Physics Theses and Dissertations

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    Lattice Quantum Chromodynamics (QCD) Calculations of Parton Physics with Leading Power Accuracy in Large Momentum Expansion
    (2023) Zhang, Rui; Ji, Xiangdong XJ; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Parton distributions describing how momenta of quarks and gluons are distributed inside a hadron moving at the speed of light, are important inputs to the Standard Model prediction of collider physics. Their non-perturbative nature makes traditional perturbative calculations from quantum field theory impossible. Besides a global fitting to experimental data, it is also possible to calculate parton physics from lattice QCD, a first-principle non-perturbative Monte Carlo simulation of the strong interaction on super computers. Among the different strategies to extract information for parton physics, the large momentum effective theory, based on a large momentum expansion of non-local Euclidean correlation functions, allows us to directly calculate the momentum-fraction or $x$-dependence. When matching the lattice-QCD calculations to the physical parton physics in the large momentum expansion, there are unavoidable power corrections in the expansion parameter $\Lambda_{\rm QCD}/P_z$, which is determined by the QCD characteristic non-perturbative scale $\Lambda_{\rm QCD}\approx300$~MeV and the hadron momentum $P_z$, and the leading term appears as $\mathcal{O}(\Lambda_{\rm QCD}/(2xP_z))$ due to the linear divergent self-energy of Wilson line in the Euclidean lattice correlators. For current lattice calculations of $P_z\sim 2-3$~GeV, this correction can be as large as $30\%$ at small $x$, dominating the uncertainties in the calculation. Achieving power accuracy in linear order of $\Lambda_{\rm QCD}/P_z$ is thus crucial for a high precision calculation of the parton physics from lattice QCD. In this dissertation, I summarize our work to eliminate this linear correction by consistently defining the renormalization for the linear divergence in lattice data and the resummation scheme of the factorially growing infrared-renormalon series in the perturbative matching. We show that the method significantly reduces the linear uncertainty by a factor of $\sim3-5$ and improves the convergence of the perturbation theory. We then apply the strategy to the calculation of pion distribution amplitude, which describes the pion light-cone wave function in a quark-antiquark pair. The method improves the short distance behavior of the renormalized lattice correlations, which is now consistent with the prediction of the short distance operator product expansion, showing a reasonable value for the moments of pion distribution amplitude. We also develop the first strategy to resum the large logarithms in the matching to physical pion distribution amplitude when the momentum of quark or antiquark in the pion are small, that could improve the accuracy of the prediction near the endpoint regions. After extracting the $x$-dependence from the large momentum expansion in mid-$x$ region, we complete the endpoint regions by fitting to the short distance correlations. Then a complete $x$-dependence is obtained for the pion distribution amplitude, which suggests a broader distribution compared to previous lattice QCD calculations or model predictions.
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    (2023) Guo, Yuxun; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this work, we study the nucleon spin structures and derive a new twist-3 partonic sum rule for the transverse spin of the proton, which involves the well-known quark spin structure function $g_T(x)$, the less-studied but known transverse gluon polarization density $\Delta G_T(x)$, and quark and gluon canonical orbital angular momentum densities associated with transverse polarization. This is the counterpart of the sum rule for the longitudinal spin of the proton derived by Jaffe and Manohar previously. We discuss in detail an important technicality related to the transverse polarization in the infinite momentum frame, i.e., separation of intrinsic contributions from the extrinsic ones. We apply our finding to the transverse-space distributions of partons, angular momentum, and magnetic moment, respectively, in a transversely polarized proton. For the experimental access to these structures, we consider the deeply virtual Compton scattering (DVCS) in twist-two generalized parton distribution (GPD) approximation. We show that different choices of light-cone vectors and gauges for the final photon polarization will lead to different higher-order kinematical corrections. The choice of light-cone vectors affects kinematic corrections at the twist-three level, accounting for the differences between the cross-section formulas in the literature. On the other hand, kinematical corrections from higher-twist GPDs eliminate the light-cone dependence at twist three. Those light-cone dependencies are studied systematically at JLab 12 GeV and future EIC kinematics. They serve as the intrinsic systematic uncertainties in extracting the Compton form factors. More importantly, they are also necessary for understanding cross-section measurements with higher-twist precision and to reconstruct higher-order Compton form factors. As DVCS is mostly sensitive to the quark structure, for the gluon, we also consider the near threshold photoproduction of heavy vector quarkonium off the proton. We show that the production amplitude can be factorized in terms of gluonic GPDs and the quarkonium distribution amplitude, similar to the high-energy limit. At the threshold, the threshold kinematics has a large skewness parameter $\xi$, leading to the dominance of the spin-2 contribution over higher-spin twist-2 operators. Thus, threshold production data are useful to extract the gluonic gravitational form factors, allowing studying the gluonic contributions to the quantum anomalous energy, mass radius, spin and mechanical pressure in the proton. We use the recent GlueX data on the $J/\psi$ photoproduction to illustrate the potential physics impact from the high-precision data from future JLab 12 GeV and EIC physics program. To put various constraints together and obtain the GPDs, we present the first global analysis of GPDs combing lattice quantum chromodynamics (QCD) calculations and experiment measurements including global parton distribution functions (PDFs), form factors (FFs) and DVCS measurements. Together with the constraints at zero skewness, we fit GPDs to global DVCS measurements from both the recent JLab and the earlier Hadron-Electron Ring Accelerator (HERA) experiments with two active quark flavors and leading order QCD evolution. With certain choices of empirical constraints, both sea and valence quark distributions are extracted with the combined inputs, and we present the quark distributions in the proton correspondingly. We also discuss how to extend the framework to accommodate more off-forward constraints beyond the small $\xi$ expansion, especially the lattice calculated GPDs.
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    Phase Transitions Affected by Molecular Interconversion
    (2023) Longo, Thomas; Anisimov, Mikhail; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Typically, pure substances may be found with only one gaseous or liquid state, while their solid state may exist in various polymorphic states. The existence of two distinct liquid forms in a single component substance is more unusual since liquids lack the long-range order common to crystals. Yet, the existence of multiple amorphous states in a single component substance, a phenomenon known as "liquid polyamorphism," has been observed or predicted in a wide variety of substances. In contrast to standard phase transitions, it has been suggested that polyamorphic liquid-liquid transitions are caused by the interconversion of molecular or supramolecular states. To investigate this phenomenon, a nonequilibrium thermodynamic model was developed to quantitatively describe the interplay between the dynamics of molecular interconversion and fluid-phase separation. The theory has been compared to a variety of interconverting systems, and has demonstrated a quantitative agreement with the results of Monte Carlo and Molecular Dynamics simulations. In this thesis, it is shown that there are two major effects of molecular interconversion on the thermodynamics and the kinetics of fluid-phase separation: if the system evolves to an equilibrium state, then the growth of one of the alternative phases may result in the destruction of phase coexistence - a phenomenon referred to as "phase amplification." It is demonstrated that depending on the experimental or simulation conditions, either phase separation or phase amplification would be observed. Previous studies of polyamorphic substances report conflicting observations of phase formation, which may be explained by the possibility of phase amplification occurring. Alternatively, if the system evolves to a nonequilibrium steady state, the phase domain growth could be restricted at a mesoscopic length scale. This phenomenon (referred to as "microphase separation") is one of the simplest examples of steady-state dissipative structures, and may be applicable to active matter systems, hydrodynamic instabilities, and bifurcations in chemical reactions, in which the nonequilibrium conditions could be imposed by an external flux of matter or energy.
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    (2023) Wagh, Kaustubh; Upadhyaya, Arpita; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The genetic information encoded within our DNA is converted into RNA in a process called transcription. This is a tightly regulated process where multiple proteins act in concert to activate appropriate gene expression programs. Transcription factors (TFs) are key players in this process, with TF binding being the first step in the assembly of the transcriptional machinery. TFs are sequence-specific DNA binding proteins that bind specific motifs within chromatin. How TFs navigate the complex nuclear microenvironment to rapidly find their target sites remains poorly understood. Technological advances over the past 20 years have enabled us to follow single TF molecules within live cells as they interact with chromatin. Most TFs have been shown to exhibit power law distributed residence times, which arise from the broad distribution of binding affinities within the nucleus. This blurs the line between specific and non-specific binding and renders it impossible to distinguish between different binding modes based on residence times alone. In this dissertation, I combine single molecule tracking (SMT) with statistical algorithms to identify two distinct low-mobility states for chromatin (histone H2B) and bound transcriptional regulators within the nucleus. On our timescales, the TF mobility states represent the mobility of the piece of chromatin that they are bound to. Ligand activation results in a dramatic increase in the proportion of steroid receptors in the lowest mobility state. Mutational analysis revealed that only chromatin interactions in the lowest mobility state require an intact DNA-binding domain as well as oligomerization domains. Importantly, these states are not spatially separated as previously believed but in fact, individual H2B and chromatin-bound TF molecules can dynamically switch between them. Single molecules presenting different mobilities exhibit different residence time distributions, suggesting that the mobility of a TF is intimately coupled with their temporal dynamics. This provides a way to identify different binding modes that cannot be detected by measuring residence times alone. Together, these results identify two unique and distinct low-mobility states of chromatin that appear to represent common pathways for transcription activation in mammalian cells. Next, I demonstrate how SMT can complement genome wide assays to paint a complete picture of gene regulation by TFs using two case studies: corticosteroid signaling and endocrine therapy resistance in breast cancer. Finally, I conclude with a roadmap for future work on examining the role of mechanical cues within the cellular microenvironment (such as stiffness and topography) in regulating TF dynamics and gene expression.
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    Topology, localization, and spontaneous symmetry breaking in nonequilibrium many-body systems
    (2023) Vu, DinhDuy Tran; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Exotic many-body phenomena are usually associated with the ground state of a time-independent Hamiltonian. It is natural to ask whether these physics can survive in a dynamic setting. Under a generic drive, the steady equilibrium state is most likely an infinite-temperature featureless thermal state. However, there exist exceptional cases where thermalization either does not happen or is delayed for a sufficiently long time, called non-equilibrium many-body systems. In this thesis, we study mechanisms that can generate non-equilibrium dynamics: many-body localization, prethermalization, and projective measurements. We then demonstrate that the resulting quantum states can host a wide variety of many-body phenomena similar to the ground state, focusing on three aspects: topology, localization, and spontaneous symmetry breaking.