Search for Heavy Top t'->Wb In Lepton Plus Jets Events
in 5.6 fb-1
J. Conway, D. Cox, R. Erbacher, W.
Johnson, A. Ivanov, T. Schwarz
University of California, Davis
Kansas State University
University of Geneva
|Upper limit, at 95% CL, on the production
rate for t' as a function of
t' mass (red). The purple curve is a theoretical cross section. The
blue band represents +/-1 standard deviation
expectation limit (light blue corresponds to +/- 2 standard deviation)
- Event Selection
- Analysis Method
- Systematic Errors
We search for the heavy top (t') quark pair production decaying to Wb
final states in 5.6 fb-1 of the CDF Run 2 data sample of
lepton+jets (requiring at least on tagged b jet). We reconstruct the
mass of the
t' quark and perform a 2D-fit of the observed (HT,Mreco)
physics signal from standard model
backgrounds. HT is defined here as the
sum of the Jet ETs, lepton ET and Missing ET. Mreco is the
reconstructed mass of the t'.
We assume that the t' quark
- has mass greater than the
promptly to Wb final states (w/ Branching ratio ~ 100% (M(t') - M(b')
The search has
theoretical motivation from several different sources.
Higgs models with conserved T-parity suggest a heavy Top
into Wb (C. Wagner et al,
fourth generation of quarks is not excluded by EWK precision data
precision data AFBb b-quark forward backward
asymmetry 2.9 sigma discrepancy with SM -> leptonic sine theta W is
~3.6 sigma away from hadronic sine theta W
We search for the t' in
events which meet several selection criteria
contributing backgrounds after these cuts are from top pair production
as well as W+jets. Much smaller backgrounds include QCD, electroweak
processes, diboson and single top production and Z + jets. All of these
processes except for QCD are modeled using MC simulation.
- one and only one high-pT
20 GeV/ c) isolated electron or muon
- large missing transverse energy (> 20 GeV)
- at least four energetic jets (ET
> 20 GeV after
corrections for detector effects)
- at least one tagged b-jet
We utilize the fact that
the t' decay chain in the regime of interest is identical to the one of
the top quark, the t' mass is reconstructed similarily to the way it is
in the top quark mass measurement analyzes. We use the template method
for top quark mass reconstruction based on the best -fit to
the kinematic properties of final top decay products. For each event
there are 4!/2 = 12 combinations of assigning 4 jets to partons. In
addition, there are two solutions to account for the unknown
z-component of the neutrino momentum. After minimization of the
expression, the combination with the lowest is
selected and the value of the top (t') mass is declared to be the
reconstructed mass Mreco of top (or t' respectively). Unlike
in the top mass measurements we do not reject events that have a poor for reconstructed events
but instead split events into a good and bad category.
In order to improve the discrimination power of our method and improve
the sensitivity to a potential t' signal we split the temnplates into
four regions, based on the number of jets: exactly 4 or greater than or
equal to 5 jets and > 8 or < 8. We use the
observed distributions of the Mreco and total
transverse energy (HT) in the event to distinguish the t'
signal from backgrounds by fitting it to a combination of t' signal,
top, w+jets, electroweak and QCD background shapes (Mreco distributions for jet / bins).
We use a binned in HT and Mreco
likelihood fit in our four regions to extract the t' signal and/or set
an upper limit on
its production rate. We calculate the likelihood as a function of the
t' cross section and use Bayes' Theorem to convert it into a posterior
density in the t' cross section. We can then use this posterior density
to set an upper limit on the production rate of t'.
The production rate for W + jets is a free parameter in the fit. Other
parameters, such as the top pair production cross section, lepton ID,
data/MC scale factors and integrated luminosity are related to
systematic errors and treated in the likelihood as nuisance parameters
constrained within their expected (normal) distributions. We adopt the
profiling method for dealing with these parameters, i.e. the likelihood
is maximized with respect to the nuisance parameters.
The sensitivity to t'
depends on knowing accurately the distribution of (HT, Mreco) in data. One of the
largest sources of uncertainty comes from the jet energy scale. Jets in
the data and Monte Carlo (MC) are corrected for various effects leaving
some residual uncertainty. This uncertainty results in possible shifts
in the HT and Mreco distributions for both
new physics and standard model templates. We take this effect into
account by generating templates with energies of all jets shifted
upwards by one standard deviation and downwards respectively. We then
use a template morphing technique which uses a shape constructed by
interpolation on the nominal and shifted templates.
W + jets Q2 Scale
The effect of the choice of the appropriate Q2 scale for W +
jets production is evalued by using the W + jets MC samples generated
with different Q2 settings. We make use of
samples generated with half and double the nominal Q2 setting. The Q2 systematic is then
incorporated into the likelihood in a manner similar to the Jet Energy
Scale systematic, except the variation is applied only to the W + jets
The systematic error associated with the initial- and final-state
radiation was determined by making use of some ttbar samples with more
more FSR and some samples with less ISR and less FSR. The IFSR error is
then incorporated into the likelihood in a manner similar to the Jet
Energy Scale and Q systematic except it only applies to the ttbar
template. In principle the IFSR also affects the t' templates, however
we found the effect of this shift on the t' templates to be small.
The integrated luminosity is taken to be 5.9% and is represented by an
additional gaussian-constrained parameter multiplying all contributions
except for the QCD background and W + jets, which is normalized from
Two components enter here: the trigger efficiencies for the individual
trigger paths in data and the lepton identification (ID) and
reconstruction Scale Factors to account for such differences between
the data and MC. We apply these errors to all MC-based backgrounds
except W + jets. The uncertainitiy due to these errors is 1% and is
applied in quadrature with the uncertainity due to the NLO theoretical
The Parton Distribution Functions (PDFs) are not precisely known, and
this uncertainty leads to a corresponding uncertainty in the predicted
cross sections, as well as the acceptance. The first is a major part of
the NLO theoretical cross section, the latter is estimated to be 1%
from the ttbar cross section analayses and is summed in quadrature with
the uncertainity due to theory.
The theory uncertainty in the t' cross section is about 10%, mainly due
to uncertainty in PDFs (~ 7%). The other effect comes from uncertainty
in the choice of the scale. We take the theoretical uncertainty in the
cross section as fully correlated with the one of t' pair and introduce
it into the likelihood as a single nuissance parameter.
The rate of b-tagging in top and t' is not perfectly modeled in MC and
so we apply a b-tagging scale factor and take an uncertainty therein.
We follow the standard joint physics scale factor of 0.95 with an
uncertainty of 0.04. Using this uncertainty we generate altered
templates at + 1 sigma
using which we apply a morphing uncertainty as is done with the JES
We use 28 bins for H and
18 bins for the reconstructed mass, with overflow bins defined for
events with H above 800 GeV and / or reconstructed mass above 500 GeV.
Thus there area total of 28 x 18 x 4 = 2016 total bins needed to
be used in the fit. Since with so many bins it is difficult to populate
all of the bins with sufficient MC statistics we developed an algorithm
that will merge contiguous bins with low MC statistics together into
super-bins. These super-bins are the ones used in the likelihood fit.
The criterion used to define the binning is the requirement that each
super-bin in the template has a relative uncertainty due to MC
statistics below 0.4
Results and Conclusions
We tested the sensitivity
of our method by drawing pseudoexperiments from standard model
distributions, i.e. assuming no t' contribution. The ranges of expected
95% confidence level upper limits with one and two standard deviation
bandwidths are shown above along with the
associated upper limit on the
t' mass. These limits are calculated assuming a true top mass of 172.5
GeV. Our measurement of the top mass may have been affeced by the
presence of a higher mass t' and thus we should treat these conclusions
Distributions of HT and Mreco
for zero signal, t'
mass of 360 GeV
Expected and observed limits for the range of t' mass points examined.
HT (left) and Mreco
showing result of the no
signal fit. The normalizations of the various sources and distortions
of kinematic distributions due to systematic effects are those
corresponding to the maximum likelihood when the cross section for t'
is set to its 95% CL upper limit.
|Distributions of HT
(left) and Mreco
showing expected signal
contribution for a t' mass of 360 GeV. The normalizations of the
various sources and
distortions of kinematic distributions due to systematic effects are
those corresponding to the maximum likelihood, the t' cross section is
modified on 02/03/11 by David Cox