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34th International Conference on High Energy Physics

34th International Conference on High Energy Physics. Single Top Quark Studies. Liang Li University of California, Riverside for the CDF, DØ and H1 Collaborations. Single Top Quark Production at Tevatron. tW production (not yet seen).  ~ 0.25 pb. Tev :  ~1 pb. t-channel (“ tqb ”).

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34th International Conference on High Energy Physics

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  1. 34th International Conference on High Energy Physics Single Top Quark Studies Liang Li University of California, Riverside for the CDF, DØ and H1 Collaborations

  2. Single Top Quark Production at Tevatron tW production (not yet seen) ~ 0.25 pb Tev :  ~1 pb t-channel (“tqb”)  ~ 2pb s-channel (“tb”) Single Top: Electroweak Interaction • Study Wtb coupling • Direct measurement of the |Vtb| CKM matrix element • Test of CKM unitarity • Anomalous Wtb couplings • New physics, example: • s-channel is sensitive to W’, H+ • t(s)-channel is sensitive to FCNC • 4th quark generation? • Study top properties: • Polarization, decay width, lifetime, CP violation … • Background study helps other physics searches, e.g. SM Higgs

  3. CKM Matrix Element |Vtb| • Direct measurement of |Vtb| from cross section measurement: • Calculate posterior in |Vtb|2 :  |Vtb|2 • Assuming standard model production: • Pure V-A and CP conserving Wtb interaction • |Vtd|2 + |Vts|2 << |Vtb|2 • Additional theoretical errors needed (top mass, scale, PDF etc…) • No need to assume three quark generations or CKM unitarity |Vtb| = 0.88 ± 0.14 (exp.) ± 0.07 (theory) |Vtb| = 1.00 + 0.00 (exp.+ theory) - 0.12

  4. Anomalous Wtb Couplings • Top couplings to the W boson is a good place to look for deviations from SM • Effective single top production cross section:  = A (f1L)2 + B (f1R)2 + C (f1L + f2R)2 + D (f2L + f1R)2 • f1: “Vector”-like, f2: “Tensor”-like • SM (left-handed vector coupling, LV ): f1L=1, rest=0 • Contributions also from left-handed 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 Standard Model:  ~ 2.9 pb f2L(R)=1, rest=0 :  ~ 10.4 pb

  5. Anomalous Wtb Couplings • Use same multivariate analysis technique and event selection as for DØ “single top evidence” analysis (Phys. Rev. Lett. 98,181802(2007)) • Simultaneous limit setting for two signals by calculating 2 dimensional posterior probability density arXiv:0807.1692 [hep-ex] First direct constraints on Wtb tensor couplings • Data favor the standard model left-handed vector hypothesis over the alternative hypotheses • Compare with SM value |Vtbf1L| = 1.31 + 0.25 - 0.21

  6. Heavy W’ Resonance • Single top quark decay channel is a promising searching background for heavy W’ with SM-like couplings • W’ decays leptonically into a single top quark • Same final states as single top production • Concentrate on s-channel production • Enhancement from W’ resonance • Consider purely left- and right-handed states • Left-handed couplings W’ • W-W’ interference • DØ includes this term in signal model • Right-handed W’R with M(W’R) > M(R) • Decays to (l, ) and (q,q) • Right-handed W’R with M(W’R) < M(R) • Decays to (q,q) only

  7. Heavy W’ Resonance • Four-vectors of all final state objects are added to obtain the invariant mass of the W’ boson • Use the mass distribution to separate W’ from background and set limit

  8. Charged Higgs Resonance • Discovery of charged Higgs boson is an unambiguous evidence for new physics: no “charged” Higgs boson in SM • Identical final state as s-channel single top quark electroweak production • Three Two-Higgs Doublet Model (2HDM) studied: • Type-I, II and III have different strategies to give mass to fermions • tb decay channel dominates for large region of parameter space • First direct search for heavy (mH+ > mtop )H+ decaying to top • Reconstructed invariant mass M(jet1, jet2, W) showed to be the single most sensitive variable, used to construct binned likelihood function • Upper limits set close to predicted cross sections • Excluded parameter region for 2HDM Type-I: 180< mH+<184 GeV with 23 < tan< 70 95%CL upper limit on 2HDM type-I,III H+

  9. Flavor-Changing Neutral Currents Any evidence for FCNC will be a signal of new physics beyond the SM • FCNC cross sections scale with (gtu/)2 and (gtc/)2 • Use same event selection as in CDF standard model single top production measurement (CDF note 9251) except: • Require exactly one hadronic jet (instead of two jets) Assume no anomalous coupling

  10. Anomalous Single Top Production at HERA Looking for new physics at HERA: anomalous single top production via FCNC • Highest sensitivity to tu • Coupling to Vtuz suppressed due to high Mz • tc suppressed due to low c-density • Events with e + PTmiss in HERA I+II e+p data • Form top/W discriminator (MVA method) 478 pb-1 @ 95% CL

  11. Summary • Single top quark production provides a rich testing ground for SM  new physics • Five studies from three collaborations so far • |vtb|, Anomalous Wtb coupling, W’/H+  tb resonances, FCNC production • Many improved limits • More studies and data to come, stay tuned!

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