Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003. / Includes bibliographical references (p. 185-197). / In this thesis, we present some results that strongly support Sen's conjectures on tachyon condensation on a bosonic D-brane. Our main tool of analysis is level truncated open bosonic string field theory We use level truncation to check that the energy difference between the local maximum and the local minimum of the open bosonic tachyon effective potential is equal to the tension of a space-filling D-brane (Sen's first conjecture). Our results prove this equality within a precision of about 0.1%. We then construct lump solutions of open bosonic string field theory, which are conjectured by Sen (third conjecture) to be D-branes of lower dimensions. We check that indeed the tensions of lumps of codimension one and two, coincide with the tensions of the respective D-branes within a precision of a few percent. We also give evidence for Sen's second conjecture; that in the nonperturbative tachyon vacuum all open string degrees of freedom must disappear. We show that this is guaranteed if we can write the identity string field I in the form I = QA, where A is some string field and Q is the BRST operator in the true vacuum. We show evidence that the identity can indeed be written in this form. We also analyze the dynamics of tachyon condensation by studying time-dependent solutions of p-adic string theory and level truncated string field theory. Although our rolling solutions conserve energy, their pressure oscillates with diverging amplitudes. These results therefore don't support Sen's proposal of a pressureless tachyon matter. / by Nicolas Moeller. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002. / Includes bibliographical references (p. 127-140). / The research presented herein is a theoretical investigation of the formation, evolution, and ultimate fate of low-, intermediate-, and high-mass X-ray binaries (L/I/HMXBs). The primary theoretical tool used throughout is binary population synthesis. Results of these calculations are used to account for the numbers and properties of observed X-ray binaries and their descendants, as well as to direct future observational and theoretical work. Combining binary population synthesis and binary stellar evolution calculations, I present a systematic population study of L/IMXBs in the Galactic plane. Since full stellar evolution calculations are used to model the X-ray binary phase, it is possible to make detailed comparisons between the theoretical models and observations. It is demonstrated quantitatively that IMXBs probably play a crucial role in shaping the population of LMXBs observed at the current epoch, as well as their descendant binary millisecond radio pulsars. Recently, a new class of HMXBs has emerged, distinguished from other HMXBs by their wide, nearly circular orbits. I show that the discovery of a significant number of such systems is at odds with the conventional wisdom that most neutron stars receive very large "kick" speeds at birth. This problem may be rectified in a self-consistent way if the kick speed depends on the rotation rate of the pre-collapse core, which I propose is strongly influenced by the evolution of the neutron-star progenitor in a binary system. The reasonable suggestion that certain globular clusters contain nearly 1000 neutron stars conflicts with the large mean kick speeds estimated from observations of isolated radio pulsars, which are 5 to 10 times the present cluster escape speeds. / (cont.) Therefore, most neutron stars born from single progenitors should have been ejected from their host clusters. I show that many more neutron stars are retained if a significant fraction are formed with massive stellar companions, but that the retained fraction is still too small to account for the inferred large numbers of neutron stars at the current epoch. Several alternative hypotheses are discussed, including the intriguing possibility that globular clusters we see today were ten times more massive in the distant past. The Chandra X-ray Observatory has revealed hundreds of previously undetected point sources in a small field about the Galactic center. I show that the majority of these sources may be neutron stars accreting from the winds of unevolved companion stars. Infrared observations are proposed to search for the stellar counterparts of the X-ray sources. / by Eric D. Pfahl. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 132-138). / Through a combination of experiments and modeling, I explored how inactivation of antibiotics by antibiotic-resistant bacteria affects the evolution of antibiotic resistance in two simple microbial communities. First, I examined the interaction between a resistant strain and a sensitive strain of the bacteria Escherichia coli in the presence of the [beta]-lactam antibiotic ampicillin. Second, I investigated whether two strains of Escherichia coli can form a cross-protection mutualism in a multi-drug environment containing the antibiotics ampicillin and chloramphenicol. In both experimental systems, I found that inactivation of antibiotics by resistant bacteria is an important cooperative behavior which enables microbes to help each other survive in otherwise lethal antibiotic concentrations. The rich dynamical behaviors that arise even in these simple systems highlight the inherent challenge in deciphering the workings of more complex microbial communities. / by Evgene Yurtsev. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 207-213). / In this thesis we formulate an effective field theory for nonlinear dissipative fluid dynamics. The formalism incorporates an action principle for the classical equations of motion as well as a systematic approach to thermal and quantum fluctuations around the classical motion of fluids. The dynamical degrees of freedom are Stuckelberg-like fields associated with diffeomorphisms and gauge transformations, and are related to the conservation of the stress tensor and a U(1) current if the fluid possesses a charge. This inherently geometric construction gives rise to an emergent "fluid space-time", similar to the Lagrangian description of fluids. We develop the variational formulation based on symmetry principles defined on such fluid space-time. Through a prescribed correspondence, the dynamical fields are mapped to the standard fluid variables, such as temperature, chemical potential and velocity. This allows to recover the standard equations of fluid dynamics in the limit where fluctuations are negligible. Demanding the action to be invariant under a discrete transformation, which we call local KMS, guarantees that the correlators of the stress tensor and the current satisfy the fluctuation-dissipation theorem. Local KMS invariance also automatically ensures that the constitutive relations of the conserved quantities satisfy the standard constraints implied e.g. by the second law of thermodynamics, and leads to a new set of constraints which we call generalized Onsager relations. Requiring the above properties to hold beyond tree-level leads to introducing fermionic partners of the original degrees of freedom, and to an emergent supersymmetry. We also outline a procedure for obtaining the effective field theory for fluid dynamics by applying the holographic Wilsonian renormalization group to systems with a gravity dual. / by Paolo Glorioso. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (p. 41-43). / Ever since the birth of the first quantum error correcting code, many error correcting techniques and formalism has been constructed so far. Among those, generating a quantum code on a locally planar geometry have lead to some interesting classes of codes. Main idea of this thesis stems from Kitaev's Toric code, which was the first surface code, yet it suffered from having a asymptotically vanishing encoding rate. In this paper, we propose a quantum surface code on a more complicated closed surface which has large genus, namely the Hurwitz surface. This code admits a constant encoding rate in the asymptotic limit that the number of genus goes to infinity. However, we give evidence that t/n, where n is the number of qubits and t is the number of correctible errors, converges to 0 asymptotically. This is based on numerically generating many Hurwitz surfaces and observing the corresponding quantum code in the limit that genus number goes to infinity. / by Isaac H. Kim. / S.B.

Thesis (S.B.)--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 (leaves 79-82). / Using an ensemble of ultracold Cesium atoms in an optical cavity we demonstrate the efficient storage and retrieval of quantum information in the form of single photons. We use a photon that has scattered into the cavity mode to herald a successful creation of a collective excitation of Cesium atoms and hence our ability to retrieve a photon from the stored excitation at a later time. Post-selecting out only data that was preceded by a heralding photon we have achieved single-photon recovery efficiencies as high as 84%. We construct an atomic quantum memory for arbitrary optical polarization states using this technique on two spatially overlapped atomic samples. The two samples constitute a quantum memory making use of a bijective mapping between a photon polarization and a shared collective excitation in the atoms. The stored state is later retrieved as a single-photon polarization state. This memory showed an average fidelity of 0.93(5) for the recovered fiducial states as well as a conditional autocorrelation function g2 = 0.24(6), indicating the single-photon nature of the retrieved photons. In this thesis, a general discussion of the techniques employed and their background theory will be given, followed by a more detailed explanation of this most recent experiment. / by Benjamin Jacob Bloom. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1998. / Includes bibliographical references (p. 91-[94]). / by Cary Robert Lapoint. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 157-170). / .Studies of the electroweak interactions using final states with leptons in proton-proton collisions at the Large Hadron Collider at [square root of] s = 7 TeV, [square root of] s = 8 TeV, and [square root of] s = 13 TeV center-of-mass energies are described. Measurements of total inclusive and fiducial W and Z boson production cross sections and their ratios are performed. The W and Z bosons are observed via their decays to electrons and muons. An indirect determination of the total width of the W boson and the B(W --> lv) from the measured cross section ratios is described. The discovery of a new boson with a mass of 125 GeV at the Large Hadron Collider in 2012 sheds a new light on understanding the nature of electroweak symmetry breaking. A question of great significance is whether the new field couples to fermions through a Yukawa coupling interaction predicted in the standard model of particles. Evidence of the 125 GeV Higgs boson decay to a pair of tau leptons with an observed significance of 3.1 standard deviations is established. The nature of the Higgs sector is probed through searches for neutral resonances decaying to a pair of tau leptons in gluon-fusion and b-quark associated production modes with no observation of a significant excess. In addition, the feasibility of measuring the standard model Higgs boson self-coupling with an expected data sample corresponding to an integrated luminosity of 3000 fb-1 is studied. / by Aram Apyan. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 149-166). / X-ray binaries are excellent test beds for studies of high-energy accretion flows and the properties of compact objects. Neutron star (NS) low-mass X-ray binaries (LMXBs) vary in brightness by almost 8 orders of magnitude and are hosts to diverse accretion flows, transporting varying amounts of energy and mass toward the central NS, as well as expelling significant mass from the binary. This thesis aims to shed light on the accretion flow properties across the mass and luminosity scale, with particular emphasis on constraining the matter accreted on the neutron star surface and the resulting heating, which has important implications for measurements of the NS mass and radius. We have utilized X-ray instruments with substantially different sensitivities in flux and resolving power, each suited to our focused study of the accretion flows in a particular luminosity regime. In our study of the accretion disk wind in GX 13+1, we analyzed the Chandra High-Energy Transmission Grating spectrum of the NS binary accreting near its Eddington limit. We found multiple plasmas with different ionization states and velocities produce the observed absorption complex, in contrast to previous analyses that only found one absorption zone. The accretion disk wind expels mass from the disk at a rate comparable to the accretion onto the NS, and is consistent with a Comptonheated outflow, the driving mechanism likely behind all accretion disk winds in NS LMXBs and, possibly, all BH LXMBs. Frequent monitoring with the Swift X-Ray Telescope allowed us to observe SAX J1750.8-2900 in the relatively short-lived transition between outburst and quiescence. We found its X-ray spectrum softens towards lower luminosities, which can either be due to a radiatively-inefficient accretion flow or an increasing contribution of the boundary layer emission as the source's flux decreases. This work contributes to the establishment of spectral softening as a common property of the accretion flow in NS LMXBs between outburst and quiescence. We also found the transition does not produce significant NS heating. In our studies of NS LMXB quiescent emission, we utilized an XMM-Newton observation of Cen X-4 while the source was at its brightest quiescent luminosity ever recorded. We found the first evidence of multi-temperature thermal emission in a non-pulsing quiescent NS. We have interpreted the hotter of the two thermal components as a potential hotspot on the NS surface, indicative of a magnetically channeled accretion flow and motivation for further studies into NS heating in quiescence. Finally, we present the results from a recent XMM observation of the extremely faint system SAX J1810.8-2609. We find that the thermal component is consistent with a cooling NS radiating heat from nuclear reactions activated during outburst. We also present a revised estimate of the time-averaged mass accretion rate based on a more detailed outburst history and a range of outburst properties, finding the outburst history is in agreement with the quiescent thermal luminosity and discounting assertions of enhanced cooling mechanisms in the NS of SAX J1810.8-260. / by Jessamyn Leigh Allen. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (leaf 43). / The human body is composed primarily of dielectric tissue with spatially varying permittivity and conductivity. Traditional MRI does not measure these properties. Instead, the conductivity of the patient is a nuisance, causing unpredictable detuning of coils and field inhomogeneities. This thesis presents a method for mapping the electrodynamic properties of the patient's body with both MR and non-MR techniques. Such mapping has direct applications for medical imaging and SAR calculation. / by Benjamin M. Schwartz. / S.B.