM prediction

14. W boson helicity at DØ Combined result from maximum likelihood fit : Dilepton selection : 2 oppositely charged leptons (e or m), at least 2 jets, large missing transverse energy. Additional kinematic cuts to suppress Z→ℓ+ℓ- events. Lepton+jets selection : at least one lepton and 4 jets. Cut on likelihood discriminant using topological and b-tagging variables to increase the signal fraction. Kinematic fit used to reconstruct the top quark pairs and the W rest-frame, assuming mtop = 175 GeV/c2 In dilepton events, the system is underconstrained and gives two possible angle measurements : both are used in the fit to increase the statistics. The resulting distribution still shows discrimination between V-A and V+A models.

15. Summary All top properties measurements consistent within errors with the Standard Model predictions The analyses are still statistically limited but were intensively optimized and already reached interesting precisions CDF and DØ have now more than 1 fb-1 of data recorded on tape currently being analyzed Updates expected from both experiments for summer conferences If the top quark is hiding something, we may figure it out next summer !