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20th International Conference on Supersymmetry and Unification of Fundamental Interactions

20th International Conference on Supersymmetry and Unification of Fundamental Interactions. Search for Anomalous tWb Couplings at DØ. Experimental setup at DØ Single top quark production Anomalous couplings in single top quark and W helicity Conclusion and outlook. Liang Li

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20th International Conference on Supersymmetry and Unification of Fundamental Interactions

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  1. 20th International Conference on Supersymmetry and Unification of Fundamental Interactions Search for Anomalous tWb Couplings at DØ • Experimental setup at DØ • Single top quark production • Anomalous couplings in single top quark and W helicity • Conclusion and outlook Liang Li Shanghai Jiao Tong University

  2. The Tevatron Accelerator Tevatron Collider • Proton-antiproton collider with s=1.96 TeV • 36x36 bunches with 396ns between crossings • ~5 collisions per bunch crossing • LpInst ~ 4x1032cm-2s-1 Run I 1992-1995 • Top quark discovered! Run II 2001-2011 • Single Top quark discovered! • Many more exciting physics results Tevatron collected significant amount of data • ~12 fb-1delivered, ~ 11 fb-1 recorded, ~ 10 fb-1 after data quality per experiment

  3. The DØ Detector • Tracking • Silicon + fiber tracker • 2T magnetic field solenoid • Pre-shower detectors • Calorimeter • Liquid argon (EM+HAD) • Muon system • Wire chambers • 1.8 T iron toroid • 3 tiered trigger system

  4. Top Quark Physics Top Quark is unique • Heaviest fundamental particle now • Mass = 173.3 ± 1.1 GeV • ~40 times heavier than bottom quark • Constraint on Higgs mass • Strong coupling to the Higgs • Higgs-top Yukawa coupling • Closely linked to EWSB: play a significant role in new physics scenarios related to the EWSB • Very short lifetime (~ 5×10–25 s) • Decays before hadronization and makes it the only “bare quark” • Decays almost 100% of the time to Wb • Top Quark Physics is a rich field • Production rates and properties • Precise test of SM and search for new phenomena MW ~ Mtop2 MW ~ lnMH

  5. Not W+ (W′,H+?) Width Tau decay FCNC (g→tu, g→tc) Lifetime Anomalous Wtb couplings Separate t-channel, s-channel cross sections CKM matrix element Vtb Single Top Quark Production

  6. Single Top Quark Event Selection Parton distributions SM=1.040.04 pb Event Topology: • High energy isolated lepton (e or mu from W) pT(lepton) > 15 GeV • Missing ET (  from W) Missing ET > 20 GeV • One b-quark jet (from t) and a light flavor jet and/or another b-jet 2–4 jets with pT > 15 GeV, || < 3.4 Leading jet pT > 25 GeV Mt = 172.5 GeV SM=2.260.12 pb PRD 74, 114012 (2006)

  7. Single Top Quark Cross Section s-channel evidence possible Tevatron legacy measurement Sensitivity to new physics!

  8. Anomalous tWb Couplings • Top quark couplings to W boson is a good place to look for deviations from SM • Effective single top production cross section:  = A (LV)2 + B (RV)2 + C (LV• LT)+ D (RV• RT) +E (LT)2 +F (RT)2 • LV,T = Vtb • fLV,T • SM (left-handed vector coupling, LV ): LV = |Vtb| ~ 1, all other =0 • Non-SM (anomalous couplings): left-handed tensor (LT), right-handed vector (RV), right-handed tensor (RT) couplings • Single top production directly sensitive to the Wtb interaction: rate, kinematics and angular distributions • Direct constraint on all four couplings

  9. Event Yields and Sensitive Variable Distributions • Same multivariate analysis • technique and event selection as • for DØ 5.4 fb-1 single top quark • analysis [PRD 84, 112001 (2011)]

  10. Discriminant Output

  11. Anomalous tWb Couplings • Simultaneous limit setting for two signals by calculating two dimensional posterior probability density PLB 708, 21 (2012) NO evidence for anomalous couplings in single top production Strong direct constraints on non-SM tWb couplings • Improved limits by a factor of 3 to 5 compared to previous 0.9 fb-1 analysis • |Vtb• fLT |2< 0.06, |Vtb • fRV |2 < 0.93, |Vtb • fRT |2 < 0.13 at 95% C.L. • More improvement when combining with W helicity measurements

  12. ν θ* t b W+ l+ Longitudinal f0 ~ 0.70 Left handed f- ~ 0.30 Right handed f+ ~ 1.4*10-3 W helicity measurements • W helicity : • Three helicity states for W boson from top quark decay • Standard model precise predictions (right-handed W suppressed) • Measure W helicity through cos(*) distribution: best-fit using templates PRD 83, 032009 (2011)

  13. Combined Results • Non-SM tWb couplings can alter W helicity fractions significantly • W helicity measurements from top pair production and single top quark measurements provide complementary information • Combination gives the best limits PLB 713, 165 (2012) Significant improvement on couplings limits • |fLT |2< 0.05, |fRV |2 < 0.30, |fRT |2 < 0.12 at 95% C.L.

  14. Summary and Outlook • No evidence for anomalous tWb couplings! • Set limits at 95% C.L. • Significant improvements over previous analyese Expect stronger constraints from LHC data soon • From both W helicity and single top quark sector • s-channel single top quark production still remains as one of Tevatron legacy measurements

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