Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002. / Includes bibliographical references (p. 245-268). / The main theme of this thesis is branes and orbifolds in string theory, with some digression towards other directions in the final chapters. After reviewing background material on D-branes and orbifolds, along with some mathematical fundamentals, I describe the following results. First, a classification of discrete torsion for a large class of orbifolds and a technique for extracting the matter content of D-brane probe gauge theories on orbifolds with discrete torsion. Subsequently, I discuss an algorithm, called stepwise projection, that illuminates the structure of the so-called exceptional quivers, and hints towards the brane realization of the associated gauge theories. Next, I present the computation of the partition function of the coset conformal field theory describing the two-dimensional black hole. This computation confirms earlier results concerning the spectrum of the black hole and it enable us to identify the physical Hilbert space. This theory appears in the exact string theory description of configurations of Neveu-Schwarz 5-branes and in the conformal field theory description of certain orbifolds in singular limits; thus, it is only a mild digression from the main theme of the thesis. This is not so, however, for the last topic discussed here. We will change gears completely and discuss our extension of Witten's construction of boundary string field theory to the superstring. As in the bosonic case, the main tool we use is the Batalin-Vilkovisky formalism. Our construction proves a recent conjecture regarding the spacetime action of the supersymmetric theory and a related conjecture concerning quantum field theories on two-dimensional spaces with boundaries. / by Nikolas Prezas. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 139-143). / Many quantum phases of matter, such as quantum spin liquids and fractional quantum hall systems, are well-described in the language of gauge theory. Until recently, most theoretical attention has been focused on systems described by familiar vector gauge theories. In this thesis, we will explore the properties of quantum phases described by higher rank tensor gauge theories. In particular, symmetric tensor gauge theories describe stable phases of matter in three dimensions. We will demonstrate that these theories lead to an exotic new class of particles which are restricted to move only in lower-dimensional subspaces, instead of being able to freely propagate in three dimensions. We call these excitations "subdimensional particles." As a special case, some models feature 0-dimensional particles, or "fractons," which are totally immobile. Subdimensional particles couple naturally to tensor electric and magnetic fields, in a form of generalized electromagnetism. We will establish the basic theoretical principles of this new tensor electromagnetism, including its Maxwell equations, force laws, and electrostatic properties. Finally, as a special case of the higher rank formalism, we will study a rank 2 phase featuring a gravity-like low-energy theory. We will show how to reconcile the restricted mobility of tensor gauge theories with the expected properties of a gravitational theory. Our toy models will thereby offer clues which may be useful for understanding more realistic gravitational theories. / by Michael Pretko. / Ph. D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 63-68). / Populations are subject to mutations conferring beneficial effects, as well as mutations conferring deleterious effects. Even if deleterious mutations occur much more frequently than beneficial mutations, the contribution of deleterious mutations to the overall rate of change of the population-wide mean fitness may be limited, due to the efficient action of selection. However, in particularly rapidly evolving populations, the stochastic accumulation of deleterious mutations may negate a significant fraction of the fitness increments provided by beneficial mutations, or even result in an overall decrease in fitness over time. Here we obtain a constraint on beneficial and deleterious mutation rates and selection pressures in order for positive adaptation to counterbalance fitness decline via Muller's ratchet. The steady state that separates parameter regimes of positive adaptation and negative adaptation is characterized by appealing to the exponential dependence of fixation probabilities on fitness effect sizes. We consider a range of fitness-mediated epistatic interactions and the corresponding implications regarding the existence, location and stability of long-term evolutionary fixed points. / by Matthew Melissa. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 55-56). / We demonstrate the use of a focused ultraviolet laser as a track calibration source in drift chambers, and specifically in a small time projection chamber (TPC). Drift chambers such as TPCs reconstruct the trajectories of charged particles by amplifying and collecting electrons produced by ionization of gas atoms with which the target particle collides. The ultraviolet laser induces a two-photon ionization of gaseous organic compounds along the beam, providing an artificially produced track which may then be used as a calibration source, particularly because it behaves as a particle of infinite momentum whose trajectory is straight in a magnetic field. To this end, we present measurements of the ionization profile of the laser for various gasses and model this ionization in terms of photon flux. / by Grant Elliott. / S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 65-66). / Dark matter makes up 85% of the known matter in the Universe, but the exact nature of dark matter remains unknown. The Cryogenic Dark Matter Search experiment, CDMS, attempts to directly detect the leading candidate dark matter particle, the Weakly Interacting Massive Particle (WIMP), recoiling off of cold germanium crystals. When particles interact with the crystals' atoms, they produce two measurable signals: phonons and ionization. The phonon signal contains information about the event such as its type, energy, and position, and has a much better resolution for lower energy events than does the ionization, especially for nuclear recoils from WIMPs. Because of this, there is a strong motivation for extracting as much information as possible from the phonon signal. For my thesis, the raw phonon pulse signal in the time domain was fit to a functional form based on phonon physics within the crystal. The functional form was carefully checked using the Markov chain Monte Carlo method. A Boosted Decision Tree (BDT) was then used to analyze the parameters from the fits to determine how well the parameters could distinguish between event types such as nuclear versus electron recoil events, and surface versus bulk events. Cuts made on the data, from results of the BDTs that were analyzed with parameters from this time-domain fitting algorithm, yielded better descrimination power than ones that were analyzed with the parameters currently used by CDMS. Applying this method to data mimicking a 15 GeV WIMP distribution produced a 34.4% signal efficiency improvement over the values currently used by CDMS. / by Chandler Schlupf. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 207-217). / Elastic form factors are fundamental quantities that characterize the electromagnetic structure of the nucleon. High precision measurements of these quantities are essential in understanding the structure of hadronic matter. Although the proton elastic form factors are well known, knowledge of the neutron form factors has been limited due to the lack of pure neutron targets. Few nucleon targets, deuterium in particular, are typically used to study the electromagnetic structure of the neutron. Cross section measurements are not sufficient for high precision determination of the electric form factor of the neutron, ..., due to its small value. Recently, experiments using polarization observables which are proportional to the product of the electric and magnetic form factors of the neutron have been used instead. Such measurements require highly polarized electron beams and either a vector polarized neutron target (typically ²H) or else a neutron final state polarimeter. The Bates Large Acceptance Spectrometer Toroid (BLAST) provides a unique opportunity to measure! the shape of the neutron electric form factor at low momentum transfers. BLAST combines a high duty-factor polarized electron beam in the South Hall Ring (SHR), an Atomic Beam Source (ABS) target of highly polarized deuterium atoms and a large acceptance detector. This work reports the results of measurements of the neutron electric form factor using the ... reaction at five 4-momentum transfer squared, Q², points of 0.14, 0.20, 0.29, 0.38 and 0.50 (GeV/c)² using data taken in 2004. The experimental setup is discussed in detail and the results for ... are presented and discussed in the context of various theoretical predictions. / (cont.) A fit to the world's data including new BLAST data determines ... to ... over 0 < Q² < 1 ((GeV/c)². The best fit includes contributions from a low Q² bump and a smooth dipole term. / by Vitaliy Ziskin. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 55-56). / Active force generation is an important class of out-of-equilibrium activity in cells. These forces play a crucial role in vital processes such as tissue folding, cell division and intracellular transport. It is important to determine the extent of such nonequilibrium activity during cellular processes to understand cell function. Here we present a framework for measuring nonequilibrium activity in biological active matter using time reversal asymmetry based on the Kullbeck-Leibler Divergence (KLD), also known as relative entropy. We estimate the KLD from a stationary time series using a k-nearest neighbors estimator, comparing the time-forwards process to the time-reversed process Using time series data of probe particles embedded in the actin cortex, we establish a lower bound for the entropy production of cortical activity. Our results demonstrate a reliable way to measure the breaking of detailed balance in mesoscopic systems. / by Garrett Watson. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. / Includes bibliographical references (leaf 51). / This thesis details a measurement setup and experimental procedures for emittance measurements using a Fourier transform infrared spectrometer. We calibrate the FTIR measurement system using measurements of a blackbody source at two temperatures to obtain the true emittance and absolute spectral radiance of our samples. / by Susannah Brown. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 88-93). / If dark matter consists of Weakly Interacting Massive Particles (WIMPs), such as the supersymmetric neutralino, various theories predict that their annihilation in the galaxy can give rise to anomalous features in the otherwise smooth spectra of charged cosmic rays. Up to now searches for these spectral anomalies have focused largely on antiparticles (p, e+) due to their lower astrophysical backgrounds. In this thesis we present results of a search for dark matter annihilation in the charge Z = -1 spectrum of AMS-01 (essentially electrons and antiprotons). To avoid model dependent complications we assume that the primary annihilation channel is through W+W- production. We use the galactic propagation software GALPROP to determine the dark matter spectra at Earth from a smooth isothermal source. Fits to the data did not reveal any contribution from dark matter and limits were placed on the rate of W+W- production in the galaxy and on the corresponding cross-section for WIMP annihilation through the W+W- channel (given a smooth isothermal distribution). / by Gianpaolo Patrick Carosi. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1994. / Includes bibliographical references. / by Charles XiaoJian Yan. / M.S.