Closing a Loophole in Factorization Proofs

We address the possibility in factorization proofs that low-energy collinear gluons can couple to soft gluons.Comment: 3 pages, 5 figures, contribution to the proceedings of Quark Confinement and the Hadron Spectrum I

Nonhistory: Slavery and the Black Historical Imagination

This dissertation examines the theoretical significance of slavery in contemporary novels by black writers of English and French expression. I contend that black authors like Gayl Jones, Edouard Glissant, Léonora Miano, Sherley Anne Williams, Jean Métellus, and Fred D’Aguiar use literature to revise historical narratives and generate new histories of slavery. By reading novels as historical texts, I theorize nonhistory as a critique of the epistemological limitations of historiography. I argue that Black Francophone and Anglophone Atlantic writers of the postcolonial and post-Jim Crow era narrate the past as a nonhistory whose discursive and aesthetic afterimages expose the disjointed experience of time engendered by the lived experience of antiblackness. This project questions the endurance of slavery in the black historical imagination. In thinking with black Anglophone and Francophone writers, I consider how literary texts explore complementary dimensions to historical inquiry. By theorizing nonhistory as a historiographical tool, I question what kinds of subjunctive knowledges might be invented to explain the often-disjointed experience of black time

Visual Dynamics Models for Robotic Planning and Control

For a robot to interact with its environment, it must perceive the world and understand how the world evolves as a consequence of its actions. This thesis studies a few methods that a robot can use to respond to its observations, with a focus on instances that can leverage visual dynamic models. In general, these are models of how the visual observations of a robot evolves as a consequence of its actions. This could be in the form of predictive models that directly predict the future in the space of image pixels, in the space of visual features extracted from these images, or in the space of compact learned latent representations. The three instances that this thesis studies are in the context of visual servoing, visual planning, and representation learning for reinforcement learning. In the first case, we combine learned visual features with learning single-step predictive dynamics models and reinforcement learning to learn visual servoing mechanisms. In the second case, we use a deterministic multi-step video prediction model to achieve various manipulation tasks through visual planning. In addition, we show that conventional video prediction models are unequipped to model uncertainty and multiple futures, which could limit the planning capabilities of the robot. To address this, we propose a stochastic video prediction model that is trained with a combination of variational losses, adversarial losses, and perceptual losses, and show that this model can predict futures that are more realistic, diverse, and accurate. Unlike the first two cases, in which the dynamics model is used to make predictions for decision-making, the third case learns the model solely for representation learning. We learn a stochastic sequential latent variable model to learn a latent representation, and then use it as an intermediate representation for reinforcement learning. We show that this approach improves final performance and sample efficiency

The Value of Soil Testing for Potassium Fertilizer in Tennessee Cotton Production

This thesis presents two separate studies focusing on the value of soil test information for potassium (K) fertilization of upland cotton. The objective of the first study was to determine the value of soil test information for available K in upland cotton production using the linear response plateau (LRP) and linear response stochastic plateau (LRSP) functions. This study uses dynamic programming to solve for optimal K fertilizer rates that maximize NPV when K carryover was and was not considered by a producer. This study extends the existing literature by comparing the value of soil testing information using a stochastic and deterministic yield response plateau functional form. Including carryover decreased the optimal K application rate and the K carryover level, while yield was optimal regardless of whether the producer considered carryover. The LRSP model Using K carryover information for K application decisions increased net present value and helped maintain steady levels of soil K. The LRSP function fit the data better than the LRP, and the value of soil testing was $11 per acre lower over ten years using the LRSP. The objective of the second study was to determine the K fertilizer application rate and temporal frequency for obtaining K soil test information that maximizes NPV to K fertilizer in cotton production in the southeastern US. This study used a dynamic programming model to determine the optimal K application rates over time. Monte Carlo simulation was used to determine NPV for cotton production using five soil test schedules ranging from soil testing annually to every fifth year. On average, optimal K application rates for all temporal frequencies varied slightly. The range of optimal K application rates increased as the producer waited longer periods of time to update their soil test information. As the temporal frequency increased, the lower bounds of their carryover levels and yields decreased due to yield limiting levels of K. NPV of returns to K was maximized at$7,580 per acre when producers updated soil testing information every two years, which was $2 per acre per year greater than annual soil testing

Disorder Enhanced Spin Polarization in Diluted Magnetic Semiconductors

We present a theoretical study of diluted magnetic semiconductors that includes spin-orbit coupling within a realistic host band structure and treats explicitly the effects of disorder due to randomly substituted Mn ions. While spin-orbit coupling reduces the spin polarization by mixing different spin states in the valence bands, we find that disorder from Mn ions enhances the spin polarization due to formation of ferromagnetic impurity clusters and impurity bound states. The disorder leads to large effects on the hole carriers which form impurity bands as well as hybridizing with the valence band. For Mn doping 0.01 < x < 0.04, the system is metallic with a large effective mass and low mobility

Effect of Hund's rule coupling on SU(4) spin-orbital system

We investigate the ground-state property of a one-dimensional two-orbital Hubbard model at quarter filling by numerical techniques such as the density-matrix renormalization group method and the exact diagonalization. When the Hund's rule coupling $J$ is zero, the model is SU(4) symmetric. In fact, both spin and orbital correlations have a peak at $q=\pi/2$, indicating an SU(4) singlet state with a four-site periodicity. On the other hand, with increasing $J$, it is found that the peak position of the orbital correlation changes to $q=\pi$, while that of the spin correlation remains at $q=\pi/2$. We briefly discuss how the SU(4) symmetry is broken by $J$.Comment: 2 pages, 2 figures, Proceedings of ICM2006 (August 20-25, 2006, Kyoto

Factorization theorems for exclusive heavy-quarkonium production

We outline the proofs of the factorization theorems for exclusive two-body charmonium production in B-meson decay and e^+e^- annihilation to all orders in perturbation theory in quantum chromodynamics. We find that factorized expressions hold up to corrections of order m_c/m_b in B-meson decay and corrections of order m_c^2/s in e^+e^- annihilation, where m_c is the charm-quark mass, m_b is the bottom-quark mass, and root-s is the e^+e^- center-of-momentum energy.Comment: 4 pages, 2 figure

Factorization of low-energy gluons in exclusive processes

We outline a proof of factorization in exclusive processes, taking into account the presence of soft and collinear modes of arbitrarily low energy, which arise when the external lines of the process are taken on shell. Specifically, we examine the process of e^+e^- annihilation through a virtual photon into two light mesons. In an intermediate step, we establish a factorized form that contains a soft function that is free of collinear divergences. In contrast, in soft-collinear effective theory, the low-energy collinear modes factor most straightforwardly into the soft function. We point out that the cancellation of the soft function, which relies on the color-singlet nature of the external hadrons, fails when the soft function contains low-energy collinear modes.Comment: 18 pages, 10 figures, 2 tables, version published in Physical Review