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A Limit on the Branching Ratio of the Flavor-Changing Top quark decay t →Zc

A Limit on the Branching Ratio of the Flavor-Changing Top quark decay t →Zc. Alexander Paramonov , Henry Frisch, Carla Pilcher, Collin Wolfe, Dan Krop CDF Collaboration Meeting. March 14, 2008 CDF 9101 + http://www-cdf.fnal.gov/~paramon/internal/. Outline. Event selection.

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A Limit on the Branching Ratio of the Flavor-Changing Top quark decay t →Zc

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  1. A Limit on the Branching Ratio of the Flavor-Changing Top quark decay t→Zc Alexander Paramonov, Henry Frisch, Carla Pilcher, Collin Wolfe, Dan Krop CDF Collaboration Meeting. March 14, 2008 CDF 9101 + http://www-cdf.fnal.gov/~paramon/internal/

  2. Outline • Event selection. • Machinery of the acceptances and the expected numbers of events. • Properties of the FCNC decays and presentation of the limit in a general (model-independent) way. • Signal regions (events with B-tags). • Details of the calculation of the limit on the Br(t → Zc) • We use 1.52 fb-1 of data.

  3. Motivation We saw a Funny Bump in June 2006. Its invariant mass was close to the top mass. We decided to test a hypothesis that the bump was due to FCNC decay of t→Zq. We froze the selection criteria. • The existing limits on the FCNC are far away from the theoretical expectations • The SM contribution is negligibly small so any signal is an indication for new physics • Some extensions of the SM predict measurable rates It turned out to be just a fluctuation :(

  4. What are we looking for? • We test a hypothesis that some fraction of top quarks decays to Zc (via Flavor Changing Neutral Current) so we consider three possible decays of the top pair: ZcZc, ZcWb, and WbWb. • We are working with two final states: A. two leptons consistent with a decay of Z and 4 jets with at least one B-tagged B. lepton + missing energy (mET) + 4 jets with at least one B-tagged • Final state A is contributed mostly by ZcZc → l+l-ccjj and ZcWb → l+l-bcjj • Final state B is contributed by WbWb → l mET bbjj and WbZc → l mET bcjj (events where Z → l+l- fakes W-decay are taken into account)

  5. What are we measuring? • Cross-section measurements suffer from uncertainties in luminosity, acceptance, and efficiency. • The key idea for this analysis is to study (tt→ZcWb), (tt→ZcZc), and (tt→WbWb) simultaneously with 2-dimensional likelihood which is a function of Br(t→Zc) and Nttbar (number of top pairs produced). • The uncertainty on luminosity does not affect the measurement directly.

  6. Event Selection • We use standard definitions for tight (> 20 GeV) and loose (> 12 GeV) leptons (including fiducial cuts). • We use electrons and muons (high-Pt lepton triggers) • We use the loose SecVTX tagger to identify decays of b- and c-quarks. Each event is required to have at least one tag. • We select two types of events: • A. Two leptons and 4 jets with at least one B-tagged jet. The leptons should be consistent with a Z-decay. • B. Lepton + missing ET and 4 jets with at least one B-tagged jet. • Using the top group fitter we can fully reconstruct the top mass in tt → WbZc→ l+l-bcjj and tt → ZcZc→ l+l-ccjj final states. • We validate our acceptances and efficiencies using inclusive W’s and Z’s via the R-ratio.

  7. Main Formulas Expected number of W + 4 jets events, where W decays leptonically: Expected number of Z + 4 jets events, where Z decays leptonically: The independent parameters are: and

  8. What do we know about FCNC decays. • The FCNC decays of the top quark can be very different from the regular t→Wb decays. • The Z is coupled to left-handed and right-handed fermions but W is coupled only to left-handed. • The exact structure of the t→Zc coupling is not known and there is a number of possibilities. • What can we do?

  9. Parameterization of the FCNC decays Longitudinally polarized Z’s decay the following way: • Please note that Z’s couple to right and left fermions but W’s couple only to left ones. • In the end any type of FCNC decay of a top quark can be described with a proper fraction of longitudinally polarized Z-bosons. Left-handed Right-handed

  10. Parameterization of the FCNC decays • For Example, a distribution of cos(Θ*) for 65% longitudinally polarized Z’s (the rest are right-handed) looks like this: cos(Θ*)

  11. Likelihood • At the end we are going to have a 2D PDF (likelihood) L(Br(t→Zc), Nttbar). The PDF is constructed for any given fraction of longitudinally polarized Z’s (in the FCNC decay). • We construct posterior distributions using the likelihood functions. These are used to compute limits on the branching fraction. • We find limits for five values of Z helicity by varying fraction of longitudinally polarized Z-bosons from 100% to 0%.

  12. W + B-tag We scale cross-section of the W+HF samples with a single coefficient to match 2-jet bin.

  13. W+ 4 jets with at least 1 B-tag The color scheme is the same as that for the previous two figures electrons muons

  14. Z+B-tag The Z+HF contributions are normalized with a single coefficient to match the 2-jets bin. The 1-jet bin is not used in this analysis at all.

  15. Mass templates We take “Z+4” jets and form mass templates which are used to make an estimate of upper limit on the number of tt->ZcWb events. We do not fit! We compute likelihood using these distributions.

  16. The Top Mass Fitter. • The top mass fitter is almost the same as that for the top mass measurement. • It utilizes a likelihood technique to constrain the jet energy scale. • The χ2 is given below

  17. Systematics. Backgrounds. • Alpgen is used to model W+HF and Z+HF events and there is an uncertainty on the N-jet distribution which contributes to the both Z’s and W’s. We estimate it as 20%. We assume that it is 100% correlated between events with W’s and Z’s with four jets. • Mistag Matrix parameterization suffers from uncertainties on the alpha-beta corrections and the total is 15%. • Normalization procedure is limited from statistics from 2-jet bin and it is on the order 2.5% (for W’s) and 8% (for Z’s).

  18. Ingredients of the likelihood. • Numbers of W+4jets + B-tag events • Top Mass profiles for Z+Btag events (data+ expectations+signal) • Acceptances Ai for any given helicity structure • Systematic uncertainties and correlations between them. + Prior distributions = Limit on Br(t→Zc) at 95% CL

  19. Results / Conclusions We extract upper limits at 95% CL on Br(t→Zc) for five fractions of longitudinally polarized Z-bosons. This allows us to make a model-independent search. In addition we present a set of posterior contours for 2D space of (Nttbar, Br(t→Zc)). The contours are presented in the backups slides. A statistical cross-check with pseudo experiments gives 8.9 ± 2.6 % for 100% longitudinally polarized Z’s and Gaussian prior. The limit goes up with decreasing fraction of longitudinally polarized Z’s because of the acceptances.

  20. Backup Slides

  21. Monte Carlo Samples • Z + Heavy Flavor (HF) and W+ HF are the official top group samples. These are generated with Alpgen and showered with Pythia. The jet matching is MLM-based. • WZ, WW, ZZ, Z+jets, W+jets, and ttbar → WbWb are generated with the standard CDF Pythia and these are standard top samples too. • We use 6.1.4mc patch b (Gen 6) to generate Monte Carlo samples of ttbar → ZcWb and ttbar → ZcZc. The tree-level decays are modeled with Madgraph and showered with Pythia.

  22. Main Formulas.. Continued

  23. How are we going to use it? We are constructing a PDF which includes the observables: where The standard Bayesian approach gives us the following 1D posteriori likelihood. This approach is generalized to include any number of observables.

  24. Parameterization of the acceptances • The acceptances A1, A2, A3, A4, and A5 can be introduced as functions of a0 (fraction of longitudinally polarized Z’s) • A4 is a constant • The other Ai have the following structure: As you can see the limit on the branching fraction is a function of a fraction of longitudinally polarized Z-bosons. This fraction can be easily estimated for any FCNC coupling.

  25. Details of Statistics Machinery. • π0(Nttbar) in a priori for Nttbar. It is a Gaussian which mean value is 7.6*L and which deviation is 1.1*L (L is the integrated luminosity in pb). The values are based on Mtop = 170.9 +- 1.8 GeV. (This the most conservative distribution. See http://www-cdf.fnal.gov/htbin/twiki/bin/view/TopMassTemplate/EMailFromMLM) • π1(BZ) is another a priori. It is 1 from 0 to 1 and it is 0 everywhere else. • P(BZ | observables) is posterior. It is used to set 1D limits on Br(t→Zc) • A limit is calculated for each fraction of longitudinally polarized Z-bosons (See next slide).

  26. Parameterization of the FCNC decays • The decay property which matters is angular distribution between the direction of the top quark and a fermion since is 2→2 decay and it can be fully described by one angular distribution. • The angle is taken between the direction of top quark and a fermion in the rest frame of Z (or W) boson and it is called Θ*.

  27. The likelihood for 100% longitudinally polarized Z’s

  28. Posterior contours

  29. R-ratio as a precision check i.e. R(theory) = 10.67 the discrepancy is within 2%

  30. Lepton ID Validation. I am showing only a few plots to save time. All the plots are in the CDF note.

  31. More cross-Checks for W’s Mostly we see a spectacular agreement

  32. Cross-Checks for Z’s

  33. Systematics on the acceptances.

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