
A
Limit on the Branching Ratio of the FlavorChanging Top Quark Decay
t→Z^{0}c

Authors: AlexanderParamonov
and Henry J. Frisch
Blessed: April 03, 2008
Data: Run II, 1.52 fb^{1}
Documentation:
CDF
Note 9101 (internal only)
Public Note: CDF Public Note 9285
Largeformat Poster: PDF
Abstract.
We measure an upper limit of 8.3%
(95%
C.L.) on the branching ratio
of
the flavorchanging top quark decay t→Z^{0}c for 100%
longitudinally polarized Zbosons using 1.52 fb^{−1} of data.
We parametrize the upper limit as a function of Zboson’s helicity to
cover the full range of possible decay structures. The analysis is
based on the comparison of two processes: pp→tt→WbWb→
l+mE_{T}+bbjj and pp→tt→Z^{0}cWb→l^{+}l^{−}cjjb.
The use of these two decay modes together allows cancellation of major
systematic
uncertainties of acceptance, efficiency, and luminosity. We validate
the MC modeling of acceptance and efficiency for lepton identification
over the multiyear dataset by a precision measurement of the ratio of
the inclusive production of W and Zbosons. To improve the
discrimination, we calculate the top mass for each event with two
leptons and four jets assuming it is a tt event with one of the
top quarks decaying to Z^{0}c. The calculation of the top mass
is performed with a fitter on an eventbyevent basis. The upper limit
on
the Br(t→Z^{0}c) is estimated from a 2dimensional likelihood
of the l^{+}l^{−}cjjb top mass distribution and the
number of
“l + mE_{T} + 4jets” events. The results are limited by
statistics at present.

1. Analysis
overview.
The analysis is
based on comparison of two final states:
 W + 4 jets with at least one btagged jet, where the W decays to
a lepton (electron or muon) and missing energy
 Z + 4 jets with at least one btagged jet, where the Z decays to
a pair of oppositely charged electrons or muons.
Simultaneous
consideration of the two final states allows cancelation
of some systematic uncertainties. All the possible structures of the
unknown FCNC decay t→Z^{0}c are
parametrized using the fraction of longitudinally polarized Zbosons in
t→Z^{0}c decays.
The
analysis of
the two final states results in measurement of toppair
production crosssection and a limit on Br(t→Z^{0}c). We focus on the
measurement of the limit on Br(t→Z^{0}c) from now on. The
statistical machinery is done using the Bayesian approach.
2. Event Selection
We select events
using highPt lepton trigger (electrons or muons), for
the tight lepton we regure Pt > 20 GeV. Each selected event is
required to have a loose Btag (a displaced secondary vertex consisten
with a decay of heavy flavour quark). The Zbosons are identified via
their delopton decays by asking for an additional loose (Pt > 12
GeV) lepton of the same flavor and opposite charge. The invariant mass
of the dilepton pair should be between 66 and 116 GeV. The Wbosons are
reconstructed in events with large missing Energy (mE_{T})
where the transverse mass of the lepton and the missing energy is
greater that 20 GeV.
2.1 Validation of
the Monte Carlo
simulations with Inclusive W and
Z bosons
We used inclusuve W
and Z bosons to test presicion of Monte Carlo simulations in modeling
of electrons and muons. The comparison of the invariant and transverse
masses of the inclusive Z and W bosons, respectively, are shown below.
In addition we estimate the Rration between the production
crosssections of the W to the Z bosons. The ratio agrees with the
NNLO predictions within 2%.
2.2 Final state
with a W
boson and four jets with at least
one Btag
We
select event with a final state containing a leptonic decay of a
Wboson and at least one btagged jet. The limit measurement relies on
event with four jets (at least one of the jets should have a btag) and
a W. The fourjet events have high contribution from decays of the
ttbar pairs.
2.3 Final state
with a Z boson and
four jets with at least one
Btag
Events with a
leptonic decay of Z bosons and at least one btagged jets are of our
interest. Consequesntly we select events with four jets in the final
state. The jet multiplicity distributions are normalized to the 2nd jet
bin.
2.3.1 Top mas
fitting
Additional
discrimination against standard mdel backgrounds can be achived by
reconstructing invariant mass of the top quark in events with a Zboson
and four jets. The reconstructed topmass information is used in the
limitsetting machinery.
3. Statistical
procedure of calculation of the limit on Br(t→Z^{0}c)
The Bayesian approach
is used to analyze events in two final states: "Z + 4jets" and "W=+4
jets". Topproduction crosssection and Br(t→Z^{0}c)
are treated as independent variables.
4. Results and
conclusions
The analysis is
performed in two stages:
First
we validate reconstruction of
inclusive W and Zbosons that play a crucial role in the study. This
test lepton ID, acceptances, and analysis code for both data and Monte
Carlo simulations over multiyear set of runs.
The
second stage if the actual
measurement of the upper limit on Br(t→Z^{0}c). To comput
the limit on Br(t→Z^{0}c) we apply Bayesian approach for a
2diminsional likelihood distribution of Br(t→Z^{0}c) and
number ot topantitop pairs produced (crosssection times
luminosity). To be assumptionindependent of the preffered FCNC
coupling we parametrize the limit as a function of longitudinal
polarization of the Zbosons. This parametrization allows us to cover
the full range of possible helicity structures of the FCNC coupling.
The results obtained with 1.52 fb^{−1} of data are presented in
the table below.
Fraction of Longitudinally
Polarized Zbosons

0.00

0.25

0.50

0.75

1.0

95% C.L. limit on Br(t→Z^{0}c)
using
theoretical pp→tt crosssectionas a prior

9.0%

8.8%

8.6%

8.5%

8.3%

95% C.L. limit on Br(t→Z^{0}c)
using
flat prior

10.2%

10.0%

9.7%

9.5%

9.2%

Page was last updated July 02, 2008
by
Alexander
Paramonov (paramon _at_ hep.uchicago.edu).
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