Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1954. / Vita. / Includes bibliographical references (leaves 82-83). / by Frank A. Aschenbrenner. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 122-136). / It was the earliest of times, it was the latest of times, it was the age of inflation, it was the age of collapse, it was the epoch of perturbation growth, it was the epoch of perturbation damping, it was the CMB of light, it was the dwarf galaxy of darkness, it was the largest of cosmic scales, it was the smallest of Milky Way subhalos, we had multiple nonminimally coupled inflatons before us, we had inelastically selfinteracting dark matter before us, we were all going direct to the Planck scale, we were all going direct the other way. Motivated by apparent discrepancies between the standard theory and observation, we analyze two astrophysical systems in the context of new particle physics. Taking a phenomenological approach, we calculate observable consequences of novel particle models during two different stages in the development of our universe. First, we explore the possibility that nonminimally coupled multifield inflation can generate a large primordial isocurvature fraction and account for the "low-multipole anomaly" in the Cosmic Microwave Background. Second, we consider the effects of dark matter that inelastically self-interacts to determine the effect on the structure and abundance of Milky Way satellites and dwarf galaxies. The disparity of time and energy scales examined in this thesis serves to highlight the range of ways to use observables in the sky as a probe of new particle physics that may be elusive at current experiments on the ground. / by Katelin Schutz. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 153-180). / The merger of topology and symmetry established a new foundation for understanding the physics of condensed matter, beginning with the notion of topological insulators (TIs) for electronic systems. For the time-reversal invariant TIs, a key aspect is the "helical" mode at the boundary of the system - that is, the ID edge of a 2D topological insulator or the 2D surface of a 3D topological insulator. These helical modes represent the extreme limit of spin-orbit coupling in that the spin-degenercy has been completely lifted while preserving time-reversal symmetry. This property is crucial for proposals realizing exotic excitations like the Majorana bound state. In this thesis, I present a series of experiments investigating electronic transport through the boundary modes of 3D and 2D topological insulators, specifically Bi1.5 Sb0.5 Te1.7 Se1.3 and monolayer WTe 2 , respectively. For the case of ultra-thin WTe 2 , I also present experiments detailing investigations of the 2D bulk states, finding a semimetallic state for the trilayer and a superconducting phase for the monolayer, both of which are strongly tunable by the electric field effect. The discovery of 2D topological insulator and 2D superconductor phases within the same material, accessible by standard solid state elecrostatic gates, places WTe2 in a unique situation among both TIs and superconductors, potentially enabling gate-configurable topological devices within a homogenous material platform. / by Valla Fatemi. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / Turbulence in tokamak plasmas is the primary means by which energy is transported from the core of the plasma to the edge, where it is lost, and is therefore the main limitation of tokamak plasma performance. Dilution of the main-ion species was found to have a stabilizing effect on ion gyroradius scale turbulence in tokamak plasmas. Dilution of deuterium tokamak plasmas is the reduction of the ratio of the deuterium ion density to the electron density, nD=ne, to less than 1.0 through the introduction of low-Z impurity species into the plasma. Controlled dilution experiments were performed on Alcator C-Mod wherein plasmas at a range of electron density and plasma current were seeded with nitrogen while a cryopump held the electron density fixed. The electron density fluctuations due to turbulence were monitored using a phase contrast imaging (PCI) diagnostic, an absolutely calibrated diagnostic that measures the line-integral of the electron density fluctuations along 32 vertical chords. In these experiments the seeding reduced the PCI density fluctuations, and had a stabilizing effect on the ion energy transport. The seeding also reversed the direction of intrinsic rotation in certain cases. Nonlinear simulations using the gyrokinetic turbulence code GYRO were performed using measured kinetic profiles from the dilution experiments both before and after the nitrogen seeding. The GYRO simulations reproduced the observed reduction in the turbulent ion energy transport with the nitrogen seeding. The GYRO simulated turbulent density fluctuations were compared to the PCI measurements using a synthetic diagnostic, and they were found to be consistent. GYRO simulations were also performed varying only the main ion dilution to explore the theoretical effects of the dilution on energy transport. Through this it was found that the dilution reduced the turbulent ion energy transport in a wide variety of cases, but primarily increased the critical gradient at low densities, and primarily reduced the stiffness of the transport at high densities. This dilution effect is related to observations of reductions in energy transport from seeding on other tokamaks, and will likely have an impact on ITER and future fusion reactors. / by Paul Chappell Ennever. / Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 79-80). / The study of gravitational micro-lensing at high optical depth has only rarely involved the close examination of the individual actual micro-images that arise as a result of the phenomenon. We discuss methods that refine on previous work done in the search for micro-images, which have been largely ignored in favor of other methods to study micro-lensing. With the help of magnification maps generated by Herr Prof. Dr. Joachim Wambsganss, we ran simulations that track positions and magnifications of micro-minima as functions of source position. We discuss the breakdown of a commonly used approximation for magnifications near fold caustics. Our results show that the approximation is noticeably broken at a caustic strength-scaled distance of 0.1. The relevance of this breakdown to work done by other authors is briefly examined. We then then discuss a few new results for the statistics of micro-images, deriving a formula for the mean micro-minimum magnification. We present a method for exactly calculating the caustic networks of micro-lensed systems, and calculate probability distributions for the caustic strength for two sets of parameters of interest. We present the creation of videos of the micro-lensing affect for pedagogical purposes. Finally, we briefly examine micro-lensing near macro-caustics and study the motion of micro-images as a point source crosses a macro-caustic. / by Luke Weisenbach. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1995. / Includes bibliographical references. / by Grace Hsiu-Ling Chen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2007. / Includes bibliographical references (p. 177-181). / We present the measurement of the ratios of branching fractions B... to B..., and B... to B... We analyze data taken with the CDF II detector that corresponds to an integrated luminosity of 355 pb - 1 in pp collisions at ... TeV at the Fermilab Tevatron. Using a novel displaced track trigger we reconstruct 494 ± 28 B... decays, 8098 ± 114 B... decays, 159 ± 17 B... decays, and 3288 ± 76 B... decays. Using the world average value of the ... production ratio fs/fd = 0.259 ± 0.038, we determine the ratios of branching fractions ... where the uncertainties labeled BR and PR refer to the uncertainty on the D meson branching fractions and the production ratio fs/fd, respectively. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (p. 32). / by Annemarie N. Grandke. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 115-121). / This work used molecular dynamics (MD) and Monte Carlo (MC) method to study the homogeneous crystal nucleation in the melts of n-alkanes, the simplest class of chain molecules. Three n-alkanes with progressive chain length were studied, n-octane (C8), n-eicosane (C20), and C150, using a united atom force field, which is able to reproduce physical quantities related to the solid-liquid phase transition in n-alkanes. Using a 3D Ising model, we proved that the size of the largest nucleus in the system, nmax, is the controlling reaction coordinate during the nucleation process. We have made direct observation of the homogeneous crystal nucleation using MD simulation at as small as 15% under-cooling. We calculated the nucleation rate and identified the critical nucleus through a mean-first-passage time (MFPT) analysis. At about 20% under-cooling, the critical nucleus size n* is around 100 united atoms, and is slightly decreasing as the chain length increases. Abnormal temperature dependence of n* against classical nucleation theory was found in C150 system. This behavior could possibly be explained by the high viscosity of the melt formed by long chain molecules. The crystal nucleus has a cylindrical shape. We have observed the change of the structure of the crystal nucleus as the chain length increases. For C8, the chains attach to and detach from the crystal nucleus as a whole, and the chains end at the end surface of the cylindrical nucleus. For C20, the partial participation of chains in the crystal nucleus became apparent, where the critical nucleus consists of a bundle of crystal segments with the tails on the same chains extending into the amorphous melt. For C150, chain folding was observed during the nucleation stage. A cylindrical nucleus model was adopted to characterize the crystal nucleus. The nucleus free energy [Delta]G(n) was sampled using MC, and was used to calculate the solid-liquid interfacial free energies based on classical nucleation theory. The end surface free energy [sigma]e is about 4 mJ/m2 and the side surface free energy [sigma]s is about 10 mJ/m 2 . Their values are insensitive to the chain length. / by Peng Yi. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (p. 189-199). / We use astrophysical data to shed light on fundamental physics by constraining parametrized theoretical cosmological and gravitational models. Gravitational parameters are those constants that parametrize possible departures from Einstein's general theory of relativity (GR). We develop a general framework to describe torsion in the space time around the Earth, and show that certain observables of the Gravity Probe B (GPB) experiment can be computed in this framework. We examine a toy model showing how a specific theory in this framework can be constrained by GPB data. We also search for viable theories of gravity where the Ricci scalar R in the Lagrangian is replaced by an arbitrary function f(R). Making use of the equivalence between such theories and scalar-tensor gravity, we find that models can be made consistent with solar system constraints either by giving the scalar a high mass or by exploiting the so-called Chameleon Effect. We explore observational constraints from the late-time cosmic acceleration, big bang nucleosynthesis and inflation. Cosmology can successfully describe the evolution of our universe using six or more adjustable cosmological parameters. There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe. We quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions. We present an accurate and robust method for measuring cosmological parameters that exploits the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. We find that a future square kilometer array optimized for 21 cm tomography could have great potential, improving the sensitivity to spatial curvature and neutrino masses by up to two orders of magnitude, to k 0.0002 and m 0.007eV, and giving a 4s detection of the spectral index running predicted by the simplest inflation models. / by Yi Mao. / Ph.D.