1. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 1 Liang Li Shanghai Jiao Tong University Search for Anomalous tWb Couplings at DØ 20th International Conference on Supersymmetry and Unification of Fundamental Interactions Experimental setup at DØ Single top quark production Anomalous couplings in single top quark and W helicity Conclusion and outlook

2. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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 L pInst ~ 4x10 32 cm -2 s -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 -1 delivered, ~ 11 fb -1 recorded, ~ 10 fb -1 after data quality per experiment

3. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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 The DØ Detector

4. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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  M W ~ M top 2  M W ~ lnM H

5. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 5 Single Top Quark Production Not W + (W ′,H + ?) FCNC (g→tu, g→tc) Separate t-channel, s-channel cross sections Width Lifetime Tau decay Anomalous Wtb couplings CKM matrix element V tb

6. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 6 Single Top Quark Event Selection Event Topology: High energy isolated lepton (e or mu from W) p T (lepton) > 15 GeV Missing E T ( from W) Missing E T > 20 GeV One b-quark jet (from t) and a light flavor jet and/or another b-jet 2–4 jets with p T > 15 GeV, |  | < 3.4 Leading jet p T > 25 GeV Parton distributions  SM =1.04  0.04 pb  SM =2.26  0.12 pb PRD 74, 114012 (2006) M t = 172.5 GeV

7. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 7 Single Top Quark Cross Section Sensitivity to new physics! s-channel evidence possible Tevatron legacy measurement

8. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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 (L V ) 2 + B (R V ) 2 + C (L V L T ) + D (R V R T ) + E (L T ) 2 + F (R T ) 2 L V,T = V tb f LV,T SM (left-handed vector coupling, L V ): L V = |V tb | ~ 1, all other =0 Non-SM (anomalous couplings): left-handed tensor (L T ), right-handed vector (R V ), right-handed tensor (R T ) couplings Single top production directly sensitive to the Wtb interaction: rate, kinematics and angular distributions Direct constraint on all four couplings

9. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 10 Discriminant Output

11. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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 |V tb f LT | 2< 0.06, |V tb f RV | 2 < 0.93, |V tb f RT | 2 < 0.13 at 95% C.L. More improvement when combining with W helicity measurements

12. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 12 W helicity measurements W helicity : Three helicity states for W boson from top quark decay Standard model precise predictions (right-handed W suppressed) Right handed f + ~ 1.4*10 -3 Longitudinal f 0 ~ 0.70 Left handed f - ~ 0.30 t b W+W+ l+l+ ν θ*θ* Measure W helicity through cos(  *) distribution: best-fit using templates PRD 83, 032009 (2011)

13. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 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 Significant improvement on couplings limits |f LT | 2 < 0.05, |f RV | 2 < 0.30, |f RT | 2 < 0.12 at 95% C.L. PLB 713, 165 (2012)

14. Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 14 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 Summary and Outlook