Theses/Dissertations - Physics
http://hdl.handle.net/2104/4793
2015-07-03T17:20:26ZHořava-Lifshitz theory of gravity and its applications to cosmology.
http://hdl.handle.net/2104/8762
Hořava-Lifshitz theory of gravity and its applications to cosmology.
Huang, Yongqing, 1984-
In this dissertation, I studied Hořava-Lifshitz gravity and its applications to inflationary cosmology. After introducing the original proposal with the projectability and detailed balance conditions, I discussed its attracting features as well as the problems it faces. An extended model without the detailed balance condition was then studied and found to be stable in de Sitter spacetime, but still possess an extra scalar mode in the gravity sector. I then studied a model with an extra U(1) symmetry dubbed as the projectable general covariant HL gravity and showed that it has the same degree of freedom as general relativity, and is free of the stability problem. It was found that the FLRW universe is necessarily flat, given that the coupling with matter takes a specific form. I also studied the scalar perturbations around the FLRW metric and presented all the possible gauge choices. Applications of the general covariant model in inflationary cosmology were studied in the second part. After deriving the slow-roll conditions in this model, I showed that in the super-horizon regions the scalar perturbations become adiabatic, and the comoving curvature perturbation is constant, because of this slow-roll condition. By using the uniform approximation technique, power spectra and indices of primordial scalar and tensor perturbations under the slow-roll approximations were expressed explicitly in terms of the slow-roll parameters and the various coupling constants. I found that they are in general different from, but reducible to, the values in the class of simplest inflation models. Next I studied the non-Gaussianities of these perturbations. For scalar perturbations, by properly choosing the coupling constants, a large nonlinearity parameter fNL is possible. I also found that the bispectrum favors the equilateral shape as a result of the higher order spatial derivatives, and that folded shape is enhanced when the vacuum is from the Bunch-Davis vacuum. Both the squeezed and the equilateral shapes appear in the bispectrum for primordial gravitational waves. In addition, the polarization tensors of the tensor fields have strong effects on the shapes of the bispectrum.
2013-09-16T00:00:00ZMeasurement of the single top quark cross section in the lepton plus jets final state in proton-antiproton collisions at a center of mass energy of 1.96 TeV using the CDF II detector.
http://hdl.handle.net/2104/8604
Measurement of the single top quark cross section in the lepton plus jets final state in proton-antiproton collisions at a center of mass energy of 1.96 TeV using the CDF II detector.
Wu, Zhenbin, 1984-
We present a measurement of the single top quark cross section in the lepton
plus jets final state using an integrated luminosity corresponding to 7.5/fb of
collision data collected by the Collider Detector at Fermilab. The single top
candidate events are identified by the signature of a charged lepton, large missing
transverse energy, and two or three jets with at least one of them identified as
originating from a bottom quark. A new Monte Carlo generator powheg is used
to model the single top quark production processes, which include s-channel, t-channel,
and Wt-channel. A neural network multivariate method is exploited to
discriminate the single top quark signal from the comparatively large backgrounds.
We measure a single top production cross section of 3.04+0.57−0.53 (stat. + syst.) pb
assuming mtop = 172.5 GeV/c^2. In addition, we extract the CKM matrix element
value |Vtb| = 0.96 ± 0.09 (stat. + syst.) ± 0.05 (theory) and set a lower limit of
|Vtb| > 0.78 at the 95% credibility level.
2013-05-15T00:00:00ZFundamental particle and wave dynamics in dusty plasmas.
http://hdl.handle.net/2104/8539
Fundamental particle and wave dynamics in dusty plasmas.
Zhang, Zhuanhao.
Dusty plasma is a low-temperature plasma containing dust particles varying in
size from nanometers to micrometers. Due to plasma fluxes to its surface, a dust particle will charge negatively or positively depending on the charging mechanism involved. The motion of a dust particle within a dusty plasma can be recorded using a video camera, allowing for examination of the particle dynamics at the kinetic level.
Any investigation of dusty plasmas first requires a proper understanding of the
fundamental particle-particle interaction, dust particle charge and screening length. Due
to the perturbative nature of the majority of diagnostics in common use, all of these are
difficult to measure properly.
Each of the fundamental parameters mentioned above are addressed in this thesis.
First a minimally perturbative technique, allowing experimental detection of each of
these parameters, will be introduced. Next, a study of vertically aligned, extended dust
particle chains employing a glass box placed on the lower powered electrode in a ground based RF plasma system will be discussed. Finally, by adjusting the discharge power and the gas pressure within the plasma chamber, it will be shown that a chaotic dust cloud can form such vertical chains as well as self-excited dust acoustic waves. An investigation of
both the particle and wave dynamics within various sizes of glass box and under a
number of plasma conditions will be presented. A theoretical model is also introduced
providing a comprehensive dispersion relationship for dust acoustic waves (DAWs) and
examining the instabilities resulting from the effects created by the dust temperature,
particle charge variation, and ion-drag-force fluctuations. It is shown that each plays a
different role depending on the wavelength regime considered.
2012-11-29T00:00:00ZFundamental physics within complex plasmas.
http://hdl.handle.net/2104/8538
Fundamental physics within complex plasmas.
Douglass, Angela Michelle.
In this work, both experimental and numerical methods are used to investigate several of the fundamental processes and assumptions commonly found in an earth-based radio-frequency (RF) complex plasma discharge. First the manner in which the dust particle charge varies with the particle’s height above the powered electrode is investigated. Knowledge of the dust particle charge is required to understand nearly all complex plasma experiments since it affects the dust particle’s levitation height and particle-particle interactions. A fluid model which includes effects due to ion flow and electron depletion (which are significant dust charging effects within the sheath where the particles levitate) is employed to determine the plasma parameters required to calculate the dust particle charge. Second, the levitation limits of the dust particles and the structure of the sheath are investigated. The CASPER GEC RF reference cell is used to perform two experiments: one to measure the dust levitation height as a function of applied RF voltage and one to determine the electric force profile. The fluid model is then used to interpret the experimental results. This study provides a better understanding of the sheath structure, particle behavior within the sheath, and provides a
new, in situ experimental method for locating the approximate height of the sheath edge in any dusty plasma system. Finally, both molecular dynamics (MD) simulations and an experiment are employed to determine the physical processes that a complex plasma system goes through as it rapidly transitions from a liquid to solid state.
2012-11-29T00:00:00Z