Theses and Dissertations from UMD
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Item Unifying Searches for New Physics with Precision Measurements of the W Boson Mass(2024) Sathyan, Deepak; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Standard Model (SM) of particle physics has been extremely successful in describing the interactions of electromagnetic, weak nuclear, and strong nuclear forces. Yet, there are both unexplained phenomena and experimentally observed tensions with the SM, motivating searches for new physics (NP). Collider experiments typically perform two kinds of analyses: direct searches for new physics and precision measurements of SM observables. For example, experimental collaborations use collider data to search for NP particles like the heavy superpartners of the SM particles, whose observation would be clear evidence of supersymmetry (SUSY). These direct searches often consider kinematic regions where the SM background is small. This strategy is unable to probe regions of the NP parameter space where the SM background is dominant. The same collaborations also measure the masses of SM particles, which not only serve as consistency tests of the SM, but can also probe effects of NP. In 2022, the Collider Detector at Fermilab (CDF) collaboration published the most precise measurement of the $W$ boson mass: $m_W$ = 80433.5 $\pm$ 9.4 MeV. This measurement is in $7\sigma$ significance tension with the SM prediction via the electroweak (EW) fit, $m_W^{\rm pred.}$ = 80354 $\pm$ 7 MeV. Many extensions to the SM can affect the prediction of $m_W$ with indirect effects of heavy NP. However, in 2023, the ATLAS re-measurement of the $W$ boson mass, $m_W$ = 80360 $\pm$ 16 MeV, was found to be consistent with the SM prediction. Both collaborations found a high-precision agreement between the measured kinematic distributions and the SM prediction of the kinematic distributions for their corresponding extracted $m_W$. We propose using the precision measurements of $m_W$ to directly probe NP contributing to the same final state used to measure $m_W$: a single charged lepton $\ell$ and missing transverse energy $\met$. This strategy is independent of modifying the EW fit, which tests indirect effects of NP on the predicted value of $m_W$. Any NP producing $\ell+\met$ which modifies the kinematic distributions used to extract $m_W$ can be probed with this method. With this strategy, since these distributions are used to search for NP while measuring $m_W$, a simultaneous fit of NP and SM parameters is required, thus unifying searches and measurements. This simultaneous fitting can induce a bias in the measured $m_W$, but only to a limited extent for our considered models. We consider three categories of NP which can be probed: ($i$) modified decay of $W$ bosons; ($ii$) modified production of $W$ bosons; and ($iii$) $\ell+\met$ scenarios without an on-shell $W$ boson. We also show that models whose signals extend beyond the kinematic region used to measure $m_W$ can be probed in an intermediate kinematic region. Our results highlight that new physics can still be discovered at the LHC, including light new physics, via SM precision measurements. Additionally, anticipated improvements in precision SM measurements at the High Luminosity LHC further enables new searches for physics Beyond the Standard Model (BSM).Item EXCITED DYONIC STATES OF MONOPOLES AND ASTRONOMICAL BOUNDS ON AN AXION-PHOTON-DARK PHOTON INTERACTION(2024) Ristow, Clayton James; Hook, Anson; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study of beyond the standard model physics can largely be broken into twocategories: theoretical and phenomenological. In the former, we study theories in depth to better understand their implications while in the latter, we hold models of our physical world to scrutiny against experimental evidence. Both are crucial to understanding physics beyond the standard model. To reflect this dichotomy, this thesis is broken into two acts, one covering theoretic research and the other discussing progress made on the phenomenological front. Chapter 2, comprising the entirety of Act 1 of this thesis, concerns the theory of magnetic monopoles. In the mid-1970’s t’Hooft and Polyakov discovered magnetic monopoles exist as generic solutions in spontaneously broken gauge theories. Since then much progress has been made in understanding these monopoles, most notably by Callan who argued that the fermion vacuum is non-trivial around the core of the magnetic monopole. These non-trivial vacuua can be interpreted as bound states of fermions with fractional fermion number. In this work, we explicitly compute these fermion bound states in an SU (2) gauge theory coupled to Nf fermions. We demonstrate there are two unique ways to grant mass to the fermions in the SU (2) theory which, after symmetry breaking, give the same UEM (1) theory of fermions. Despite this low energy equivalence, we show that the two theories exhibit very different physics at low energy scales around a magnetic monopole. We show that there may exist stable excited dyonic states with differing charges and energies between the two theories. We find the ground states can also differ in energy and charge between the two theories. We demonstrate the monopole can inherit a mass correction and charge distribution that depends on the topological θ angle even if one of the fermions is massless. This effect is present in one of the theories and is completely absent in the other. Finally, we discuss the implications of these effects on the SU (5) GUT monopole. Act two, comprising of chapters 3 and 4, focuses on the phenomoenological side of beyond the standard model physics. In these chapters, we consider two highly motivated beyond the standard model particles, the axion, φ, and the dark photon AD which are coupled to the standard model photon via a coupling φF ̃FD. In some models, this coupling can provide the leading order coupling between our sector and the dark sector containing the axion and dark photon. In chapter 2, we demonstrate the effect this coupling has on the Cosmic Microwave Background (CMB) in the scenario where either the axion or the dark photon constitutes dark matter. Depending on which we choose to be dark matter, we show that this interaction leads to the conversion of the CMB photons into the other dark sector particle, leading to a distortion in the CMB spectrum. We present the details of these unique distortion signatures and the resulting constraints on the φF ̃FD coupling. In particular, we find that for a wide range of masses, the constraints from this effect are stronger than on the more widely studied axion-photon-photon coupling. We also demonstrate that CMB distortions of this type can a exhibit unique, non-thermal frequency profile which could be detected by future experiments. In chapter 3, we consider the astrophysical effects of the φF ̃FD coupling, in particular, its effect on supernova cooling rates. We show that the bound on this interaction due to supernova cooling exhibits two unusual features. If there is a large mass difference between the axion and dark photon, we show both production and scattering become suppressed and the bounds from bulk (volume) emission and trapped (area) emission both weaken exponentially. We show that these bounds do not intersect leading to a larger area of excluded parameter space than may have otherwise been expected. The other unusual feature occurs because the longitudinal modes of light dark photons couple more weakly than their transverse modes. As a consequence, the longitudinal modes can still cause excessive cooling even if the transverse modes are trapped. Thus, the supernova constraints for massive dark photons look like two independent supernova bounds super-imposed on top of each other. We also briefly consider the effect of this interaction on white dwarf cooling and Big Bang Nucleosynthesis.Item PHENOMENOLOGY OF ULTRALIGHT FIELDS(2024) Brzeminski, Dawid; Hook, Anson; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Standard Model is an amazing success of particle physics, a success further cemented by the discovery of the Higgs boson. While its picture is incredibly satisfying, there are still a few mysteries it cannot address, one of which is the nature of dark matter. While we have overwhelming evidence for its existence, we still do not know its basic properties such as mass or spin. Ultralight fields are among the most exciting dark matter candidates. Their large occupation number allows us to treat them as classical fields, while their non-relativistic velocities ensure that the field oscillates at an angular frequency equal to its mass with a long coherence time. In this dissertation, we discuss some challenges associated with constructing successful models of ultralight dark matter and discuss new detection strategies. In the first part of this dissertation, we address the underlying issue with ultralight scalars, namely the naturalness problem. Generally, requiring the scalar to couple to the Standard Model introduces radiative corrections to its mass, which conflicts with the requirement of a small mass. We present an ultraviolet-complete model that avoids this issue by employing $Z_N$ symmetry, which suppresses corrections to the mass while retaining relatively large couplings to photons, making the model testable by current and future experiments looking for the time-variation of the fine structure constant. In the second part of this dissertation, we focus on the experimental aspects of ultralight scalars. The general experimental landscape is divided into two categories: experiments assuming a dark matter background, and experiments measuring the fifth force associated with the new scalar.The former provides strong constraints for the lightest scalars due to their large abundance, while the latter provides more conservative but robust limits on scalar interactions across many decades in scalar mass. We propose a novel approach based on measuring scalar potential using atomic and nuclear clocks, which complements fifth force measurements and offers significant improvements over current bounds. In the third part of the dissertation, we shift our attention to vector dark matter. Specifically, we consider a scenario where some of the lepton generations are charged under a new gauge field. In this case, neutrino decays in the early universe impose strong constraints on their couplings, particularly for the lightest vectors. At higher masses, neutrino oscillations become a leading constraint due to the sourcing of the field by electrons affecting their oscillations. We demonstrate that in the presence of vector dark matter, the influence of the background field on neutrinos is even more pronounced, significantly enhancing constraints on the lightest vectors by several orders of magnitude.Item Quantization of causal diamonds in (2+1)-gravity(2024) Andrade e Silva, Rodrigo; Jacobson, Theodore A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We develop the non-perturbative reduced phase space quantization of causal diamondsin (2+1)-dimensional gravity with a nonpositive cosmological constant. The system is defined as the domain of dependence of a spacelike topological disk with fixed boundary metric. By solving the constraints in a constant-mean-curvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of Diff+(S1)/PSL(2,R). Classically, the states correspond to causal diamonds embedded in AdS3 (or Mink3 if Λ = 0), with fixed corner length, and whose Cauchy surfaces have the topology of a disc. Because the phase space does not have a natural linear structure, a generalization of the standard canonical (coordinate) quantization is required. As the configuration space is a homogeneous space for the Diff+(S1) group, we apply Isham’s group-theoretic quantization scheme. We propose a quantization based on (projective) unitary irreducible representations of the BMS3 group. We find a class of suitable quantum theories labelled by a choice of a coadjoint orbit of the Virasoro group and an irreducible unitary representation of the corresponding little group. The most natural choice, justified by a Casimir matching principle, corresponds to a Hilbert space realized by wavefunctions on Diff+(S1)/PSL(2,R) valued in some unitary irreducible representation of SL(2,R). A surprising result is that the twist of the diamond corner loop is quantized in terms of the ratio of the Planck length to the corner perimeter.Item TENSOR NETWORK APPROACHES IN NON-EQUILIBRIUM QUANTUM MANY-BODY DYNAMICS(2024) Yoo, Yongchan; Swingle, Brian; Sau, Jay D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the dynamics of non-equilibrium quantum systems has been a longstanding challenge in a wide range of physical phenomena. Recent advancements in various quantum technologies have driven theoretical investigations of non-equilibrium quantum many-body dynamics. A crucial aspect of the theory is the development of computational methods, which have led to significant results and show promising directions for further development. However, simulating these complex systems using classical algorithms remains extremely difficult, primarily due to the challenge of identifying physically principled and efficient approximations capable of reducing their computational complexity.This thesis discusses theoretical and numerical studies on the non-equilibrium dynamics of quantum many-body systems, particularly focusing on the steady-state transport of conserved quantities in various one-dimensional spin systems. We study transport phenomena employing boundary-driven open quantum setups and investigate various aspects of the non-equilibrium dynamics induced by these systems. Our goal is to understand steady-state phenomena and to push the boundaries of simulation capacity using state-of-the-art technologies. In the first part, we examine the non-equilibrium steady-state (NESS) phases of an interacting Aubry-Andr´e-Harper model. This model involves a quasiperiodic potential, leading to interesting emergent collective phenomena. The observed spin transport and quantum correlation structure suggest the presence of multiple dynamical phases between the well-studied thermal and many-body-localized phases. In the second part, we study the impact of operator weight dissipation on the scaling behavior of transport in various spin models. Our findings suggest that dissipation’s effect on transport depends on the system’s conserved quantities. When dissipation preserves these symmetries, it maintains the scaling of the system’s transport properties. However, when it disrupts these conserved quantities, it leads the system towards diffusive scaling of transport. In the third part, we investigate energy transport within the non-integrable regime of the Z3 chiral clock model, utilizing Lindblad operators with adjustable size and temperature. Through scaling analysis, we extract the model’s transport coefficients at relatively high temperatures, both above its gapless and gapped low-temperature phases. Furthermore, we calculate the temperature dependence of the energy diffusion constant across various model parameters, including the regime where the model exhibits quantum critical behavior at low temperatures.Item Harnessing Quantum Systems for Sensing, Simulation, and Optimization(2024) Bringewatt, Jacob Allen; Gorshkov, Alexey V; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum information science offers a remarkable promise: by thinking practically about how quantum systems can be put to work to solve computational and information processing tasks, we gain novel insights into the foundations of quantum theory and computer science. Or, conversely, by (re)considering the fundamental physical building blocks of computers and sensors, we enable new technologies, with major impacts for computational and experimental physics. In this dissertation, we explore these ideas through the lens of three different types of quantum hardware, each with a particular application primarily in mind: (1) networks of quantum sensors for measuring global properties of local field(s); (2) analog quantum computers for solving combinatorial optimization problems; and (3) digital quantum computers for simulating lattice (gauge) theories. For the setting of quantum sensor networks, we derive the fundamental performance limits for the sensing task of measuring global properties of local field(s) in a variety of physical settings (qubit sensors, Mach-Zehnder interferometers, quadrature displacements) and present explicit protocols that achieve these limits. In the process, we reveal the geometric structure of the fundamental bounds and the associated algebraic structure of the corresponding protocols. We also find limits on the resources (e.g. entanglement or number of control operations) required by such protocols. For analog quantum computers, we focus on the possible origins of quantum advantage for solving combinatorial optimization problems with an emphasis on investigating the power of adiabatic quantum computation with so-called stoquastic Hamiltonians. Such Hamiltonians do not exhibit a sign problem when classically simulated via quantum Monte Carlo algorithms, suggesting deep connections between the sign problem, the locality of interactions, and the origins of quantum advantage. We explore these connections in detail. Finally, for digital quantum computers, we consider the optimization of two tasks relevant for simulating lattice (gauge) theories. First, we investigate how to map fermionic systems to qubit systems in a hardware-aware manner that consequently enables an improved parallelization of Trotter-based time evolution algorithms on the qubitized Hamiltonian. Second, we investigate how to take advantage of known symmetries in lattice gauge theories to construct more efficient randomized measurement protocols for extracting purities and entanglement entropies from simulated states. We demonstrate how these protocols can be used to detect a phase transition between a trivial and a topologically ordered phase in $Z_2$ lattice gauge theory. Detecting this transition via these randomized methods would not otherwise be possible without relearning all symmetries.Item Quantum Advantage in Sensing and Simulation(2024) Ehrenberg, Adam; Gorshkov, Alexey V; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since the discovery of Shor's factoring algorithm, there has been a sustained interest in finding more such examples of quantum advantage, that is, tasks where a quantum device can outperform its classical counterpart. While the universal, programmable quantum computers that can run Shor's algorithm represent one direction in which to search for quantum advantage, they are certainly not the only one. In this dissertation, we study the theory of quantum advantage along two alternative avenues: sensing and simulation. Sensing refers to the task of measuring some unknown quantity with the smallest possible error. In many cases, when the sensing apparatus is a quantum device, this ultimate achievable precision, as well as specific protocols producing estimators with this precision, are unknown. In this dissertation, we help close this gap for both qubit-based and photonic quantum sensors for the specific task of measuring a linear function of unknown parameters. We use quantum Fisher information and the quantum Cramér-Rao bound to derive limits on their ultimate precision. We further develop an algebraic framework that allows us to derive protocols saturating these bounds and better understand the quantum resources, such as entanglement, that are necessary to implement these protocols. In doing so, we help clarify how quantum resources like entanglement lead to more precise sensing. We also study a specific form of simulation called Gaussian Boson Sampling, which is a member of the broad framework of random sampling tasks that have become a popular method for demonstrating quantum advantage. While many of the theoretical underpinnings of these random sampling tasks, including Gaussian Boson Sampling, are well understood, many questions remain. Anticoncentration, which is strongly related to the moments of the output distribution, is a particularly relevant property when it comes to formally proving the existence of quantum advantage. We develop a graph-theoretic framework to calculate these moments, and we show that there is a transition in the strength of anticoncentration as a function of how many of the photonic modes are initially squeezed. We therefore demonstrate a transition in the evidence for the so-called approximate average-case hardness of Gaussian Boson Sampling, hence clarifying in what regimes we have the strongest evidence for quantum advantage. Finally, we also discuss the simulation complexity of Many-Body Localized systems. Many-Body Localization is a widely studied phase of matter that is often characterized by the appearance of a large number of quasilocal integrals of motion (operators that commute with the Hamiltonian) that interact via exponentially decaying interactions. In this dissertation, we study a phenomenological form of Many-Body Localization and show three main results. First, we demonstrate that, for polynomially long evolution times under a Hamiltonian in the Many-Body Localized phase, there is a quasipolynomial-time classical algorithm that can perform strong simulation of the output state. On the flip side, our second result is that, when the evolution time is exponentially long, weak simulation of the output state becomes formally classically hard. Finally, as a consequence of our classical results, we show the approximate quantum circuit complexity of these Hamiltonians grows sublinearly in the evolution time (in contrast with the proposed linear growth for chaotic Hamiltonians). Thus, this work helps clarify whether and how we might find quantum advantage via simulating certain types of condensed matter systems.Item QUANTUM INFORMATION SCRAMBLING AND PROTECTION IN MANY-BODY SYSTEM(2023) Cheng, Gong; Swingle, Brian; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work is focused on two main topics in quantum information theory: the scramblingof quantum information and the preservation of quantum information in many-body system. In terms of information scrambling, the main focus of this work is on the Out-of-time-order corre- lator (OTOC), which is used to probe the dynamics of quantum information as it spreads from localized degrees of freedom to those that are distributed throughout the system. On the other hand, the aim of the study of quantum information protection is to construct a system that can preserve quantum information for a sufficiently long time when coupled to a finite-temperature environment. The many-body systems analyzed in this work belong or are related to a class of stronglyinteracting systems known as holographic quantum models. The standard examples in this class are believed to be equivalent to gravitational theory in spacetime that is one-dimensional higher than that the quantum model lives in. Therefore, the results may also provide insights into topics in quantum gravity. The first part of the thesis explores the scrambling dynamics close to a critical point whereconformal symmetry emerges. The second case deals with the scrambling dynamics with con- servation law constraints in holographic quantum field theory. The result also clarifies how con- served charges influence the dynamics in the bulk dual. The third part of the thesis presents a matrix model with a large matrix rank N that belongs to the class of approximate quantum error correction codes. We investigate its thermal stability by coupling it to a thermal bath and demonstrate that it behaves as a self-correcting quantum memory at finite temperature. The coherent memory time scales polynomially with the system size N.Item Phase Transitions in Random Quantum Circuits(2023) Niroula, Pradeep; Gorshkov, Alexey; Gullans, Michael J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Random circuits have emerged as an invaluable tool in the quantum computing toolkit. On the one hand, the task of sampling outputs from a random circuit has established itself as a promising approach to experimentally demonstrate the superiority of quantum computers using near-term, noisy platforms. On the other hand, random circuits have also been used to deduce far-reaching conclusions about the theoretical foundations of quantum information and communication. One intriguing aspect of random circuits is exemplified by the entanglement phase transition that occurs in monitored circuits, where unitary gates compete with projective measurements to determine the entanglement structure of the resulting quantum state. When the measurements are sparse, the circuit is unaffected and entanglement grows ballistically; when the measurements are too frequent, the unitary dynamics is arrested or frozen. The two phases are separated by a sharp-phase transition. In this work, we discuss an experiment probing such phases using a trapped-ion quantum computer. While entanglement is an important resource in quantum communication, it does not fully capture the non-classicality necessary to achieve universal quantum computation. A family of measures, termed "magic", is used to quantify the extent to which a quantum state can enable universal quantum computation. In this dissertation, we also discuss a newly uncovered phase transition in magic using quantum circuits that implement a random stabilizer code. This phase transition is intimately related to the error correction threshold. In this work, we present numerical and analytic characterizations of the magic transition. Finally, we use a statistical mechanical mapping from random circuits acting on qubits to Ising models to suggest thresholds in error mitigation whenever the underlying noise of a quantum device is imperfectly characterized. We demonstrate the existence of an error-mitigation threshold in dimensions D>=2.Item Holographic Cosmological Models and the AdS/CFT Correspondence(2023) Antonini, Stefano; Swingle, Brian; Jacobson, Theodore; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The formulation of a quantum theory of gravity is a central open problem in theoretical physics. In recent years, the development of holography---and in particular the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence---provided a new framework to investigate quantum gravity and led to consistent advancement. However, how to describe cosmology within holography remains an unanswered question whose solution could determine whether holography is able to capture physics in our universe. This dissertation describes a new proposal for embedding cosmological physics in the holographic paradigm. This is articulated in two different but related approaches, both involving time-symmetric Big Bang-Big Crunch cosmologies with negative cosmological constant $\Lambda$.In the first approach, the cosmological universe is given by a four-dimensional end-of-the-world brane moving in a five-dimensional AdS black hole spacetime. The proposed holographic dual description is given by a boundary conformal field theory. Under specific conditions, gravity is localized on the brane and effectively four-dimensional: an observer living on the brane is unaware of the existence of the extra dimension. In this dissertation, I show how these conditions can be met in an AdS-Reissner-Nordstr\"om background while retaining a holographic dual description. The second approach focuses on spatially flat $\Lambda<0$ cosmologies which analytically continue to Euclidean wormholes connecting two asymptotic AdS boundaries. The proposed dual theory is given by two holographic 3D CFTs coupled by non-holographic 4D degrees of freedom on a strip. A different analytic continuation of the Euclidean wormhole leads to a Lorentzian traversable wormhole. After discussing the general features of these holographic cosmologies, I describe how the traversable wormhole can be reconstructed from the dual theory and how the existence of the former constrains the latter. Finally, I show that these $\Lambda<0$ cosmologies can undergo phases of accelerated expansion and match observational data for the scale factor evolution. The results presented in this dissertation should be regarded as the initial steps on a new line of research which will hopefully lead to a description of quantum gravity in a cosmological universe via holography. Achieving this goal would render holography a viable candidate to describe quantum gravity in our universe.