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Title: Search for the Flavor Changing Neutral Current Decay $$t\rightarrow qZ$$ in $$p\overline{p}$$ Collisions at $$\sqrt{s}$$ = 1.96 TeV

Abstract

This dissertation presents a blind search for the flavor changing neutral current decay of the top quark t → qZ in pp collisions at [special characters omitted] = 1.96TeV using a data sample corresponding to an integrated luminosity of 1.12 fb-1 collected by the Collider Detector at Fermilab (CDF), This decay is extremely rare in the standard model, and a signal at the Tevatron would be an indication of new physics. Dividing candidate events with a Z boson and four or more jets into a sample of those which have a heavy flavor jet identified by a secondary vertex algorithm and those which do not, we observe data yields consistent with background expectations. We set a 95% C.L. upper limit on the branching fraction B (t → qZ) of 11.3%, consistent with an expected upper limit of 7.8% ± 3.3%.

Authors:
 [1]
  1. Harvard U.
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1342790
Report Number(s):
FERMILAB-THESIS-2007-95
1243484
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Zaw, Ingyin. Search for the Flavor Changing Neutral Current Decay $t\rightarrow qZ$ in $p\overline{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV. United States: N. p., 2007. Web. doi:10.2172/1342790.
Zaw, Ingyin. Search for the Flavor Changing Neutral Current Decay $t\rightarrow qZ$ in $p\overline{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV. United States. doi:10.2172/1342790.
Zaw, Ingyin. Mon . "Search for the Flavor Changing Neutral Current Decay $t\rightarrow qZ$ in $p\overline{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV". United States. doi:10.2172/1342790. https://www.osti.gov/servlets/purl/1342790.
@article{osti_1342790,
title = {Search for the Flavor Changing Neutral Current Decay $t\rightarrow qZ$ in $p\overline{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV},
author = {Zaw, Ingyin},
abstractNote = {This dissertation presents a blind search for the flavor changing neutral current decay of the top quark t → qZ in pp collisions at [special characters omitted] = 1.96TeV using a data sample corresponding to an integrated luminosity of 1.12 fb-1 collected by the Collider Detector at Fermilab (CDF), This decay is extremely rare in the standard model, and a signal at the Tevatron would be an indication of new physics. Dividing candidate events with a Z boson and four or more jets into a sample of those which have a heavy flavor jet identified by a secondary vertex algorithm and those which do not, we observe data yields consistent with background expectations. We set a 95% C.L. upper limit on the branching fraction B (t → qZ) of 11.3%, consistent with an expected upper limit of 7.8% ± 3.3%.},
doi = {10.2172/1342790},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Thesis/Dissertation:
Other availability
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  • This thesis reports the results of a search for the flavor changing neutral current decay of the top quark, t → Zq, in decays of tmore » $$\bar{t}$$ pairs produced in p$$\bar{p}$$ collisions at a center-of-mass energy of 1.96 TeV. This search is performed on a data sample recorded by the Collider Detector at Fermilab (CDF), corresponding to an integrated luminosity of 1.9 fb -1. This search follows a previous CDF analysis that resulted in an upper limit for the branching fraction β(t → Zq) of 10.4% at 95% C.L. using a dataset equivalent to 1.1 fb -1 of integrated luminosity. This thesis extends to 1.9 fb -1 of data, and has improved sensitivity to the small signal with the introduction of a template fit technique that includes systematic uncertainties by a linear interpolation between templates. Using a Feldman-Cousins construction, an upper limit at 95% C.L. is set on β(t → Zq) of 3.7%, with the expected upper limit in absence of a signal is 5.0 ± 2.2% for a top mass of 175 GeV/c 2.« less
  • In hadronic collisions, photons (more » $$\gamma$$) with high energies emerge unaltered from the hard parton-parton interaction and therefore provide a clean probe of the underlying hard-scattering dynamics. Photons produced in these interactions (called direct or prompt) in association with one or more bottom ($b$)-quark jets provide an important test of perturbative Quantum Chromodynamics (QCD) predictions at large hard-scattering scales $Q$ and over a wide range of parton momentum fractions. In addition, the study of these processes also provides information about the parton density functions of $b$ quarks and gluons ($g$), which still have substantial uncertainties. In $$p\bar{p}$$ collisions, \gb-jet events are produced primarily through the Compton process $$gb\to \gamma b$$, which dominates for low and moderate photon transverse momenta ($$\Ptg$$), and through quark-antiquark anni hilation followed by $$g \to b\bar{b}$$ gluon splitting $$q\bar{q}\to \gamma g \to \gamma b\bar{b}$$, which dominates at high $$\Ptg$$. The final state with $b$-quark pair production, $$p\bar{p} \rightarrow \gamma+b\bar{b}$$, is mainly produced via $$q\bar{q}\to \gamma b\bar{b}$$ and $$gg\to \gamma b\bar{b}$$ scatterings. The $$\gamma+2~b$$-jet process is a crucial component of background in measurements of, for example, $$t\bar{t} \gamma$$ coupling and in some searches for new phenomena.\\ This thesis presents the first measurements of the differential cross section $${\rm d}\sigma/{\rm d}\Ptg$$ for the production of an isolated photon in association with at least two $b$-quark jets. The ratio of differential production cross sections for $$\gamma+2~b$$-jets to $$\gamma+b$$-jet as a function of \ptg is also presented. The measurement of the ratio of cross sections leads to cancellation of various experimental and theoretical uncertainties, allowing a more precise comparison with the theoretical predictions. The results are based on the proton-antiproton ! collision data corresponding to an integrated luminosity of $8.7$~fb$$^{-1}$$ at $$\sqrt{s}=$$1.96~\TeV collected with the \DO detector at the Fermilab Tevatron Collider. %In this measurement, we follow an inclusive approach by allowing the final state with any additional jet(s) on top of the studied $b$-quark jets. Inclusive $$\gamma+2~b$$-jet production may also originate from partonic subprocesses involving parton fragmentation into a photon. However, photon isolation requirements significantly reduces the contributions from such processes. The large data sample and use of advanced photon and $b$-jet identification tools enable us to measure the $$\gamma+2~b$$-jet production cross section differentially as a function of $$\Ptg$$. This allows for probing the dynamics of the production process over a wide kinematic range not studied before in other measurements of a vector boson + $b$-jet final state. The measured cross sections and their ratios are compared to the NLO perturbative Q CD calculations as well as predictions based on the $$k_{\rm T}$$-factorization approach and those from the {\sc sherpa} and {\sc pythia} Monte Carlo event generators. We also measure the ratio of cross sections, $$\sigma(p\bar{p}\rightarrow Z+2~b~\text{jets})$$/$$\sigma(p\bar{p}\rightarrow Z+\text{2~jets})$$, for associated production of a $Z$ boson with at least two jets. This measurement uses data corresponding to an integrated luminosity of 9.7 fb$$^{-1}$$ collected by the \DO experiment. The measured integrated ratio is in agreement with predictions from NLO perturbative QCD and the Monte Carlo event generators {\sc alpgen} and {\sc pythia}. A good theoretical description of this process is essential since it forms a major background for a variety of physics processes, including standard model Higgs boson production in association with a $Z$ boson, $$ZH(H\rightarrow b\bar{b})$$, and searches for supersymmetric partners of the $b$ quark. These results will improve our theoretical understanding as we search for phenomena beyond the standard! model us! ing the data from similar collider experiments in the case where the final states of the interaction involved the production of vector bosons in association with two $b$-quark jets.\\ This work has been done in collaboration with \DO experiment but the analyses and results presented in this thesis are my contribution.« less
  • We report a search for the flavor changing neutral current (FCNC) decay of the top quark t {yields} Zq (q = u, c) in p{bar p} collisions at {radical}s = 1.96 TeV using a data sample corresponding to an integrated luminosity of 1.9 fb{sup -1} collected by the CDF II detector. This decay is strongly suppressed in the standard model (SM) and a signal at the Tevatron would be an indication of physics beyond the SM. Using Z+ {ge} 4 jet final state candidate events, both with and without an identified bottom quark jet, we discriminate signal from background bymore » exploring kinematic constraints present in FCNC events and obtain an upper limit of {Beta}(t {yields} Zq) < 3.7% at 95% C.L.« less
  • This dissertation presents the results of the first search for heavy, neutral, longlived particles that decay to photons at a hadron collider. We use a sample of +jet+missing transverse energy events inmore » $$p \bar{p}$$ collisions at √s = 1.96 TeV taken with the Collider Detector at Fermilab. Candidate events are selected based on the arrival time of a high-energy photon at the electromagnetic calorimeter as measured with a timing system that was recently installed. The final result is that we find 2 events, using 570±34 pb -1 of data collected during 2004-2005 at the Fermilab Tevatron, consistent with the background estimate of 1.3±0.7 events. While our search strategy does not rely on model-specific dynamics, we interpret this result in terms of cross section limits in a supersymmetric model with $$\vec{X}$$$0\atop{1}$$ eG and set a world-best e 01 mass reach of 101 GeV/c 2 at e = 5 ns. We can exclude any γ+jet+missing transverse energy signal that would produce more than 5.5 events.« less