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Study of Rare Top Decays at the Tevatron

Study of Rare Top Decays at the Tevatron

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Study of Rare Top Decays at the Tevatron

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  1. Study of Rare Top Decays at the Tevatron John Strologas University of New Mexico for the CDF and D0 collaborations John Strologas, Top Quark Symposium

  2. Does top always decay to bottom? • According to the SM, the top quark almost always decays to a b quark • B(tWb)~1 • Most of the SM rare decays of the top are really rare • B(tWs)<0.18%, B(tWd)<0.02% (these are the larger ones!!) • An observation of a B(tWb) considerably different than unity will be an indication of new physics • Non-SM top decay • Non-SM background to top decay • Fourth generation • ??? John Strologas, Top Quark Symposium

  3. Current Rare Top Decay Results from the Tevatron • The R=B(t  Wb)/B(t  Wq) measurement • Run II results (CDF and D0) • The t H+b • Run II result (CDF) • FCNC t Zq or t γq • Run I result (CDF) John Strologas, Top Quark Symposium

  4. The Detectors D0 CDF John Strologas, Top Quark Symposium

  5. The ratio R=B(tWb)/B(tWq) • The top quark decays to a b-quark almost always, given: • The unitarity of CKM matrix with 3 flavors • The small measured values of |Vub| and |Vcb| • |Vtb|~ 0.999 • R = |Vtb|2 / (|Vtd|2 + |Vts|2 + |Vtb|2) = |Vtb|2 =99.8% • up to phase space factors, given 3 flavors (SM) • Any significant deviation from R=1, would be an indication of new physics! John Strologas, Top Quark Symposium

  6. How to measure R=B(tWb)/B(tWq)? • Just count the events with b-tagged jets (jets that are associated with b-quarks) • The number of b-tagged jets we expect to see from ttbar decays depends on • R (if low, fewer b-jets are produced) • The tagging efficiency (if low, fewer b-jets are tagged) • We classify the ttbar based on the number of b-tagged jets • The relative rates of events with 0/1/2 b-tags is more sensitive to R John Strologas, Top Quark Symposium

  7. Simple relation between R and tag-multiplicity Assuming zero backgrounds and only b-tagging In reality the relation is more involved and a likelihood is used John Strologas, Top Quark Symposium

  8. Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II • Study tt  Wq + Wq  lνq + qqq(lepton+jets events) • Integrated luminosity of 169 pb-1 (e+jets) and 158 pb-1 (μ+jets) • Isolated ETe(PTμ)>20 GeV, MET>20(17) GeV, Dφ(ΜΕΤ,lepton) cut. • 3-jet and >=4-jet subsets are considered • Two methods of b-tagging used • CSIP (Counting Signed Impact Parameter) • SVT (Secondary Vertex Tagger) • The probability to observe n-tags is calculated for three possible decay modes of the t-tbar pair: • tt  Wb +Wb • tt  Wq +Wb (where q is a non-b quark) • tt  Wq +Wq John Strologas, Top Quark Symposium

  9. Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II • The overall probability to observe n b-tags in an event: Pn-tag = R2Pn-tag(ttWbWb) + 2R(1-R)Pn-tag(ttWqWb) + (1-R)2Pn-tag(ttWqWq) • From that, calculate the expected number of events in 8 samples (e/μ, 3-jet/4-jet, 1-tag/2-tag), which is a function of an input σtt • Construct a likelihood, consisting mainly of Poissons for the 8 samples. The σtt is a floating input to the likelihood) • Maximize the likelihood to extract R. John Strologas, Top Quark Symposium

  10. Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II Use observed events (σttis an input) Maximize Likelihood (for R and σtt simultaneously) Measure R John Strologas, Top Quark Symposium

  11. Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II • 68% and 90% CL contours in the (R,σtt) phase space CSIP SVT Central measurement Central measurement John Strologas, Top Quark Symposium

  12. Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II • Integrated luminosity of 162 pb-1 • At CDF we study both tt  Wq + Wq  lνq + qqq(lepton+jets) and tt  Wq + Wq  lνq + lνq (dilepton) events • Use SVX b-tagging (separate 0-tag, 1-tag and 2-tag sets) • σtt – independent measurement • Lepton+jets set require: • Isolated lepton with ETe(PTμ)>20 GeV, MET>20 GeV and at least 4 jets with ET>15 GeV • Dilepton set require • At least two leptons (ee, μμ, eμ) with ETe(PTμ)>20 GeV, MET>20 GeV, and at least two jets with ET>15 GeV. • Greater statistical significance comes from the lepton+jets sample John Strologas, Top Quark Symposium

  13. Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II • At CDF, we use the 0-tag sample as well to further constrain R. • This means that we have to measure the top-content in a sample that has no b-tags ! • We do that by utilizing a Neural Network (NN), to measure Ntop(0-tag) • The QCD background is independently estimated • We have also NN measurements of Ntop(1-tag) and Ntop(2-tag), but the statistics are not that great. We prefer to use an a-priori method (based on MC normalized to the lepton+jets data) to estimate the 1-tag and 2-tag backgrounds John Strologas, Top Quark Symposium

  14. Measuring the n-tag top content with a NN at CDF Run II 1-tag 0-tag 2-tag John Strologas, Top Quark Symposium

  15. Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II • We first determine the b- c- and q- jet tagging efficiencies εb, εc and εq , defined as (# tagged jets/# taggable jets) • using MC and correcting with scale factors • We then determine the fraction of MC events with i-taggable b-jets, j-taggable c-jets and k-taggable ql-jets • From the above, using combinatorics we determine the efficiency to have 0, 1 or 2 tags in a particular top event. • We explicitly set the tagging efficiency for a jet coming from a top to εbR +(1-R)εq • Multiply the efficiency to the expected top events, given the estimated background, to get the expected top content in 0/1/2 tags. • Compare the expected top with the observed top in the 0/1/2 tag subsets and extract R by maximizing the likelihood. John Strologas, Top Quark Symposium

  16. Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II Calculate expected events as a function of R Compare to observed and Maximize Likelihood Measure R Set FC lower limit , assuming 3 generations John Strologas, Top Quark Symposium

  17. Top Decay to a Charged Higgs • If we assume two Higgs doublets, then EWK symmetry breaking produces 5 Higgs fields, three neutral and two charged. • The top quark will couple to H+ if mt > mH++mb • B(tH+b) ~ (mt2cotβ + mb2tanβ) + 4mt2mb2 at tree level • tanβ is the ratio of vev for the two Higgs doublets • The coupling of top to H+ will be strong, if tanβ>>sqrt(mt/mb) or tanβ<<sqrt(mt/mb) • If tanβ is low • H+cs is the dominant decay • Unless the mH+ is high enough to dominantly decay as H+  t*b  Wbb • If tanβ is high • H+τν is the dominant decay John Strologas, Top Quark Symposium

  18. H+/topbranching ratios John Strologas, Top Quark Symposium

  19. ~193 pb-1 from MC from XS meas. σtheo=(6.7±0.7)pb hep-ph 0303085 Branching fractions of each decay mode Search for tH+b at CDF Run II • Luminosity of 193 pb-1 – Tree level analysis • Utilizing lepton+jets, dilepton, and lepton+τhadtop cross section analyses • For the lepton+τhad sample require • An electron(muon) with ET(pT)>20 GeV and also MET>20 GeV • τ cuts (track requirements in a jet, calorimetry e/μ vetos) • Zveto, HT>205, >= 2 jets • τ charge (determined from the tracks) opposite of that of e or μ • Calculate the estimate number of top events decaying to H+, with the charged Higgs decaying to any of the three modes. John Strologas, Top Quark Symposium

  20. Search for tH+b at CDF Run II(tree-level analysis) Expected sensitivity (for expected 11 dilepton, 66 lepton+jets and 2 lepton+τ events) John Strologas, Top Quark Symposium

  21. Search for tH+b at CDF Run II(tree-level analysis) Paremeterizing the likelihood as a function of BR tH+b, for tau final states John Strologas, Top Quark Symposium

  22. Search for tH+b(Run I / Run II comparison) John Strologas, Top Quark Symposium

  23. Flavor changing neutral currentsFCNC t Zq or t γq • FCNC at tree level are forbidden by the SM • always cancel if left-handed fermions appear in iso-weak doublets • They are allowed in second-order processes, like penguin diagrams • SM rate: O(10-12). Any observation of top FCNC would be a strong indication of new physics. John Strologas, Top Quark Symposium

  24. Searching for t γq and t Zq at CDF Run I • Run I analysis, 110 pb-1 • F. Abe et al. (CDF), Phys. Rev. Lett. 80, 2525 (1988) • Normalization sample of lepton+jets top candidates • An electron(muon) with ET (pT) > 20 GeV • MET>20 GeV • At least three jets with ET>15 GeV • 34 t-tbar candidates with an estimated background of 9 +/- 1.5 in our data • ISAJET MC is used for the calculation of relative acceptances (FCNC/lepton+jets) John Strologas, Top Quark Symposium

  25. Searching for t γq at CDF Run I • In the search for t γq, we assume that the other top quark decayed to Wb • If the W decayed hadronically • >=4 jets with ET>15 GeV • A photon with ET>50 GeV, • b-tag of a jet related to top decay • a photon-jet mass consistent with a top (140-210) • The rest of the jets should have total ET>140 GeV (consistent with a top) • This channel carries 40% of our acceptance • If the W decayed leptonically • A lepton with ET(pT)>20 GeV and MET>20 GeV • >= 2 jets with ET>15 GeV • A photon with ET>20 GeV • The jets should have total ET>140 GeV (consistent with a top) • This channel carries 60% of our acceptance • Background of 0.5 events expected in both hadronic and leptonic channels. John Strologas, Top Quark Symposium

  26. Searching for t Zq at CDF Run I • In the search for t Zq, we assume that the other top quark decayed to Wb • We require 2 electrons or 2 muons, 4 jets with at ET>20 GeV and dilepton mass between 75 and 105 GeV • Expected background is 1.2 events (Z+jets (0.5), residual dilepton-t-tbar (0.6), diboson(0.1)) John Strologas, Top Quark Symposium

  27. Relative Run I acceptances John Strologas, Top Quark Symposium

  28. t γq and t Zq CDF Run I limits • Do set conservative limits, the backgrounds are not subtracted • We see one event in the leptonic t γq sample • Kinematically consistent with radiative t-tbar lepton+jets • B(t γc) +B(t γu)<3.2% at 95% CL • We also see a dimuon t Zq event • Kinematically consistent with Z+jets • B(t Zc) +B(t Zu)<33% at 95% CL John Strologas, Top Quark Symposium

  29. FUTURE: Vtb reach (CDF) and Charged Higgs/FCNC sensitivity (LHC) • LHC Sensitivity (100 fb-1) • B(t γq) ~ 10-4 • B(t Zq) ~ 10-4 • B(t H+q) ~ 5 10-4 • (ATLAS studies) CDF II Assuming R=1 and 3 generations (same analysis) John Strologas, Top Quark Symposium

  30. Conclusions and Plans • Status of rare top decays at the Tevatron • New Run II R=B(tWb)/B(tWq) results • New Run II tH+b result • Run I FCNC result • Current D0 analyses/plans • New R=B(tWb)/B(tWq) under review • H+ search using σ(tt dileptons)/σ(tt lepton+jets) • Hope to start a FCNC analysis in the near future • Current CDF analyses/plans • Improved charged Higgs analysis • Inclusion of all QCD, SUSY-EWK, SUSY-QCD corrections • Separate 1-tag and 2-tag lepton+jets analyses (more sensitivity in the low tanβ) • the analysis is close to blessing • Top FCNC analysis in the tZq sector • the analysis just started John Strologas, Top Quark Symposium

  31. Back-up John Strologas, Top Quark Symposium

  32. CDF charged Higgs analysis implicit assumptions There are at least four important assumptions implicitly taken in the method • The tt production cross section is not affected by the inclusion of the MSSM.Claimed by CDF. No reason against that. • Is the background in the XS measurements affected by the inclusion of the MSSM ? Those processes involving SUSY particles are ignored here.The Higgs sector is considered ahead. • The efficiencies ei,j do not depend in MSSM parameters.This can be shown by analyzing the decay topologies and MSSM coupling constants. • Other H+ decays, besides the three final states mentioned, have negligible branching ratios.True for large fraction of MSSM parameter space. Q: Do the width of top and Higgs modify the efficiencies ?Yes, slightly, but they are corrected for that in the method. John Strologas, Top Quark Symposium

  33. H+/top widths John Strologas, Top Quark Symposium

  34. Contributions to the Posterior probability density(three charged Higgs CDF analyses) John Strologas, Top Quark Symposium

  35. D0 RunI Charged Higgs analyses • Direct [PRL 88, 151803 (2002)] • 62.2 pb-1 (multijet+MET trigger) • H+  τν • Loose selection MET>25 GeV, >=4 jets with ET>20 GeV (<=8 with ET>8 GeV) • Use of a neural network cut • Background 5.2 +/- 1.6 (observed 3) • Indirect [PRL 82, 4975 (1999)] • 110 pb-1 • Lepton+jets [ET(pT)>20 GeV, MET>25 GeV, >=4 jets (ET>15 GeV), HT>180 GeV] • Lepton+jets+μ-tagged jet [ET(pT)>20 GeV, MET>20 GeV, >=3 jets (ET>20 GeV), HT>110 GeV] • Background 30.9 +/- 4 (Observed 30) John Strologas, Top Quark Symposium

  36. CDF RunI Charged Higgs analysis • Direct [PRL 79, 357 (1997)] • 100 pb-1 • H+  τν • Hadronic tau cuts, with ET>20 GeV for 1 tau, or ET>30 GeV for 2 taus. • MET>30 GeV • Z veto • Background of 7.4 +/- 2 (7 events observed) John Strologas, Top Quark Symposium

  37. NN for the top content measurement(CDF R measurement) 9 input Variables , 10 hidden nodes 1 Output Ranked based on KS significance John Strologas, Top Quark Symposium

  38. CDF top event tagging efficiencies(efficiencies to tag 0/1/2-jets in a top event) Fijk are the fraction of MC top events with i-taggable q-jets, j-taggable c-jets and k-taggable b-jets εb,, εc, εq are the single jet tagging efficiencies, defined as (#tagged/#taggable) John Strologas, Top Quark Symposium

  39. QCD background size estimation • We estimate the QCD background using the ISO vs MET scatter plot of the actual data: • We define 4 regions: • Signal region: MET>20 GeV & ISO<0.1 • A: MET<10 GeV & ISO<0.1 • B:MET<10 GeV & ISO>0.2 • C: MET>20 GeV & ISO>0.2 • The estimated QCD background fraction in the signal region is: NANC/(NBNsignal) John Strologas, Top Quark Symposium