Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 120-130). / Inflation is a paradigm for the physics of the early universe, and it has received a great deal of experimental support. Essentially all inflationary models lead to a regime called eternal inflation, a situation where inflation never ends globally, but it only ends locally. If the model is correct, the resulting picture is that of an infinite number of "bubbles", each of them containing an infinite universe, and we would be living deep inside one of these bubbles. This is the multiverse. Extracting meaningful predictions on what physics we should expect to observe within our bubble encounters severe ambiguities, like ratios of infinities. It is therefore necessary to adopt a prescription to regulate the diverging spacetime volume of the multiverse. The problem of defining such a prescription is the measure problem, which is a major challenge of theoretical cosmology. In this thesis, I shall describe the measure problem and propose a promising candidate solution: the scale-factor cutoff measure. I shall study this measure in detail and show that it is free of the pathologies many other measures suffer from. In particular, I shall discuss in some detail the "Boltzmann brain" problem in this measure and show that it can be avoided, provided that some plausible requirements about the landscape of vacua are satisfied. Two interesting applications of the scale-factor cutoff measure are investigated: the probability distributions for the cosmological constant and for the curvature parameter. The former turns out to be such that the observed value of the cosmological constant is quite plausible. As for the curvature parameter, its distribution using the scale-factor measure predicts some chance to detect a nonzero curvature in the future. / by Andrea De Simone. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2007. / Includes bibliographical references (leaves 125-128). / In this thesis, several novel techniques are proposed and demonstrated for measuring the coherent properties of materials and testing aspects of quantum information processing using a single crystal neutron interferometer. In particular we introduce methods for reciprocal space encoding of spatial information systematically in a neutron interferometer. First, a proof of principle experiment is conducted for coherent reciprocal space neutron imaging. This newly proposed technique overcomes the limitation of spacial resolution of current two dimensional neutron detectors. Second, an experiment to measure the vertical coherent length of a neutron interferometer is reported, which extends the previously achieved path separation. Third, we propose a new interferometry geometry that reduces the sensitivity of the neutron interferometer to environmental vibrational disturbances. The method is based on a quantum error compensating algorithm. Finally, a new method that is capable of measuring the autocorrelation function of a sample is proposed. This new technique can extend the capability of traditional neutron scattering experiments. All experiments are conducted at the neutron interferometry facility at the National Institute of Standards and Technology (NIST). / by Dmitry A. Pushin. / 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 (pages 159-167). / In order to gain a deeper understanding of the Standard Model of particle physics and test its limitations, it is necessary to carry out accurate calculations to compare with experimental results. Event shapes provide a convenient way for compressing the extremely complicated data from each collider event into one number. Using eective theories and studying the appropriate limits, it is possible to probe the underlying physics to a high enough precision to extract interesting information from the experimental results. In the initial sections of this work, we use a particular event shape, C-parameter, in order to make a precise measurement of the strong coupling constant, s. First, we compute the e+e- C-parameter distribution using the Soft-Collinear Eective Theory (SCET) with a resummation to N³LL' accuracy of the most singular partonic terms. Our result holds for C in the peak, tail, and far-tail regions. We treat hadronization effects using a field theoretic nonperturbative soft function, with moments [omega]n, and perform a renormalon subtraction while simultaneously including hadron mass effects. We then present a global fit for [alpha]s(mZ), analyzing the available C-parameter data in the resummation region, including center-of-mass energies between Q = 35 and 207 GeV. We simultaneously also fit for the dominant hadronic parameter, [omega]1. The experimental data is compared to our theoretical prediction, which has a perturbative uncertainty for the cross section of ~/= 2:5% at Q = mZ in the relevant t region for [alpha]s(mZ) and [omega]1. We find [alpha]s(mZ) = 0:1123 +/- 0:0015 and [omega]1 = 0:421 +/- 0:063 GeV with X² / =dof = 0:988 for 404 bins of data. These results agree with the prediction of universality for [omega]₁ between thrust and C-parameter within 1-[sigma]. The latter parts of this study are dedicated to taking SCET beyond leading power in order to further increase the possible precision of calculations. On-shell helicity methods provide powerful tools for determining scattering amplitudes, which have a one-to-one correspondence with leading power helicity operators in SCET away from singular regions of phase space. We show that helicity based operators are also useful for enumerating power suppressed SCET operators, which encode subleading amplitude information about singular limits. In particular, we present a complete set of scalar helicity building blocks that are valid for constructing operators at any order in the SCET power expansion. We also describe an interesting angular momentum selection rule that restricts how these building blocks can be assembled. / by Daniel W. Kolodrubetz. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (leaf 25). / At the MIT-Bates Linear Accelerator Center a comprehensive study of low-Q2 spin-dependent electron scattering from deuterium has been carried out using the Bates Large Acceptance Spectrometer Toroid (BLAST). This experiment has employed an 850 MeV polarized electron beam from the MIT-Bates linear accelerator incident on an internal polarized deuterium target and the BLAST detector. Deuterium's simple composition is an important factor in understanding the structure of the inter-nucleon potential. The pion production region has a resonant structure and is a promising location to search for pre-existing delta particles in deuterium. While, theoretical calculations predict that delta resonant states account for anywhere from 0.25 to 3.60% of the nuclear wave function, more realistic predictions for deuterium range from 0.3 to 1.0%. This thesis presents the determination of the (e,e'++) event rate from deuterium, the first of many steps towards a complete study of delta resonant states. / y Chana M. Greene. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. / Includes bibliographical references (p. 62-63). / We present a brief review of the AdS/CFT correspondence and the progress made toward building a realistic gravity/gauge theory duality for a non-relativistic conformal field theory. In particular, we highlight many of the computational tools necessary for such a program before introducing one such model duality. The model presented exhibits the symmetry group of Schrodinger's equation along with conformal symmetry. A black hole can be placed in this spacetime to study a finite temperature duality. In the low-frequency, long-distance limit at finite temperature classical hydrodynamics can be used to determine the retarded Green's functions of the field theory, which can be computed from the gravity dual. This facilitates the calculation of several characteristic quantities including the shear viscosity and the shear diffusion constant giving results consistent with other hydrodynamic analyses of the system. / by Connor W. McEntee. / S.B.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1998. / Includes bibliographical references (leaves 28-29). / by Karen Thibault. / B.S.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 49). / Novel compact X-ray sources using coherent ICS have the potential to positively impact a wide range of sectors by making hard x-ray techniques more accessible. However, the analysis of such novel sources requires improvements to existing simulation routines to incorporate Coulomb forces among particles and effects related to the phase of emitted radiation. This thesis develops a numerical routine for calculating the radiation scattered by electrons counter-propagating with a linearly-polarized, Gaussian laser pulse. The routine takes into account electron-electron repulsion and the constructive and destructive interference between the radiation emitted by each electron, making it suitable for characterizing the properties of inverse Compton scattering (ICS) sources where the electron density varies on the order of the laser wavelength. Finally, an analysis of the emission characteristics for an example ICS source with coherent emission at 10 nm wavelength is included. The source uses a 2 MeV electron bunch and a 1 /pm wavelength laser. The coherent emission demonstrates a significantly narrowed linewidth and greatly increased output power when compared to traditional ICS. / by Giacomo Rosario Resta. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. 112-115). / by Witold Skiba. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 98-109). / Photonic crystals are recently discovered meta-materials whose optical properties arise from periodic refractive index variations. In this thesis I examine various aspects of photonic crystals including a self-assembled photonic crystal, anomalous properties arising from periodicity, and tailoring absorption and emission spectra. Fabricating photonic crystals with the desired properties in the infrared and optical frequencies, including a complete photonic bandgap, is an experimental challenge. Self-assembly can provide a solution. In Chapter 2, I examine a new type of colloidal photonic crystal of tetrahedral building blocks in a fcc lattice that is found to possess a robust and complete bandgap. In Chapter 3, I explore the photonic states that exist around a zero-group velocity point. Motivated by negative refraction, a measure of the effective wavevector is constructed that distinguishes various types of zerogroup velocity modes. Around one type of zero-group velocity mode, an anomalous region of backward effective wavevector is found that enables superior light confining properties of a mirror-less cavity. In the last two chapters I look at the problem of efficiently converting radiant energy to electrical power. In Chapter 4, I explore the extent to which ID multi-layer thin films can enhance the short circuit current of a 2 [mu]-thick silicon solar cell. Though such cells are limited by their size, for two front-layers a relative boost of 45% is possible. Finally, in Chapter 5, motivated by the problem of low efficiency in thermophotovoltaics, I look at selective emissivity of a 2D metallic photonic crystal. A semi-analytical theory is developed using only the material dispersion and geometrical parameters. Applications of the selective emitter, including power generation and lighting, are discussed. / by Michael Ghebrebrhan. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 53-54). / We describe a setup to search for the existence of a massive gauge boson A', the "dark photon," mediating dark matter interactions with coupling strength [alpha]'. In certain models, the A' decays promptly but invisibly and might be discoverable in high luminosity collider experiments. Following a proposed setup by Wojtsekhowski et al. to use the VEPP-3 electron-positron storage ring at the Budker Institute for Nuclear Physics, we study whether an e+ + e- - A'+ -[gamma] signal could be seen over the quantum electrodynamic background. The proposed VEPP-3 setup is sensitive to an A' within the mass range mA = 5-20 MeV. Out of the two backgrounds, e++e- - 3[gamma] and e+ + e- - e+ + e- + [gamma], we find that the former process provides the dominant background for the A' signal. While positron bremsstrahlung events can be detected and suppressed in Wojtsekhowski's apparatus, the 3[gamma] cross-section has a large cross section in this range, "faking" an A'. We use Monte Carlo numerical integration techniques to calculate the cross sections and obtain reach plots, determining which values of mA' and a' could be discovered at 5[sigma] confidence. This background study can be used to improve the VEPP-3 proposal, and provides a valuable comparison study with the MIT-led DarkLight proposal to search for a dark photon in the same mass range. / by Dustin R. Katzin. / S.B.