Physics - Theses, Dissertations, and other Required Graduate Degree Essays

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Measurement of single top quark production in the Tau+jets channel using boosted decision trees at D0

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The top quark is the heaviest known matter particle and plays an important role in the Standard Model of particle physics. At hadron colliders, it is possible to produce single top quarks via the weak interaction. This allows a direct measurement of the CKM matrix element Vtb and serves as a window to new physics. The first direct measurement of single top quark production with a tau lepton in the final state (the tau+jets channel) is presented in this thesis. The measurement uses 4.8 fb^−1 of Tevatron Run II data in p\bar{p} collisions at \sqrt{s}=1.96 TeV acquired by the D0 experiment. After selecting a data sample and building a background model, the data and background model are in good agreement. A multivariate technique, boosted decision trees, is employed in discriminating the small single top quark signal from a large background. The expected sensitivity of the tau+jets channel in the Standard Model is 1.8 standard deviations. Using a Bayesian statistical approach, an upper limit on the cross section of single top quark production in the tau+jets channel is measured as 7.3 pb at 95% confidence level, and the cross section is measured as 3.4+2.0−1.8 pb. The result of the single top quark production in the tau+jets channel is also combined with those in the electron+jets and muon+jets channels. The expected sensitivity of the electron, muon and tau combined analysis is 4.7 standard deviations, to be compared to 4.5 standard deviations in electron and muon alone. The measured cross section in the three combined final states is sigma(p\bar{p}->tb+X,tqb+X) = 3.84+0.89−0.83 pb. A lower limit on |Vtb| is also measured in the three combined final states to be larger than 0.85 at 95% confidence level. These results are consistent with Standard Model expectations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Microwave flux-ow impedance measurements of Type-II superconductors

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The electrodynamic properties of type-II superconductors have been studied in strong magnetic elds using microwave spectroscopy to access the ux- ow regime of vortex motion. The measurements give insight into two important physical quantities: ux- ow resistivity, which is intimately related to dissipation from electronic states near the vortex cores; and pinning forces, from the collective interactions of the vortex lattice with material defects. A notable aspect of this work is the ability to accurately separate contributions from viscous and elastic forces. This has been made possible by the development of microwave apparatus that allows the separate measurement of in-phase and out-of- phase components of surface impedance, and through the use of well-controlled, high quality samples. Measurements on conventional superconducting systems Nb, NbSe2 and V3Si have been used to test both the experimental technique and the subsequent data analysis, with the results in good accord with established data on these materials. In addi- tion, some interesting new features have been observed, likely associated with the multiband nature of these systems. The primary focus of this work is on the ux- ow dynamics of cuprate high temperature superconductors. Two material systems, YBa2Cu3O6+x and Tl2Ba2CuO6+x, have been used to carry out measurements that span the entire superconducting region of the cuprate phase diagram. For each sample, the pinning force constant and ux- ow resistivity have been extracted across the superconducting temperature range. As has been previously reported by our group, there is an anomalous logarithmic up- turn in ux- ow resistivity at low temperatures, previously attributed to localization physics. The main discovery of this work is that the logarithmic behaviour persists across the phase diagram, even in highly overdoped Tl2Ba2CuO6+x samples in which the normal state is known to be a metallic, conventional Fermi liquid. This suggests,for the frst time, that the resistivity upturns in cuprates are in fact intimately con- nected to the presence of vortices, which in turn has strong implications for the nature of the underdoped cuprate normal state. i

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Active shielding of cylindrical transmit-coils for low-field magnetic resonance imaging

Author: 
Date created: 
2005
Abstract: 

Simple yet effective cylindrical transmit and receive coils for low frequency magnetic reso nance imaging (MRI) can be fabricated from a sine-phi distribution of windings. This thesi introduces the novel concept of an actively-shielded transmit coil. This coil produces a uni form oscillating magnetic field (a "B1 field") transverse to the bore of a cylindrical volum (the patient volume) yet at the same time produces very little magnetic field outside of thi region. In practice, this allows one to minimize undesirable inductive coupling betweei the B1 coil and the myriad of other coils (Bo, gradient, shim) that are needed to implemen MRI. Exact analytical solutions are derived for active shielding of the elementary build ing blocks of typical B1 coils. Specifically, solutions for a single straight wire, a completl current loop, and a sine-phi current distribution on the surface of an infinite cylinder ar~ derived. Following this, the results of an experimental study examining the magnetic field produced by actively shielded B1 coils is presented. The concepts described in this thesi are expected to be useful for emerging applications in low field nuclear magnetic resonance and magnetic resonance imaging.

Document type: 
Thesis
File(s): 
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Structural investigation of Carbopol ETD2050 by light scattering

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Carbopol, a family of cross-linked acrylic acid-based polymers and a well-known thickener used in personal care, household and industrial products, is often used as a model yield-stress fluid because it is transparent and its rheological properties can be precisely tuned by sample preparation conditions. Carbopol dispersions behave as an elastic solid, but they will flow when the applied stress exceeds a sample-dependent yield value. Both the yield-stress phenomena and the range of potential applications motivate the study of the microscopic structure and properties of yield-stress fluids. These investigation will lead to a fundamental understanding of the yield-stress phenomenon of Carbopol. Light scattering experiments investigating Carbopol ETD2050 dispersed in water have revealed that the Carbopol microstructure consists of a small and a large length scale, possibly described by different cross-linking densities. Experiments also show that the length scales and mass fractal dimension depend on the sample concentration and pH.

Document type: 
Thesis
File(s): 
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

The effect of cholesterol on the partitioning of 1-octanol into POPC vesicles

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Microcalorimetry has become a method of choice for sensitive characterization of biomolecular interactions. In this study, isothermal titration calorimetry (ITC) was used to measure the partitioning of 1-octanol into lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a semi-unsaturated lipid, and cholesterol, a steroid, as a function of cholesterol molar concentration. The ITC instrument measures the heat evolved or absorbed upon titration of a liposome dispersion, at concentrations ranging from 0 to 40% cholesterol, into a suspension of 1-octanol in water. A model function was fit to the data in order to determine the partition coefficient of octanol into POPC bilayers and the enthalpy of interaction. I found that the partition coefficient increases and the heat of interaction becomes less negative with increasing cholesterol content, in contrast to results found by other groups for partitioning of alcohols into lipid-cholesterol bilayers containing saturated lipids. The heat of dilution of vesicles was also measured.

Document type: 
Thesis
File(s): 
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Discovery of single top quark production

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The top quark is by far the heaviest known fundamental particle with a mass nearing that of a gold atom. Because of this strikingly high mass, the top quark has several unique properties and might play an important role in electroweak symmetry breaking—the mechanism that gives all elementary particles mass. Creating top quarks requires access to very high energy collisions, and at present only the Tevatron collider at Fermilab is capable of reaching these energies. Until now, top quarks have only been observed produced in pairs via the strong interaction. At hadron colliders, it should also be possible to produce single top quarks via the electroweak interaction. Studies of single top quark production provide opportunities to measure the top quark spin, how top quarks mix with other quarks, and to look for new physics beyond the standard model. Because of these interesting properties, scientists have been looking for single top quarks for more than 15 years. This thesis presents the first discovery of single top quark production. An analysis is per- formed using 2.3 inverse fb of data recorded by the D0 detector at the Fermilab Tevatron Collider. Boosted decision trees are used to isolate single top signal from background, and the single top cross section is measured to be sigma( ppbar -> tb + X, tqb + X ) = 3.74 +0.95 −0.74 pb. Using the same analysis, a measurement of the amplitude of the CKM matrix element Vtb, governing how top and b quarks mix, is also performed. The measurement yields: | Vtb f_L1 | = 1.05+0.13 −0.12, where f_L1 is the left-handed Wtb coupling. The separation of signal from background is improved by combining the boosted decision trees with two other multivariate techniques. A new cross section measurement is performed, and the significance for the excess over the predicted background exceeds 5 standard deviations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

In-place bonding of III-V semiconductor heterostructures

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Elastically strain-relaxed GaAs/In$_{0.08}$Ga$_{0.92}$As/GaAs heterostructures on GaAs(001) substrates were fabricated. Pseudomorphic heterostructures grown by MOCVD were patterned using conventional photolithography and a sacrificial AlAs layer was removed by selective etching. As etching proceeds, the structure is released from the substrate, elastic strain relaxation occurs and the strain-relaxed structures are weakly bonded in-place to the substrate. The bond between the structure and the substrate was then strengthened by annealing under conditions similar to those used for whole wafer bonding of GaAs. The strain, composition and thickness of the layers were determined using high resolution X-ray diffraction and the sample surface quality was examined using atomic force microscopy. The degree of strain relaxation of the InGaAs layer is determined by the relative thickness of the GaAs and InGaAs layers in agreement with a force balance model. The increase in the in-plane lattice parameter of the bonded structures as compared to GaAs is 0.25-0.44%.

Document type: 
Thesis
File(s): 
Senior supervisor: 
P
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

The electroluminescence and scanning tunneling microscopy of single molecules

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The scanning tunneling microscopy (STM) of single molecules has become a prominent experimental method in the field of molecular electronics. It has been found that in STM experiments, when an electric current flows through a single molecule, the molecule may luminesce. This electroluminescence, in conjunction with traditional STM data, provides a potentially important additional degree of freedom for understanding nanoscale systems. This thesis describes exploratory theoretical work on the newly emerging phenomenon of molecular electroluminescence, and its relationship to the scanning tunneling microscopy of single molecules. A local electrode framework is progressively developed, in order to explain single-molecule electroluminescence data as well as simulating STM current-voltage characteristics and topographic maps for molecules on complex substrates. The molecule Zn(II)-etioporphyrin I is chosen for detailed analysis and comparison with experiment. Electron transport is studied using Landauer theory that relates electric current to the transmission probability for an electron to scatter through the molecule. The theoretical approach utilizes tight binding and extended Huckel approaches for the electrodes and molecule, a charge-conserving scheme to self-consistently model the influence of electric fields and electric currents on the molecular energy level structure, and Fermi's golden rule in calculating electroluminescence. A single coherent framework is ultimately achieved that explains for the first time both the electric current data and molecular electroluminescence in a molecular system and elucidates the physics underlying a rich and previously puzzling array of interlinked optical and transport phenomena.

Document type: 
Thesis
File(s): 
Senior supervisor: 
G
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Using boosted decision trees for tau identification in the ATLAS experiment

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The ATLAS detector will begin taking data from $p$-$p$ collisions in 2009. This experiment will allow for many different physics measurements and searches. The production of tau leptons at the LHC is a key signature of the decay of both the standard model Higgs (via H $\rightarrow \tau \tau$) and SUSY particles. Taus have a short lifetime ($c \tau= 87$ $\mu$m) and decay hadronically ~65\% of the time. Many QCD interactions produce similar hadronic showers and have cross-sections about 1 billion times larger than tau production. Multivariate techniques are therefore often used to distinguish taus from this background. Boosted Decision Trees (BDTs) are a machine-learning technique for developing cut-based discriminants which can significantly aid in extracting small signal samples from overwhelming backgrounds. In this study, BDTs are used for tau identification for the ATLAS experiment. They are a fast, flexible alternative to existing discriminants with comparable or better performance.

Document type: 
Thesis
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Anisotropies and spin dynamics in ultrathin magnetic multilayer structures

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

High quality magnetic films were prepared by Molecular Beam Epitaxy (MBE) using Thermal Deposition (TD) and Pulse Laser Deposition (PLD) techniques. Ferromagnetic Resonance (FMR) and Mossbauer studies have shown that the Fe films prepared by PLD exhibited a more intermixed interface lattice structure than those prepared by TD. Dramatic decrease of the inplane interface uniaxial anisotropy for the PLD films compared to those prepared by TD has shown that the in-plane uniaxial anisotropy is caused by magnetoelasticity driven by the Fe/GaAs(001) interface lattice shear. Magnetization dynamics of the ultrathin Fe/Au,Ag/Fe films was studied using Time-Resolved Magneto-Optical Kerr Effect (TRMOKE) and FMR in the frequency range from 1 to 73 GHz. The Gilbert damping was studied in the Au/Fe/GaAs(001) structures as a function of the Fe and Au layer thickness, respectively. The observed increase in magnetic damping in the Fe film covered with thick Au capping layers was explained by spin pumping at the Fe/Au interface accompanied by spin relaxation and diffusion of the accumulated spin density in the Au layer. The spin diffusion length in Au was found to be 34 nm at room temperature. Significant increase of the Gilbert damping was observed in the Au/Fe/GaAs structures with decreasing Fe film thickness. Its origin lies in the additional damping at the Fe/GaAs interface. Direct detection of the spin current propagating across the Ag spacer in Fe/Ag,Au/Fe/GaAs(001) structures was carried out with stroboscopic TRMOKE measurements. The Fe layer grown on GaAs served as a spin pumping source and the Fe layer grown on the Au,Ag spacer was used as a probe for detection of the spin current propagating across the Au and Ag spacers. The experimental results were interpreted using self-consistent solution of the Landau Lifshitz Gilbert (LLG) equations of motion with the spin diffusion equation for the accumulated spin density in the Au and Ag spacers. The spin diffusion length in Ag was found to be 150 nm.

Document type: 
Thesis
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)