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Measurements of Beyond Standard Model Interactions with the UCNA and nEDM@SNS Experiments
Get PDFUltracold neutrons (UCNs) are neutrons that have been cooled such that their kinetic energy is on the order of their gravitational potential energy. Experimentally, ultracold neutrons are valuable because at these energies they are trappable and provide experimenters with long observation times. In fact, their observation times are on the order of the free neutron decay lifetime --- allowing direct observation of neutron β-decay. Many contemporary experiments measuring high-precision processes involving neutrons use UCNs. Two such experiments are UCNA and nEDM@SNS, both of which form the basis of this work.
UCNA is an experiment that took place at Los Alamos National Laboratory. In this work, we analyze the 2010-2013 UCNA datasets on neutron β-decay using UCNs. These datasets were originally designed to measure the asymmetry parameter, A, in neutron β-decay. However, there was also sensitivity to another physical parameter in the neutron β-decay rate: the Fierz interference term, b. The Fierz interference term in neutron β-decay acts as a probe of beyond Standard Model (SM) physics interactions, specifically scalar and tensor couplings in the weak interaction. Due to the vector - axial-vector nature of the weak interaction in the SM, any non-zero measurements of b would be indicative of new, beyond SM couplings. In this work, we present the extraction of the Fierz interference term as measured by neutron β-decay for the 2010, 2011-2012, and 2012-2013 UCNA datasets. We present these measurements using two methods: a direct extraction by measuring shape distortions in the β-decay electron spectrum, and an energy dependence in the asymmetry, A0. These two methods across the three datasets yield six new measurements of b from neutron β-decay data. Our final result is the weighted average of the three asymmetry-extracted b results.
The UCNA datasets were also sensitive to another type of beyond SM interaction: neutron decaying to dark matter with an accompanying positron-electron pair (first proposed in [FG18a]. This decay channel was originally proposed in order to resolve the discrepancy between two measurement methods of the neutron lifetime: bottle experiments which measure neutron population as a function of time, and beam experiments which measure the decay protons from conventional neutron β-decay. Due to the experimental setup of the UCNA apparatus, the UCNA dataset was sensitive to such a decay channel. Using the 2012-2013 UCNA dataset which had functioning timing data, we effectively rule out this decay channel as the sole explanation for the neutron lifetime discrepancy for ≈ 84% of the available decay phase space. Furthermore, we set branching ratio limits on this decay channel as compared to the conventional weak interaction mediated decay.
The last project in this work is the construction of a large scale magnet for the nEDM@SNS experiment. The nEDM@SNS experiment is an experiment designed to measure the neutron electric dipole moment (nEDM) and will take place at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory (ORNL). This experiment will take place in 2027 and make a leading precision measurement on the nEDM. Part of the experiment is the magnetic system and, within that system, the B0 magnet which will provide a DC holding field to UCNs within the experiment's measurement volume. The assembly procedure for constructing the B0 magnet is detailed and intermediate quality checks as well as a post-construction room temperature magnetic field map are presented. The preliminary results indicate that the completed B0 magnet satisfies the specifications and will be useable in the nEDM@SNS experiment.</p
Mechanical Approach to Active Matter: Reverse Osmotic Effect and Motility-Induced Phase Separation
Get PDFThe defining feature of active matter, self-propulsion requires constant consumption of energy to be maintained. As a result, active matter systems are inherently out of equilibrium and some principles that are accepted as common knowledge, particularly from thermodynamics, do not apply to the active matter systems. Arguably the most popular example is the motility-induced phase separation (MIPS) -- active matter can spontaneously phase separate into liquid-like dense phase and gas-like sparse phase even without any attractive interactions between the self-propelling constituents. In this thesis, I demonstrate the utility of a mechanical perspective in revealing and understanding the underlying physics of seemingly confounding behaviors of active matter systems. In Chapters 2 and 3, I consider the mechanics of a suspension of active colloidal particles when the transport properties (self-propelling speed and diffusivities) vary spatially. The mechanical analysis reveals the reverse-osmotic nature of active matter systems with a spatial variation in activity. I provide an explanation for why physical processes governed by the osmotic pressure of particles can appear in a reversed manner in active matter systems, e.g. a fluid can flow from regions of high concentration to low in a suspension of active colloids. In Chapter 4, I develop a mechanical theory of phase coexistence that applies to both equilibrium and nonequilibrium systems. By applying the mechanical theory to MIPS, I find phase coexistence conditions of the MIPS that allow a construction of a phase diagram, which excellently agrees with the results from computer simulations. The mechanical theory also allows access to the microscopic structure of phase interfaces. By investigating the interfacial structure, I discover interesting nonequilibrium interfacial behavior of the MIPS. I find that the width of the MIPS interface varies nonmonotically with the activity of particles and provide a mechanical explanation for the phenomena
Theory of Mathematical Optimization for Delegated Portfolio Management
Get PDFWe study the optimization problem of finding closed convex sets Γ ⊆ Rd containing the origin that minimize F(Γ) = ∑i=1k wi | θi/2 - pΓ(θi) | 2, where w1, ..., wk > 0, θ1, ..., θk in Rd are given, and pΓ(θi) are the closest points in Γ to θi, i = 1, ..., k. This problem is motivated by the topic of delegated portfolio management in finance. In Chapter 2, we will explore this connection. To approach the problem, we first prove existence of a solution for the general problem. To further study properties of the solution, we next introduce the semidefinite programming relaxation, for which we have a first-order characterization of optimality. We then explore the question of exactness of this relaxation, which turns out to be equivalent to the notion of localizability: the shape optimization problem embedded in higher dimensions must have solutions in the original dimension. Finally, we present special cases for which localizability holds.</p
Principles of Addressing Specificity in Promiscuous Ligand-Receptor Systems
Get PDFIn multicellular organisms, a relatively small number of highly conserved signaling pathways are used to enable intercellular communication. While the underlying molecular components and interactions are increasingly well understood, a fundamental mystery is how the diverse cell types of the body can be so precisely coordinated by so few pathways. It has long been known that different cell types exhibit varied responses to molecular signals, and it is unclear how this cell type specificity arises. In this work, we take a different perspective on this question and explore how cell type specificity can be generated at the level of intracellular signal. We refer to this ability to selectively activate different cell types as "addressing." By eliminating the complexity of considering downstream pathway effectors, we are able to more comprehensively understand how cell type specificity can arise in spite of—or because of—promiscuity in ligand-receptor interactions. We focus on the bone morphogenetic protein (BMP) pathway as an ideal example. This pathway is essential in development, is of therapeutic interest in an array of pathologies, and has proven amenable to theoretical and experimental analysis. We first describe a minimal model of the pathway and identify what types of response functions can be achieved. We show that each layer of computation, from the formation of signaling complexes to the activation of downstream second messenger, can provide nontrivial integrations of ligand inputs. We then extend this analysis to systems with multiple cell types that may vary in receptor expression profile. The diverse response functions of this pathway enable systems in which different cell types or sets of cell types may be addressed with high specificity. In particular, the BMP pathway can address multiple cell types with high capacity, flexibility, and robustness. Taken together, these results provide a framework for understanding how molecular promiscuity in signaling pathways can, in fact, enable cellular specificity in pathway responses.</p
Asymmetric Total Synthesis of (–)-Myrifabral A and B, Havellockate, and New Strategies for Acyclic Stereocontrol
Get PDFResearch in the Stoltz group aims, generally, to develop novel technologies for the preparation of stereochemically rich molecules and, further, to apply these technologies in the context of complex natural product total synthesis. Chapter 1 of this thesis describes the strategic utilization of a Pd-catalyzed asymmetric allylic allylation and N-acyl iminium ion cyclization to accomplish short, enantioselective total syntheses of (–)-myrifabral A and (–)-myrifabral B. Chapter 2 describes the development of a Pd-catalyzed asymmetric allylic alkylation to generate acyclic α-quaternary carboxylic acid derivatives from geometrically defined fully substituted N-acyl indole-derived allyl enol carbonates. While ester-derived enol carbonates could be prepared with a high degree of enolate geometry control, they were ineffective in the asymmetric allylic alkylation reaction. Thus, N-acyl indole substrates served as excellent carboxylic ester equivalents. Chapter 3 discusses an unusual Pd-catalyzed decarboxylative α,β-dehydrogenation reaction of N-acyl indole-derived enol carbonates enabled by a novel, highly electron-deficient phosphinooxazoline ligand. Research presented in Chapter 4 delineates a globally diastereoconvergent approach to the Ireland–Claisen rearrangement for the synthesis of α-tetrasubstituted amino acids bearing vicinal tertiary stereogenic centers. Computational investigation of the diastereoconvergence in Ireland–Claisen rearrangement revealed key intramolecular interactions which enable the reaction to proceed in exceptional diastereoselectivity without the need for a selective enolization protocol. Additionally, a diastereodivergent approach for the Ireland–Claisen rearrangement in acyclic systems to generate vicinal quaternary/tertiary and quaternary/quaternary stereogenic centers in good to high diastereoselectivity is discussed. Enolate geometry control and substrate design are critical for achieving high diastereoselectivity in these transformations.</p
Vat Photopolymerization Additive Manufacturing of Functional Materials: from Batteries to Metals and Alloys
Get PDFIn recent years, additive manufacturing (AM), also known as 3D printing, has emerged as a uniquely powerful tool for rapid prototyping and for creating complex, high value structures. Vat polymerization (VP) is an AM technique which forms parts through light-initiated polymerization, capable of achieving both high resolution and high throughput. While VP has been utilized to fabricate a wide variety of polymeric materials, fabricating functional materials such as ceramics, metals, and inorganic composites has remained a challenge. This thesis focuses on developing fabrication methods for a range of functional materials, from battery active materials to metals and ceramics, via vat polymerization additive manufacturing, taking advantage of chemical reactions within an AM part after fabrication to form target materials in situ.
We demonstrate the use of emulsions to introduce aqueous active material precursors into organic photopolymer resins to create architected lithium sulfide/carbon composites for use as lithium-sulfur battery cathodes. Such architected cathode materials are promising for mitigating mechanical degradation in high volume-change battery materials such as the sulfur cathode. We additionally performed nanome- chanical experiments on lithium sulfide powders to determine how lithium sulfide yields, deforms, and fails in the context of volume-change-induced stress during battery cycling. Because lithium sulfide is present as a discharge product in all lithium sulfur batteries, these nanomechanical particle compressions have bearing on the entire field, beyond the realm of 3D architected cathodes.
We additionally demonstrate the use of organogel templates to streamline the AM process by enabling the fabrication of many materials starting with a single resin composition, followed by infiltration of appropriate metal precursors and post-processing heat treatment to convert the polymer/precursor matrix to the target metal via calcination and reduction reactions. We fabricate and characterize copper, nickel, silver, cobalt, cupronickel alloys, tungsten, and more to highlight the wide-ranging versatility of achievable materials and microstructures.</p
Conformational Plasticity of HIV-1 Env and Implications for Vaccine Design
Get PDFThe human immunodeficiency virus (HIV) envelope glycoprotein (Env), a (gp120/gp41)₃ trimer, is present on the surface of the viral envelope membrane. Env binding to the host cell receptor, CD4, and the co-receptor, CCR5 or CXCR4, triggers a cascade of Env conformational changes and structural rearrangements which ultimately leads to the viral and host cell membrane fusion, marking the initiation of a viral infection. In this work, we present findings of the conformational changes of an Env trimer from a closed, pre-fusion state to an asymmetrically open state when bound to receptor CD4 and a co-receptor mimicking antibody, E51. We showed the importance of tyrosine sulfation in gp120 binding. The EM structures also indicate the existence of Env’s multiple conformational states. Based on the structural information, we modeled the order of conformations on the path to co-receptor binding and viral-host cell membrane fusion.
As the sole viral protein present on the virion surface, the Env acts as the target for anti-HIV antibodies. Using various types of engineered Env as the immunogen, researchers made attempts to elicit anti-HIV neutralizing antibodies in animals. In this work, we identified and analyzed two neutralizing antibodies, Ab1303 and Ab1573, that target the Env CD4 binding site (CD4bs), one of the conserved epitopes on the Env gp120 surface. Using biophysical and structural methods, we described a novel recognition mechanism of these antibodies and proposed a model about the unique behavior of Env under physiological conditions. This study proved that CD4bs Abs that recognize an "occluded open" Env can be raised by sequential animal immunizations, thereby guiding the future immunogen design and therapeutic applications.</p
Electrical Impedance Spectroscopy-Derived 3D Conductivity Tomography for Atherosclerosis Detection
Get PDFElectrical impedance tomography (EIT) utilizes voltage/current data measured from the surface of interest to reconstruct the electrical conductivity distribution. This results in a noninvasive medical imaging procedure with many applications. Some examples would be: lung ventilation monitoring, breast cancer detection, and fatty liver detection. Non-alcoholic fatty liver disease (NAFLD) is one of the most common causes of cardiometabolic diseases in overweight individuals. The gold standard for NAFLD diagnosis is a liver biopsy which is a risky and invasive procedure. A non-invasive and cost effective method to detect fatty liver is an important unmet clinical need. Due to the distinct electrical properties of fatty tissue versus normal tissue, EIT can be applied to detect the fat infiltrate in the liver. We conducted EIT measurements and reconstructions on 19 subjects where the fat infiltrate was validated by MRI proton-density fat fraction (PDFF). The liver EIT conductivity was shown to be inversely correlated with MRI PDFF, demonstrating the ability of EIT to detect fatty infiltrate in the liver.
This thesis also extends the EIT reconstruction to detect atherosclerosis, which is a build-up of fatty tissue in the arteries (plaque). Some plaques are prone to rupture and the current gold standard has a false negative rate of 20 % when distinguishing between vulnerable plaque and stable plaque. We sought to use EIT to detect the fatty content (mainly oxidize LDL) inside these vulnerable plaques. Therefore, the reconstruction method was modified into an outward setting that can measure from the inner surface of interest. Ex vivo experiments have demonstrated the ability to detect the location of fatty tissue in swine aorta. This technique has the potential to detect vulnerable plaque. However, the dimension of the device and the required electrode number limits the application from in vivo animal artery experiments.
Finally EIS-derived EIT, a new method we proposed, utilizes impedance values at a fixed frequency to solve for the conductivity distribution. This approach circumvents the mathematically ill-posed problem found when performing traditional EIT methods. We designed a 6-point EIS electrode array that was circumferentially configured to a balloon catheter and deployed in Yorkshire mini-pigs with induced stenosis in the right carotid artery. The EIS spectra demonstrated an elevated impedance in the right carotid arteries and the EIS-derived EIT mappings were reconstructed. The low conductivity regions in the EIS-derived EIT mappings were correlated with the positive E06 immunostaining for oxLDL-laden regions. Thus, we establish the capability of 3D EIS-derived EIT to detect oxLDL-laden arterial walls with translational implication to predict metabolically active plaques prone to acute coronary syndromes.</p
Extending the Capability of Classical Quantum Many-Body Methods
Get PDFThis thesis discusses several topics in extending the capability of conventional quantum many-body methods. The first project focuses on extending quantum chemical methods, namely coupled cluster theory, to the correlated systems in the condensed phase. We consider bulk nickel oxide and manganese oxide, which are two paradigmatic correlated electron materials that pose challenges to traditional density functional theory-based simulation framework. We adapted molecular coupled cluster singles and doubles theory using Gaussian basis sets with translational symmetry and norm-conserving pseudopotential. This allowed us to carry a detailed study on the ground and excited states of the two materials.
The second project investigates numerical optimization techniques for Abelien group symmetric tensor contractions. In many-body quantum simulations, group symmetries in states and operators often lead to block sparse structure in the representing tensors. Exploiting this opportunity can significantly reduce the computation cost and memory footprint in tensor contractions. We consider cyclic group symmetry and introduce an efficient remapping scheme to express the sparse tensor contractions almost fully in terms of dense tensor operations.
The third project is devising a wavefunction-based method for coupled electrons and phonons. We are interested in simulating the interacting electrons and phonons at the same footing using coupled cluster methods. The ground state and excited state of two types of systems are investigated in this work: the Hubbard Holstein model and diamond crystal in ab initio setting.
Finally, the fourth project is to develop a generic framework for tensor network simulation on fermionic systems. Tensor network methods are powerful tools to study strongly correlated physical systems. However, traditionally these methods have been developed with commutative algebraic rules, which are commensurate with bosons but not compatible with anti-symmetric fermions. Our approach encodes the fermion statistics directly in the block sparse tensor backend so the tensors behave just like anti-commuting fermion operators.</p
Aqueous Aerosols in Atmospheric Chemistry and Airborne Diseases
Get PDFAqueous atmospheric aerosols are small droplets (typically smaller than 5 μm) suspended in the air that are comprised of water and water-soluble components. These aerosols provide an air-water interfacial reaction environment on their surfaces, and act as a medium for airborne disease transmission. In this thesis, Chapters II and V explore atmospherically relevant reactions on the aqueous aerosol surface using an online mass spectrometry, while Chapter III investigates the SARS-CoV-2 airborne transmission considering suspended virus-laden aerosols as the transmission media. Spinning off this SARS-CoV-2 work, Chapter IV describes a newly developed quantitative RNA amplification test kit for COVID-19, with an emphasis on the amplification result photo recognition component