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
Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 2
The Tevatron AcceleratorTevatron 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-1 delivered, ~ 11 fb-1 recorded, ~ 10 fb-1
after data quality per experiment
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
Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 4
Top Quark PhysicsTop 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
gHt 2 mt VEV 2 173.1 GeV 246 GeV 1
MW ~ Mtop2 MW ~ lnMH
Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 5
Single Top Quark ProductionNot W+ (W ′,H+?)
FCNC (g→tu, g→tc)
Separatet-channel, s-channel
cross sections
Width
Lifetime
Tau decay
Anomalous Wtb couplings
CKM matrix element Vtb
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) 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
Parton distributionsSM=1.040.04 pb
SM=2.260.12 pbPRD 74, 114012 (2006)
Mt = 172.5 GeV
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 possibleTevatron legacy measurement
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 (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
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 asfor DØ 5.4 fb-1 single top quark analysis [PRD 84, 112001 (2011)]
Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 10
Discriminant Output
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 productionStrong 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
Search for Anomalous tWb Couplings at D0, L. Li (Shanghai Jiao Tong University) SUSY 2012, August 16, 2012 12
W helicity measurementsW 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 f0 ~ 0.70
Left handed f- ~ 0.30
t bW+
l+
νθ*
• Measure W helicity through cos(*) distribution: best-fit using templates
PRD 83, 032009 (2011)
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• |fLT |2< 0.05, |fRV |2 < 0.30, |fRT |2 < 0.12 at 95% C.L.
PLB 713, 165 (2012)
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
