Physics Department Dissertations Collection

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  • Publication
    Do Dibosons Dream of Semileptonic Sheep? Searching for Heavy WH Resonances and Optimizing Track Reconstruction with the Atlas Detector
    (2024-09) Vessella, Makayla
    A search for new heavy vector resonances decaying into a $W$ boson and a Standard-Model Higgs boson ($h$), targeting a semileptonic final state where the $W$ boson decays into a lepton-neutrino pair and the Higgs boson decays into a b-quark pair, is performed using 139 fb\textsuperscript{-1} of $\sqrt{s}$ = 13 TeV proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider (LHC) during LHC Run 2 (2015-2018). The search probes a wide range of potential heavy resonance masses, from 400 GeV to 5 TeV, by examining the invariant and transverse mass distributions of $Wh$ candidates for a localized excess. Many beyond Standard Model (BSM) theories generically predict such diboson vector resonances, such as the Heavy Vector Triplet (HVT) benchmark model through which the results are interpreted. No significant excess over Standard Model predictions is observed, and 95\% confidence level upper limits are placed on the production cross-section times branching ratio. These limits are also converted into constraints on the parameter space of the HVT model. Furthermore, reconstructing the trajectories of charged particles through the ATLAS Inner Detector is a critical component of both this search and the broader ATLAS physics program, but represents an enormous computational challenge due to its inherent combinatorial complexity. This work also presents successful efforts to improve and optimize this process, in both computational resources and physics performance, for LHC Run 3 (2022-2025). Under design accelerator conditions, the improved ATLAS track reconstruction is twice as fast as the legacy version, with no significant reduction in overall efficiency and a more than two-fold reduction in ``fake" tracks.
  • Publication
    Progress towards Tesseract: Calibrations and Backgrounds
    (2024-09) Patel, Pratyush Kumar
    We have just begun expanding our exploration for leptophobic Dark Matter to the sub-GeV scale and below using Direct Detection methods. We are utilizing a variety of advanced target materials and capitalizing on the most recent advancements in quantum sensing, including the implementation of TES (Transition Edge Sensors) for the detection of athermal-phonons. Detecting leptophobic dark matter in different target materials relies on sensing the recoil of a nucleus. Therefore, it is essential to precisely assess the response of a target material to a nuclear recoil. Performing tagged neutron scattering experiments is a commonly employed method for such studies. Nevertheless, sub-GeV dark matter kinematically transfer energy in the range of eV to the target materials. To accurately mimic the response of a such a low-energy recoiling nucleus, it is crucial to possess both a neutron source and a neutron detector that can effectively detect neutrons in the keV range. In this thesis, I have presented a detailed account of the design approach used to create a facility for calibrating such low-energy nuclear-recoil. This facility possesses a remarkable capability to generate pulsed quasi-monoenergetic neutrons. The design capitalizes on the narrow anti-resonances observed in the neutron-nucleus scattering cross-sections of materials like Scandium. Furthermore, this thesis discusses the procedure of designing and characterizing a prototype neutron backing detector, emphasizing its exceptional efficiency in detecting neutrons with energies in the keV range. Subse- quently, I embarked on addressing the issue of observed low energy excess events that has been puzzling numerous low-threshold DM detectors. These phenomena could be attributed to lattice stress. To investigate this, we chose to employ a liquid target material, superfluid 4He, progress on this has also been discussed.
  • Publication
    Quantum Chaos and Emergent Chirality in Correlated Degenerate Systems
    (2024-09) Wei, Chenan
    Correlated degenerate systems represent a fascinating subset of quantum manybody systems where the interaction effects among particles lead to emergent phenomena beyond conventional perturbation theory. In this dissertation, we survey several such systems, including flat band systems, moat band systems, and interacting conformal field theories (CFTs), elucidating their chaotic, integrable, and topological properties. We begin by exploring the Sachdev-Ye-Kitaev (SYK) model, which captures chaotic non-Fermi liquid behavior and holographic duality properties. Leveraging a system of spinless fermionic atoms in an optical Kagome lattice with flat band spectra, we demonstrate the emergence of the SYK Hamiltonian, providing a platform for experimental exploration of its exotic behavior. Transitioning to magic-angle twisted bilayer graphene (TBG) under strong Coulomb disorder, we unveil an emergent quantum chaotic strange metal (SM) phase characterized by weakly coupled SYK bundles. We propose a finite-temperature phase diagram for TBG, discussing implications for experimental observations. In the realm of strongly interacting bosons with moat band dispersion in two dimensions, we investigate the propensity for stabilizing a chiral spin liquid (CSL) ground state. Through Monte Carlo simulations and variational analysis, we identify parametric windows favoring the uniform CSL state and provide density estimates for experimental relevance. Lastly, we delve into the low-energy properties of the one-dimensional spin-1/2 XXZ chain with time-reversal symmetry-breaking pseudo-scalar chiral interaction, unveiling a comprehensive phase diagram using thermodynamic Bethe ansatz. We identify emergent conformal field theories describing phases with distinct ground state symmetries and investigate finite-size effects around the critical transition point.
  • Publication
    Phase Transitions and Self-Assembly of Charged Polymer Solutions
    (2024-09) Li, Siao-Fong
    Charged polymers ubiquitously play a crucial role in numerous biological and synthetic systems, exhibiting diverse phases and self-assembly behaviors due to the entanglement of polymer connectivity and long-range electrostatic interactions. Charged polymer physics delves into understanding their highly coupled and non-linear response, shedding light on comprehending biological systems composed of charged biopolymers. Inspired by biomolecular condensates, membrane-less organelles assembled by intrinsically disordered proteins within cells, we aim to explore the phases and self-assembly in complex charged polymer solutions, where complexities stem from the chemical sequences and physical associations of polymers. These complexities cause difficulties in responding to the growth interest of biomolecular condensates. Employing the field theory formalism and statistical properties of conformations, we address the connectivity and interaction into the spatial correlation of monomer concentration, leading to the Landau free energy predicting the formations of phases and self-assembly. This thesis comprises two works. The first work focuses on predicting the stability, size, and morphology of microphase separation for sequence-specified charged polymers, resulting in machinery for microphases for general sequences. In the second work, utilizing polyzwitterions as a simplified model for associative charged polymers, we explore the thermoreversible behaviors of electric-dipole-driven macrophase separation and gelation. Furthermore, our use of the renormalization group reveals that concentration fluctuations near critical points deviate from the Ising universality class due to the presence of associations. These results can facilitate future investigations on more complex systems in biological and synthetic realms with suitable modifications.
  • Publication
    Two-Qubit Gates with Superconducting Fluxonium Qubits
    (2024-09) Dogan, Ebru
    The field of quantum computing with superconducting qubits has achieved considerable success, yet the challenge of realizing high-fidelity multi-qubit operations remains central to its advancement. Fluxonium qubits, with their long coherence times and strong anharmonicity, are promising building blocks for such systems. In this thesis, we present the two-fluxonium gate implementations we worked on, with a particular focus on the first realization of a cross-resonance CNOT gate with a capacitively coupled two-fluxonium system in a 3D cavity. This fully microwave-driven gate scheme confines qubit dynamics within the computational space and preserves coherence while enabling high-fidelity operations. We present our experimental results that achieve two-fluxonium gate fidelities up to 99.5% despite challenging setup conditions. Our findings suggest pathways for further reducing the error rates, with implications for future research into more efficient quantum gate implementations.
  • Publication
    Optical Spectroscopy of Quantum Materials
    (2024-09) DeCapua, Matthew
    The isolation of monolayer graphene and the subsequent development of fabrication techniques to stack it and other two-dimensional materials in highly customizable ways has ushered in a new era of materials synthesis. We now have unprecedented control over the atomic structure of materials, which we can use to better understand complex quantum many-body effects. In this dissertation, we use optical spectroscopy to investigate the physics of superstructures of angle-aligned, two-dimensional layers. Using Raman spectroscopy, we characterize excitons formed in a twisted bilayer graphene, and investigate the effects of moire band reconstruction on its electronic properties. We use photoluminescence spectroscopy to probe interlayer excitons in a transition metal dichalcogenide moire heterobilayer, shedding light on their formation and polarization in the moire potential landscape. We also present Raman characterization studies of AgFeO2 and MnBi2Te4, laying the groundwork for future studies on these novel materials.
  • Publication
    Geometrically Frustrated Assembly at Finite Temperature
    (2024-09) Hackney, Nicholas
    Geometric frustration refers to the incommensurability between locally preferred order and global geometry. The inclusion of such frustration in systems of self-assembling particles has been shown to give rise to unique, scale-dependent states characterized by the self-limitation of domain size and the presence of topologically defective ground states. In this dissertation, we introduce a minimal lattice model of geometrically frustrated assembly and use a variety of numerical and theoretical techniques to explore its behavior at finite temperature. In chapter \ref{chapter: intro}, we review the literature of frustrated assembly and identify the key questions that we seek to address throughout this dissertation. In chapter \ref{chapter: model_intro}, we introduce our minimal model and develop the numerical and theoretical machinery that we will use throughout the following chapters. In addition to this, we will derive several key predictions for the effect of temperature on frustrated assembly. In chapter \ref{chapter: self-limiting assembly} and \ref{chapter: bulk condensation}, we use our numerical techniques to test these predictions and explore the self-limiting and defect bulk phase of assembly under fairly dilute conditions. In chapter \ref{chapter: equilibrium paths}, we investigate the role of entropy in stabilizing self-limiting assembly. After that, we relax the dilute restriction and study our model over the entire range of allowable concentration. Here, we show the existence of a percolation transition at high concentration and compare the structure of this resultant phase to the defective bulk structure. In chapter \ref{chapter: soft gauge model}, we generalize our model to allow the effects of subunit elasticity to be studied. Finally, in chapter \ref{chapter: conclusion}, we summarize the key results of this work and discuss several future directions that are motivated by experiment.
  • Publication
    MANIPULATING AND CHARACTERIZING INTERACTION OF BACTERIA WITH SURFACES
    (2024-05) Xu, Zhou
    Motivated by observations of cell orientation at biofilm–substrate interfaces and reports that cell orientation and adhesion play important roles in biofilm evolution and function, this thesis investigated the influence of surface chemistry on the orientation of Escherichia coli cells captured from flow onto surfaces that were cationic, hydrophobic, or anionic. We characterized the initial orientations of nonmotile cells captured from gentle shear relative to the surface and flow directions. The broad distribution of captured cell orientations observed on cationic surfaces suggests that rapid electrostatic capture of cells to oppositely charged surfaces preserve the instantaneous orientations of cells as they rotate in the near-surface shearing flow. By contrast, on hydrophobic and anionic surfaces, cells were oriented slightly more in the plane of the surface and in the flow direction compared with that on the cationic surface. This suggests slower development of adhesion at hydrophobic and anionic surfaces, allowing cells to tip toward the surface as they adhere. Once cells were captured, the flow was increased by 20-fold. Cells did not reorient substantially on the cationic surface, suggesting a strong cell–surface bonding. By contrast, on hydrophobic and anionic surfaces, increased shear forced cells to tip toward the surface and align in the flow direction, a process that was reversible upon reducing the shear. These findings suggest mechanisms by which surface chemistry may play a role in the evolving structure and function of microbial communities. The viability and growth of captured Escherichia coli cells on cationic and hydrophobic surfaces were tested. This thesis confirms cells can grow on both cationic and hydrophobic surfaces, especially for cationic surfaces which contrast to fact that cationic surfaces are often used as bactericide. When bacteria adhere to surfaces, the chemical and mechanical character of the cell-substrate interface guides cell function and the development of microcolonies and biofilms. Alternately on bactericidal surfaces, intimate contact is critical to biofilm prevention. The direct study of the buried cell-substrate interfaces at the heart of these behaviors is hindered by the small bacterial cell size and inaccessibility of the contact region. Here, this thesis presents a total internal reflectance fluorescence depletion approach to measure the size of the cell-substrate contact region and quantify the gap separation and curvature near the contact zone, providing an assessment of the shapes of the near-surface undersides of adhered bacterial cells. Resolution of the gap height is about 10%, down to a few nanometers at contact. Using 1 and 2 µm silica spheres as calibration standards we report that, for flagella-free Escherichia coli (E. coli ) adhering on a cationic poly-L-lysine layer, the cell-surface contact and apparent cell deformation vary with adsorbed cell configuration. Most cells adhere by their ends, achieving small contact areas of 0.15 µm2, corresponding to about 1-2% of the cell’s surface. The altered Gaussian curvatures of end-adhered cells suggest the flattening of the envelope within the small contact region. When cells adhere by their sides, the contact area is larger, in the range 0.3-1.1 µm2 and comprising up to ∼12% of the cell’s total surface. A region of sharper curvature, greater than that of the cells’ original spherocylindrical shape, borders the flat contact region in cases of side-on or end-on cell adhesion, suggesting envelope stress. From the measured curvatures, precise stress distributions over the cell surface could be calculated in future studies that incorporate knowledge of envelope moduli. Overall, the small contact areas of end-adhered cells may be a limiting factor for antimicrobial surfaces that kill on contact rather than releasing bactericide. Furthermore, this thesis studied the swimming patterns of bacteria in quiescent condition which is the pre-step to investigate bacterial swimming in flow near surfaces. Bacterial swimming in flow near surfaces is critical to the spread of infection and device colonization. Understanding how material properties affect flagella- and motility-dependent bacteria surface interactions is a first step in designing new medical devices that mitigate the risk of infection. In this part, the thesis reported statistics of motile swimmers and no-motor, nonmotile E. coli cells in the quiescent bulk solution. The run and tumble character of the motile swimmers was confirmed by particle tracking. The swimming velocities were consistent for multiple run phases of each cell, but the swimming speed itself was cell dependent, with some cells swimming faster than others, mostly in the range 3-6 µm/s. Molly Shave further reported the run and engagement motion pattern of motile swimmers in steady flow near the interfaces which driven by tumbling via flagellar unbundling.
  • Publication
    SEARCHING FOR EXOTIC HIGGS BOSON DECAYS IN THE bb𝜏𝜏 FINAL STATE IN THE ATLAS DETECTOR
    (2024-05) Wagner, Cooper P
    For my dissertation, I explain how I have contributed to several areas of the ATLAS experiment for both hardware and analysis. On the detector side, this paper will examine my contributions to the New Small Wheel, part of the muon system installed for Run 3, and the ongoing development of the Inner Tracker, a silicon pixel upgrade for the High Luminosity LHC. My analysis work on exotic Higgs decays, probing physics beyond the Standard Model, will also be covered in a description of the $H rightarrow aa rightarrow bbtautau$ analysis.
  • Publication
    STUDIES OF e+e- PAIR PHOTO-PRODUCTION ON PROTON TARGET AT 8 GEV IN THE GLUEX EXPERIMENT
    (2024-05) Schick, Andrew Conrad
    Lepton pair production has played an important role in both nuclear and particle physics, being among the earliest calculations utilizing QED, and seen most famously in the discovery of the $J/psi$ at BNL. A technique is presented for measuring the linear polarization of GeV scale photon beams through the detection of $e^+e^-$ pairs photo-produced in the target. This technique is applied to the analysis of GlueX data on proton target. Simulation predicts the analyzing power for pair production to be $.5725 pm 0.0025$ for the GlueX data, and the analysis of experimental data gives a linear polarization of approximately 35%, in good agreement with other measurements of beam polarization. The pair production technique is complementary to other electromagnetic and hadronic measurements of beam polarization, and is generally applicable in experiments that allow for forward angle electron and positron identification and tracking. To facilitate this study, a neural net was trained for $ e/pi $ separation to eliminate the pion background. Further, it is demonstrated that these $ e^+ e^- $ pairs are sensitive to the proton charge form factor, which opens up the possibility for a new method to measure the proton RMS charge radius.
  • Publication
    ALUMINUM QUASIPARTICLE DIFFUSION MEASUREMENTS IN VACUUM AND SUPERFLUID HELIUM 4
    (2024-02) Osterman, David Z
    Dark matter, one of the greatest mysteries in physics, continues to elude direct detection even after decades of effort. Physicists, in more recent years, are looking toward smaller mass ranges (sub-GeV), and a slew of new detector ideas have emerged. The HeRALD experiment seeks to use superfluid helium as a target to detect low-mass dark matter through the production of phonons and helium excimers as well as the resulting photons and quantum-evaporated helium atoms. HeRALD and many other experiments across particle physics use transition edge sensors (TESs) to detect small energy deposits - for HeRALD, such events are characteristic of a collision between a low-mass dark matter particle and a target He atom. Energy from impinging photons and He atoms are funneled to the TES through athermal phonons in a silicon substrate followed by quasiparticles (broken Cooper pairs) in thin-film superconducting aluminum fins. For all such calorimeters, energy efficiency is reduced by quasiparticles (QPs) becoming trapped by impurities in the Al. Additionally, immersed sensors could potentially lose QP energy to the surrounding superfluid He. In this thesis, I present studies of quasiparticle diffusion in superconducting Al fins using a laser-scanning microscopy-based technique. The characteristic QP trapping length (or diffusion length) is measured with the TES-fin device both in vacuum and immersed in superfluid He, to measure the QP energy lost to the superfluid. QP are produced at a localized origin in the Al film using a focused 1550nm laser coupled to a single-mode optical fiber mounted on piezoelectric nanopositioners. The resulting QP propagation is then monitored using a TES, and described using a simple 1D diffusion model. The measurements of 100µm-scale quasiparticle diffusion determine that the Al fins - fabricated at Argonne National Laboratory - work sufficiently well to be used in TES-based detectors. Additionally, no significant drop in QP collection efficiency due to device immersion in superfluid helium was measured. I conclude this thesis with the detailed design considerations for - and some preliminary testing results of - the upgrade to the HeRALD cryogenic photodetector (CPD), which attempts to eliminate the low energy excess background.
  • Publication
    QUANTUM CHAOS, INTEGRABILITY, AND HYDRODYNAMICS IN NONEQUILIBRIUM QUANTUM MATTER
    (2024-02) Lopez Piqueres, Javier
    It is well-known that the Hilbert space of a quantum many-body system grows exponentially with the number of particles in the system. Drive the system out of equilibrium so that the degrees of freedom are now dynamic and the result is an extremely complicated problem. With that comes a vast landscape of new physics, which we are just recently starting to explore. In this proposal, we study the dynam- ics of two paradigmatic classes of quantum many-body systems: quantum chaotic and integrable systems. We leverage certain tools commonly employed in equilibrium many-body physics, as well as others tailored to the realm of non-equilibrium scenar- ios, in order to address various problems within this evolving field. Our contributions are the following: Inspired by random matrix theory and random unitary circuits subject to projective measurements, we first uncover a novel phase transition in a model of random tensor networks separating an area-law from a logarithmic-law in the scaling of entanglement entropy of a many-body wavefunction. Next, we study transport in the Rule 54 cellular automaton, a paradigmatic integrable model displaying just two species of solitons of different chiralities. Our contribution here is a sound numerical verification of some of the formulas for transport coefficients recently derived within a generalized hydrodynamic approach valid for integrable systems. Using the equations of generalized hydrodynamics as a starting point we then propose a new phenomenological scheme based on a relaxation-time approximation widely used in kinetics, but fundamentally different, to study the experimentally relevant regime where only a few conservation laws are present. We then aim at uncovering the hydrodynamics of integrability-breaking starting from fully microscopic dynamics. To do so we study a noisy version of the Rule 54 model and of the hard-rod gas, where the source of noise in both models is backscattering of solitons. We find that these models of integrability-breaking are atypical in that in the former relaxation occurs at long time scales owing to the presence of kinetic constraints, and the latter displays singular transport signatures as a result of infinitely many conserved charges despite the model being nonintegrable. Finally, we conclude by studying operator spreading in both integrable and chaotic quantum chains. Using hydrodynamics and tensor net- work simulations we find distinctive signatures of these two classes of models when looking at their operator front.