From the sublime to the ridiculous: top physics and minimum bias events in the ATLAS detector at the LHC

Wraight, Kenneth (2011) From the sublime to the ridiculous: top physics and minimum bias events in the ATLAS detector at the LHC. PhD thesis, University of Glasgow.

Abstract

This thesis is comprised of two separate physics themes, both of which involve the ATLAS
detector situated at the LHC at CERN. The first constituent is a study of the top quark signal
in the fully-leptonic channel for proton-proton collisions at a centre-of-mass energy of
10 TeV. Here an event counting analysis is performed based on Monte Carlo simulation.
This is supplemented by a study into one of the sources of systematic error. The second
component is forward-backward correlations in minimum bias events. For this, there is a
Monte Carlo hadron-level comparison of the correlation for 900 GeV centre-of-mass collisions,
followed by a comparison of Monte Carlo predictions to data for 900 GeV and 7 TeV
collisions.
Top Physics
A measurement of the fully-leptonic ttbar cross-section in the three decay channels ee, mumu, and
emu is performed on ATLAS produced fully simulated pseudo-event data-samples. Selection
rates for signal and background events consistent with ATLAS results are found along
with the kinematic distributions of selected events. A calculation of the non-hadronic ttbar
cross-section, based on the measured cross-sections, will then return the theoretical value of
217:06pb used to generate the original samples, showing the closure of the pseudo-analysis
process.
A more detailed study is made of the systematic uncertainty arising from variations in the
initial (ISR) and final (FSR) state showering models, based on the Pythia event generator. A
fast simulation of the ATLAS detector is used with similar object and event selection to the
fully simulated case. The effect of ISR variations on the signal is found to be negligible as it
is washed out in the subsequent decays of the ttbar system. However, the effect of FSR is found
to cause 5% uncertainty in the selected signal events. In addition, in the main background of
each of the selection channels the effect of FSR is found to produce variations of up to 30%
in well populated channels. The variations in signal and background measurements will then
be used to calculate a new estimate of the systematics on the measured ttbar cross-section for
each channel.
Minimum Bias
A detailed study of the forward-backward (FB) correlation and event shapes of a selection
of Pythia tunes for pp collisions with CoM = 900 GeV is performed. This includes an investigation
into the sources of particle production in generated minimum bias events as well
as the component sub-processes in generated minimum bias events. The tunes are found to
be practically degenerate (within 10 - 20% variation) for the 'standard' distributions. The
inclusion of a new observable, namely the forward-backward correlation, to the standard
set is recommended. The study finds that the FB-correlation and its pT and dependent
variations are able to discern differences between the selected tunes to a greater degree than
the usual inclusive distributions. Further, the FB-correlation is found to be sensitive to the
particle production processes within the tunes, an invaluable property for the purposes of
generator tuning.
A measurement of the forward-backward correlation for pp collision of CoM = 900 GeV
and 7 TeV at the LHC using the ATLAS detector is made. The measured correlation is
compared to the predicted correlation of several ATLAS centrally produced generator tunes.
A correction procedure is developed and validated on the generator samples to correct the
generated correlation to the hadron-level correlation. This is then applied to the measured
correlation and a comparison of corrected data to the hadron-level predictions of the generated
tunes made. The corrected correlations at the two collision energies are compared as
well as the calculation of a global correlation at both energies. The measured and corrected
correlations are found to lie above the predicted distributions at both energies and across the
eta-range. Further investigation of measured correlation using augmented FB-correlations is
recommended.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Chapter 6 based on Eur.Phys.J.C71:1628,2011
• Keywords: Particle Physics, Top Physics, Minimum Bias, Forward-Backward Correlations, ATLAS, LHC
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Buttar, Dr. Craig M.
• Date of Award: 2011
• Depositing User: Mr Kenneth G Wraight
• Unique ID: glathesis:2011-2723
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 04 Jul 2011
• Last Modified: 10 Dec 2012 13:59
• URI: https://theses.gla.ac.uk/id/eprint/2723