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Top / Exotics Physics
Top Quark
Pair Production Cross Section with 35 pb-1 of 2010 data
Top Quark Pair
Relative Differential Cross Section with 2 fb-1 of 2011
data
Search for Top Pair
Resonances (Boosted Top Pairs) with 2 fb-1 of 2011 data
Search for Top Pair
Resonances (Resolved Top Pairs) with 2 fb-1 of 2011
data
Combined Search for Top
Pair Resonances with 5 fb-1 of 2011 data
Ongoing: Search for Exotic Same Sign Dilepton Signatures 2012 - 13
Ongoing: Search for Exotic Vector Like Tops with 2012 - 13 data
Ongoing: Search for Top Pair Resonances with 2012 - 13 data
Top Quark
Pair Production Cross Section
Top quark pair production cross section was our
first ATLAS top physics measurement. We analyzed 35 pb-1
of ATLAS collider data collected at a center-of-mass energy √s = 7
TeV. Candidate events with top quark pairs decaying into lepton
plus jets final state were selected using cut based selection. The
primary Standard Model backgrounds for this signal are the W boson
plus jets (W+jets) production and QCD multi-jets production. Other
minor backgrounds include the Z boson plus jets, single top and
diboson production. With the exception of QCD multi-jets, the rest
of the backgrounds and signal were modeled using Monte Carlo
simulation.
In order to extract the number of signal events (signal fraction),
a method of maximum likelihood was used with the likelihood ratio
as the estimator. Probability density functions (PDF's) were
constructed by choosing a few kinematic variables with good
discriminating power to reject background, small correlation
between the variables and low sensitivity to systematic
uncertainties associated with their measurement. The signal
fraction was extracted using a maximum likelihood fit. Minor
background contributions (such as the fractions of single top and
diboson production) were first subtracted from the observed number
of collider events before the fit. The fractions of top pairs
signal, W+jets and QCD multi-jets were varied simultaneously and
the resulting templates were fit to the observed likelihood
distribution. Statistical and systematic uncertainties were
evaluated and the resulting cross section measurement was found to
be consistent with that predicted by the Standard Model.
Publication:
Phys. Lett. B711
(2012) 244-263
Analysis Code: SVN
link
Top Quark
Pair Relative Differential Cross Section
The measurement of differential top pair production cross section is
a precision measurement of the Standard Model (SM). Theoretical
predictions of the invariant mass of the top pair system are
calcuated in the SM framework to high degree of accuracy and any
deviation in the shape of measured differential cross section could
indicate new physics beyond it. This measurement was carried out
with three characteristic observables of the top pair system - its
invariant mass, transverse momentum and rapidity in the lepton plus
jets final state. In order to enable direct comparison with
theoretical models, the differential distributions of these
variables were unfolded for detector resolution effects and
corrected for acceptance effects.
My contribution toward this measurement was to lay the ground work
for a baseline analysis in:
- Optimizing the top pair invariant mass observable by comparing
three different reconstruction methods in simulated events and
comparing the mass resolution (difference of true and
reconstructed invariant mass)
- Developing a binning
algorithm for optimal choice of mass bins (variable-sized
bins) which are used in response matrix. Response matrix maps
the binned true events to binned reconstructed events. It is an
input to unfolding
- Choosing an unfolding
method among Singular Value Decomposition (SVD),
bin-by-bin or simple matrix inversion methods so as to minimize
two figures of merit, viz., bias and unfolding error
- Providing reconstructed (nominal and systematic variations)
invariant mass distributions of candidate events for unfolding
Documentation/talks can be found in baseline
analysis, bin-width,
kinematic
fitter and unfolding
configuration
Publication: arXiv
Analysis code: SVN
link
Search for
Highly Boosted Top Pair Resonances
Search for Top
Pair Resonances with Resolved Approach
Combined (Resolved
and Boosted Approaches) Search for Top Pair Resonances