The ATLAS experiment is used to observe the √s=7 TeV proton-proton collisions produced by the LHC at CERN. This gives an unprecedented opportunity to search for physics beyond the Standard Model at hitherto unexplored kinematic regimes. Supersymmetry (SUSY) provides interesting solutions to a variety of theoretical problems that may be encountered in the Standard Model at high energy scales, while providing signatures that may be observed at the LHC. However, in order to produce a search that is sensitive to SUSY it is vital to understand how the physics that has been discovered to date may produce signatures that mimic those expected from SUSY. Statistical models are constructed using both Monte Carlo and data-driven predictions of various background processes. The expectations are compared to the observed data for selections containing one electron or muon, each in association with jets and missing transverse momentum. Kinematic variable shapes, in the form of histograms, are used to enhance the sensitivity of the search. Squark and gluino masses in a MSUGRA SUSY model are excluded up to 1200 GeV, while gluino masses up to 900 GeV are excluded in a simplified SUSY model. Model-independent limits are also set, excluding theoretical models with efficiency times cross section above 1 fb.

<p>At the ATLAS experiment at CERN’s Large Hadron Collider, bunches of protons will cross paths at a rate of 40 MHz, resulting in 14 TeV head-on collisions. During these interactions, calorimeters, spectrometers and tracking detectors will look for evidence that can confirm or disprove theories about the smallest constituents of matter and the forces that hold them together. In order for these sub-detectors to sample the signals from exotic particles correctly, they rely on a constant phase between a clock signal and the bunch crossings in the experiment.</p><p>On each side of the detector, 175 m away from the interaction point, electrostatic button pick-up detectors are installed along the accelerator ring to monitor the beam. A model describing how these detectors function as beam information transducers is constructed and analyzed in order to understand the signal.</p><p>The focus of this thesis is the design, implementation and testing of a system that uses this signal to monitor the phase between the clock signal and the arrival time of the bunches in the center of the detector. In addition, the system extracts information about the proton beam structure as well as the individual bunches. Given the interaction rate and the complexity of the processes the experiment wants to study, vast amounts of data will be generated by ATLAS. To filter out well-understood phenomena, a trigger system selects only the most interesting events to be saved for further offline analysis. A proposal for how the signals from the button pick-ups can be used as input to the trigger system is therefore also presented.</p>

At the ATLAS experiment at CERN’s Large Hadron Collider, bunches of protons will cross paths at a rate of 40 MHz, resulting in 14 TeV head-on collisions. During these interactions, calorimeters, spectrometers and tracking detectors will look for evidence that can confirm or disprove theories about the smallest constituents of matter and the forces that hold them together. In order for these sub-detectors to sample the signals from exotic particles correctly, they rely on a constant phase between a clock signal and the bunch crossings in the experiment. On each side of the detector, 175 m away from the interaction point, electrostatic button pick-up detectors are installed along the accelerator ring to monitor the beam. A model describing how these detectors function as beam information transducers is constructed and analyzed in order to understand the signal. The focus of this thesis is the design, implementation and testing of a system that uses this signal to monitor the phase between the clock signal and the arrival time of the bunches in the center of the detector. In addition, the system extracts information about the proton beam structure as well as the individual bunches. Given the interaction rate and the complexity of the processes the experiment wants to study, vast amounts of data will be generated by ATLAS. To filter out well-understood phenomena, a trigger system selects only the most interesting events to be saved for further offline analysis. A proposal for how the signals from the button pick-ups can be used as input to the trigger system is therefore also presented.

One of the primary goals of the CERN Large Hadron Collider is to search for new physics. Many such searches have been carried out, in particular searches for supersymmetry, yet no new physics beyond the Standard Model has been found. With a large number of free parameters introduced by frameworks such as supersymmetry, it can be difficult to interpret the null results of searches. The first analysis presented in this thesis attempts to tackle this difficulty head-on, and gives a summary of the constraints from the Run-1 ATLAS searches. A combination of 22 searches were used, with integrated luminosities of up to 20.3 inverse femtobarns of 7 and 8 TeV data. The results are interpreted in the context of the 19-dimensional phenomenological MSSM, and are presented in terms of the masses of supersymmetric particles. Constraints from dark matter, heavy flavour and precision electroweak measurements were incorporated, and results are also interpreted in terms of these observables. Properties of models missed by the Run-1 searches are also shown. The second analysis presented in this thesis documents a direct search for new physics, using 18.2 inverse femtobarns of 13 TeV data collected during 2015 and 2016 by the ATLAS detector. The search targets final states with large jet multiplicity (at least 7 to at least 10 jets), which can arise from the pair production of gluinos decaying via a cascade. Further requirements are imposed on the sum of masses of reclustered large-radius jets. No evidence for new physics is found, and the results are interpreted in both a model-independent way and in terms of two simplified supersymmetric models, one of which was inspired by the results of the first study. Limits on the gluino mass of up to 1600 GeV are set at the 95 &percnt; confidence level, extending previous limits.

This thesis reports an analysis of 3.0fb^-1 of pp collision data collected by the LHCb experiment in 2011 and 2012. Decays of neutral B mesons to neutral D mesons and excited neutral kaons are reconstructed, because of their sensitivity to the CP-violating phase of the CKM matrix, γ.

The top quark is the most massive fundamental particle in the Standard Model of particle physics. Only experimentally observed in 1995, it can be used as a precise test of Standard Model predictions, and it could lend insight to the problem of what lies beyond the Standard Model. This thesis presents a measurement of top-quark pair production using data collected at a center-of-mass energy √s=7 TeV in 2011, and a search for production of vector-like quarks using data collected at √s=8 TeV in 2012. Both datasets were recorded by the ATLAS detector, a multipurpose proton-proton collider located at the CERN LHC outside of Geneva Switzerland. The top-quark pair production cross-section is measured as a function of four different variables and the results are presented as normalized, differential spectra. The variables considered are the transverse momentum of the top quark, and the mass, rapidity, and transverse momentum of the top-quark pair system. Events are selected in the lepton+jets channel, and the measured spectra are corrected for detector resolution and efficiency. The final results are compared with predictions from various Monte Carlo generators, theoretical calculations and proton parton distribution functions and found to be in reasonable agreement. Data is found to be softer than all predictions, particularly for high values of top-quark transverse momentum and the top-quark pair invariant mass. The search for vector-like quarks focuses on new heavy quarks that decay with a large branching ratio to a Z boson and a third generation Standard Model quark. Events are selected with at least two leptons (electrons or muons), and two of the leptons are required to reconstruct a Z boson with high transverse momentum. No significant excess of events is observed above the Standard Model prediction. Upper limits on the masses of vector-like T and B quarks are derived for various branching ratio hypotheses.

Many ATLAS analyses study events with muons in them including those searching for the Higgs boson and new physics. Cosmics muons, however, can also occasionally enter the detector and mimic the trajectory of a muon from one of the collisions produced by CERN’s Large Hadron Collider. By understanding the different ways ATLAS triggers on, collects, reconstructs, and analyses data from cosmic rays and collisions, the flux of cosmic muons with transverse momenta above 20 GeV in the central region of the detector was measured to be 1.34 ± 0.06 (stat.) s^−1 m^−2 . At the same time the cosmic muon charge ratio has been measured to be 1.3 ± 0.1 (stat.). This measurement of the cosmic muon flux in ATLAS is the first step in quantifying the sizes of the cosmic muon backgrounds to various physics analyses that look for events with muons. / Graduate

This research project searches for new physics in the τ sector that would resolve the tension between BABAR measurement for the pion-photon transition form factor Fπ0(Q^2)and the standard model asymptotic prediction. This behaviour could be explained by a new light pseudo-scalar state that mixes with the π 0 and enhances its coupling to the c and b quarks or the τ lepton, or by a new spin-0 particle with mass very close to the π 0 . We examine one channel to test for existence of such objects: their creation in association with τ pairs in e+ e− collisions. The analysis uses a typical cut-based approach as the large predicted cross-sections and the kinematics of the final states allow for a direct selection of signal events and background suppression. Current simulation studies predict a 90% CL limit on the production cross-section on the order of 100 fb in case of no signal, while the theory predicts production cross-sections on the order of 1 pb to 100 pb depending on the model. / Graduate / 0798 / beaa@uvic.ca

In the high energy environment of the Large Hadron Collider, there is a finite probability for the longitudinal tail of the hadronic shower represented by a jet to leak out of the calorimeter, commonly referred to as longitudinal hadronic shower leakage, or jet 'punchthrough'. This thesis prescribes a method for identifying such 'punch-through' jets via the use of muon activity found behind a jet in the ATLAS muon spectrometer, finding an occurrence rate of up to 18% in the worst affected regions. 'Punch-through' jets were found to degrade the measured jet energy scale by up to 30%, and jet energy resolution by a factor of 3. A correction to remove these effects was developed in Monte Carlo and validated in data, with associated systematic uncertainties derived. The correction was found to negate the degradation of the measured jet energy scale, improving the jet energy resolution by up to 10% in the worst affected regions, and up to 1.6% overall. The correction was integrated into the final 2012 ATLAS jet energy calibration scheme as the fifth step of the Global Sequential corrections. The prescription developed in this thesis to derive the correction is currently being used by ATLAS in Run II of the Large Hadron Collider.

Neutral current Drell-Yan production in proton-proton collisions at the LHC was studied with the ATLAS detector. The 20.1 inverse femtobarn data set used in this precision measurement was collected in 2012 during which the LHC collided protons at a centre-of-mass energy of 8 TeV. The production rate or differential cross-section was measured in three-dimensions: invariant mass, absolute rapidity, and cosine of the polar angle in the Collins-Soper frame. A measurement of the forward-backward asymmetry was obtained from the differential cross-section by summing over the forward and the backward events and taking their difference. The three-dimensional differential cross-section measurement presented in this dissertation can be used to constrain the invariant mass- and rapidity-dependent parton distribution functions of the proton and the forward-backward asymmetry results can be used to extract a measurement of the weak mixing angle. / Graduate