Theses/Dissertations - Physics

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    First search for electroweak supersymmetry in final states with hadronic decays of WW, WZ, or WH boson pair and missing transverse momentum.
    (2023-08) Kanuganti, Ankush Reddy, 1993-; Hatakeyama, Kenichi.
    This dissertation presents a new search for particle physics beyond the standard model in the form of electroweak production of supersymmetric particles, specifically charginos and neutralinos. The novel search is performed using Run 2 data from the CMS experiment at the LHC with a total integrated luminosity of 137 fb−1 . The targeted final states include a pair of bosons (WW, WZ, or WH) with large missing transverse momentum arising from the lightest neutralinos (χe 0 1 ). Deep neural network large-radius jet taggers are used to identify hadronic decays of the bosons. The standard model backgrounds are estimated using data-driven techniques. No significant excess of events is observed relative to expectations from the standard model. Results are presented in terms of constraints on the cross sections for production of mass-degenerate wino-like superpartners of SU(2) gauge bosons, χe ± 1 /χe 0 2 , and higgsino-like supersymmetric particles, χe ± 1 /χe 0 2 /χe 0 3 , in both cases with the bino-like superpartner of U(1) gauge boson as the lightest supersymmetric particle. In the wino-bino scenario, wino masses are excluded up to 960 GeV. In the higgsino-bino scenario, the search excludes higgsino masses from 300 to 650 GeV. These exclusions are most stringent to date set by the CMS experiment. Sensitivity projections with future high-luminosity LHC data are also presented.
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    Quantization of black holes and singularity resolution in loop quantum gravity.
    (May 2023) Gan, Wen-Cong, 1992-; Wang, Anzhong.
    In this dissertation, we study the properties of quantum black holes in the framework of loop quantum gravity. Loop quantum gravity is based on the canonical quantization of holonomies and fluxes of densitized triads. In loop quantum cosmology (LQC), the effective Hamiltonian can be obtained from the classical Hamiltonian by polymerization. The interior of Schwarzschild black hole is isometric to Kantowski-Sachs cosmological model with symmetry group R × SO(3). Thus loop quantization techniques of LQC can be used in loop quantization of black holes. On the other hand, different choices of quantum parameters δb, δc correspond to different quantization schemes and will lead to different loop quantum black hole solutions. In particular, we investigate global and local properties of Bodendorfer, Mele, and Münch (BMM) model, Alesci, Bahrami and Pranzetti (ABP) model and Böhmer-Vandersloot (BV) model. We find that different choice of parameters will lead to different asymptotic behaviors. Specifically, for appropriate parameters, BMM model has black hole/white hole structure, ABP model has asymptotic de Sitter solution, while in BV model, black hole/white hole horizon never forms due to large quantum effects.
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    Modeling the influence of ion wakes on the self-organization of dust in a complex plasma.
    (December 2022) Vermillion, Katrina, 1985-; Matthews, Lorin Swint.
    The interaction between charged dust grains and streaming plasma leads to the formation of ion wakes, which are thought to be responsible for the self-organization of dust into complex structures. These structures have been observed to exhibit long-range stability both in ground-based and microgravity experiments. The response of dust in complex plasma to changes in experimental conditions is similar to the atomic-level ordering in traditional materials, but occurs at more easily observable spatial and temporal scales due to the larger dust grain size. The purpose of this work is to expand the current understanding of stable configurations of dust grains in a streaming plasma environment by implementing a molecular dynamics simulation that models dust and ions on their individual timescales. The model is used to determine plasma parameters that are currently unable to be directly measured experimentally and quantify their impact on dust structures, to compare and evaluate existing theoretical models of electric potential and resulting interactions between charged dust grains, and to quantify plasma conditions that lead to transitions between stable configurations of dust.
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    The study of overlapping sheath and dust particle structures in dusty plasma.
    (December 2022) Chen, Mudi, 1985-; Hyde, Truell Wayne.
    Complex (dusty) plasma consists of plasma and dust particles which vary in size from nanometer to millimeter in diameter. Complex plasma exists across many different environments, and can be found in environments from the cosmos to industry. Due to plasma fluxes to its surface, a dust particle embedded in a complex plasma will charge negatively or positively depending on the charging mechanism. It should be noted that unless the electron are very energetic, the dust particle is usually negatively charged. When a surface is immersed in plasma, the region closest to the surface where a quasi-neutral plasma no longer exists is called the plasma sheath. Due to the perturbative nature of the majority of diagnostics in common use, such sheaths, especially the sheath forming near surfaces, can exhibit complex geometries making their physics difficult to diagnose. In such plasma sheath, the surface most often has a negative electric potential, which allows negatively charged dust particles to levitate against the force of gravity. These dust particles can behave as minimally perturbative electrostatic probes. This technique is relatively simple since the necessary measurements are simply the position of the particle and its motion after perturbation. A novel plasma diagnostic technique, the freefall particle technique, will be introduced in this thesis. This technique allows measurement of the sheath profile with small perturbations, high spatial resolution and minimal equipment requirements. This technique will be employed to investigate the sheath produced at a planar surface allowing investigation of the overlapping sheaths formed inside a glass box commonly used to confine dust in laboratory experiments. The first experimental measurements of the plasma sheath inside a trench-like structure will also be discussed, as will the ion-wake inside a glass box. Finally, the various dust particle structures will be studied. Through measurement of the external confinement force at each position of the dust particle structures, the relationship between dust particle structure formation and confinement is also examined.
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    Nonlinear responses in dusty plasmas.
    (2020-09-29) Ding, Zhiyue, 1990-; Hyde, Truell Wayne.
    Dusty plasma as a system containing both plasmas and dust particles. Dusty plasma systems are found throught out the industy and the space, for example, dust are found in plasma etching and in TOKAMAKs, as well as Saturn rings. Thus, the study of dusty plasma helps to understand many systems in reality. In this dissertation, the interaction of dust particles in a plasma sheath has been studied to determined the nature of the non-linear interaction. Theoretical model for describing the nonlinear dust interaction has been established and used to explain experimental observations. This dissertation is arranged in the following way. In chapter one, a background introduction of dusty plasmas is provided. In chapter two, a theoretical model describing the motion of two coupled dust particles considering nonlinear particle-particle interaction is established, and a perturbation method is used to analytically solve this model. In chapter three, experiments measuring amplitude-frequency responses are introduced and the nonlinear interaction is studied based on the model established in chapter two. In chapter four, the model established in chapter two is extended to a higher degree of freedom, which explains the ‘internal resonance’ that is observed for the first time in dusty plasma. In chapter five, a Bayesian optimization-based automatic method for response analysis of a single dust particle is proposed. In chapter six, a quick estimation method is proposed for measuring the charge of dust particles in the plasma sheath.
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    Enhancing hadron jet reconstruction in the CMS Level-1 trigger using machine learning.
    (2022-05-03) Hasan, Syed Mahedi, 1992-; Brinkerhoff, Andrew.
    Level–1 Trigger (L1T) algorithms used in the Compact Muon Solenoid (CMS) experiment for detecting different physics objects must be optimized to ensure that CMS continues to collect the most interesting proton–proton collision events for analysis. In this thesis, a new machine learning based approach using boosted decision trees (BDTs) is presented, which improves the jet detection performance in the L1T. In the first step, a BDT is trained using 12 features of L1T jets to generate an importance ranking of the features. The results indicate that a new algorithm for mitigating the effect of simultaneous collisions (‘pileup’) called the ‘phi–ring’ algorithm could be better at detecting L1T jets than the current ‘chunky donut’ algorithm. New BDTs are then trained separately using phi–ring and chunky donut energies as input, to confirm the previous finding. Outputs of the BDTs that use phi–ring energies as input are found to be more stable in energy scale under varying pilepup conditions, with resolution similar to the current jet detection algorithm. Hence, we propose to use the phi–ring algorithm to calibrate jet energies and improve jet detection in the CMS L1T in Run 3 (2022–2025).
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    Search for new physics using top quark pairs produced in association with a boosted Z or Higgs boson in effective field theory.
    (2022-03-29) Caraway, Bryan David, 1993-; Hatakeyama, Kenichi.
    A data sample containing top quark pairs (t ̄t) produced in association with a boosted Z or Higgs boson is used to search for signs of new physics within the framework of effective field theory. The data correspond to an integrated luminosity of 138 fb−1 of proton-proton collisions produced at a center-of-mass energy of 13 TeV at the LHC and collected by the CMS experiment. Selected collision events contain a single lepton and hadronic jets, including two identified with the decay of bottom quarks, plus an additional large-radius jet with high transverse momentum identified as a Z or Higgs boson decaying to a bottom quark pair. Machine learning techniques are employed to discriminate t ̄tZ and t ̄tH events from background processes, which are dominated by t ̄t + jets production. The signal strengths of boosted t ̄tZ and t ̄tH processes are measured, and upper limits are placed on the t ̄tZ and t ̄tH differential cross sections as a function of the Z or Higgs boson transverse momentum. In addition, effects of physics beyond the standard model are probed using a framework in which the standard model is considered to be the low-energy effective field theory of a higher- scale theory. Eight possible dimension-six operators are added to the standard model Lagrangian and their corresponding coefficients are constrained via a fit to the data.
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    Extremal conditions in early universe cosmology.
    (2021-10-22) Lee, Jeff S., 1964-; Cleaver, Gerald B.
    Some aspects of Special Relativity have remained largely unresolved and unexplored even after more than a century since its formulation; this is particularly true in the case of relativistic thermodynamics. Attempts to derive a relativistic temperature transformation have met with limited success, particularly when trying to transform a scalar temperature. Much more credible results have emerged when the inverse temperature (a van-Kampen Israel future-directed timelike 4-vector) was invoked. In this dissertation, the first self-consistent formulations of the relativistic Wien’s Displacement Law and the relativistic Stefan-Boltzmann Law are presented. Also examined is the use of occupation number and the inverse temperature 4-vector to justify temperature inflation of the Cosmic Microwave Background for any relativistic observer. The interaction of the Hawking spectrum of a 1 attometer (10-18 m) primordial black hole with an incoming composite particle reveals that when a primordial black hole reaches the Planck scale, its absorptivity and emissivity cause it to effectively become a white hole for the final instant of its existence.
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    Gravitational waves in Einstein-aether theory.
    (2021-05-24) Zhao, Xiang, 1991-; Wang, Anzhong.
    We study gravitational waves produced by N-body systems in Einstein-aether theory. In particular, we calculate the gravitational waveforms, polarizations, response functions of the detectors and the radiation power by using the post-Newtonian approximations to the lowest order. Applying the general formulas to three different triple systems with periodic orbits, we find that the scalar mode and the longitudinal mode (hb and hL) are all suppressed by a factor of c14 < O(10−5) with respect to the transverse-traceless modes (h+ and h×), while the vector modes (hX and hY ) are suppressed by a factor of c13 < O(10−15). We also find that gravitational waves depend sensitively on the configurations of the triple systems, their orientations with respect to the detectors, and the binding energies of the three compact bodies. The result for the first relativistic triple system, PSR J0337+1715, shows that the quadrupole emissions in different theories of gravity have almost the same amplitude, but the dipole emission can be as big as the quadrupole emission in Einstein-aether theory. This provides a very promising window to obtain severe constraints on Einstein-aether theory by multi-band gravitational wave astronomy.
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    Investigation of electrically-induced light emission in epsilon near zero (ENZ) heterostructure.
    (2021-05-04) Rokunuzzaman, Md Kazi, 1991-; Lee, Ho Wai (Howard); Zhang, Zhenrong.
    Electrically driven plasmonic nanostructures can generate and guide highly confined light. Light emission and the excitation of surface plasmon polaritons by inelastic electron tunneling have been shown in metal-insulator-metal heterostructures. Similar to metals, thin films of conducting oxides whose real part of permittivity (“epsilon”) goes to zero, or epsilon-near-zero (ENZ) materials, support plasmon polariton modes. In this work I study the inelastic electron tunneling and possible ENZ mode excitation and light emission from ENZ heterostructures. Indium doped tin oxide (ITO) and HfO2 has been used as the ENZ material and insulator, respectively. When the hot electrons injected by the means of electrical biasing across the junction of the heterostructure, they will emit the extra energy in terms of photons. The photons excite the ENZ mode in the ITO and can potentially be emitted and enhanced by the surface scattering or output coupling from nanoantenna.
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    Gravitational waves and cosmology.
    (2021-04-14) Fier, Jared Robert, 1992-; Wang, Anzhong.
    The recent discovery of GW140915 and the confirmation of the existence of gravitational waves (GWs) has garnered the attention of many physicists as they seek to understand their behavior as they travel across the universe. In this dissertation, one will find the study singularities which may arise in plane GWs, and cosmological perturbations may affect GWs as they propagate through an expanding, inhomogeneous universe. It is found that in the BJR coordinates, singularities arise at the focused point u = us, except in the two cases: (i) α = 1/2, ∀ χn, and (ii) α = 1, χi = 0, where χn are the coefficients in the expansion and α is a parameter. When observing GWs produced from remote astrophysical sources, one finds that there are three scales to consider, λ, Lc, and L which denote the typical wavelength of the GW, the scale of the cosmological perturbations, and the size of the observable universe, respectively. The Einstein equations were calculated for GWs on the cosmic scale, and the geometric optics approximation found the gravitational integrated Sachs-Wolfe effects created by both the cosmological scalar and tensor perturbations.
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    Plasmonics and epsilon-near-zero photonics in optical fiber and thin film platforms : from extreme light nanofocusing to enhanced spontaneous emission.
    (2021-04-12) Minn, Khant, 1985-; Zhang, Zhenrong.; Lee, Ho Wai (Howard)
    The major challenges in the study of light-matter interaction in the deep subwavelength regime are the inefficient conversion of nearfield to farfield energy, low signal-to-noise ratio, complicated device designs requiring complex multi-step fabrication processes. Metallic nanowires supporting surface plasmon polaritons (SPP) can localize optical fields at nanoscale tapered ends for near-field imaging. Similarly, epsilon-near-zero (ENZ) resonance, which is the behavior of light inside the medium with vanishing permittivity, in transparent conducting oxide thin films possesses strong light confinement properties. In addition, both SPP and ENZ resonances enhance the local density of optical states. Due to this property, dipole emitters near the plasmonic and ENZ medium experience greatly enhanced spontaneous emission. In this dissertation, I have applied these unique optical properties to overcome the current challenges in the field of nanoscale light-matter interaction such as nearfield scanning probe spectroscopy, nanoscale waveguiding and enhancement of light emission. I have reported four main results in this dissertation. Firstly, I have developed a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot at the metallic tip apex. Secondly, as a proof-of-concept demonstration of the probe described above, I have fabricated a plasmonic nanoantenna on the end facet of a photonic crystal fiber and subsequently demonstrated the coupling of light from the fiber waveguide mode to the nanoantenna plasmonic mode. Thirdly, I have designed a novel optical waveguide of a hollow step index fiber modified with a thin layer of indium tin oxide (ITO) that supports highly confined waveguide mode at the ENZ wavelength of ITO. Lastly, I have observed the room temperature photoluminescence (PL) enhancement of molybdenum disulfide monolayers on epitaxial titanium nitride (TiN) thin films at excitation wavelengths covering the ENZ regime where TiN films transition from dielectric to plasmonic. The first two results provide an important step toward widespread application of optical fibers incorporated with plasmonic tips in nearfield spectroscopic techniques such as tip-enhanced Raman and fluorescence microscopy, single photon excitation and quantum sensors, nanoscale optical lithography, and lab-on-fiber devices. The third result provides new understanding of coupling between ENZ and fiber waveguide modes which has potential applications in nonlinear and magneto-optics, in-fiber light manipulation, and biosensing. The fourth result enriches the fundamental understanding of how emission properties are modulated by ENZ substrates that could be important for the development of advanced nanoscale lasers and light sources, bio-sensors, and nano-optoelectronic devices.
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    Advanced light manipulation and waveguiding in plasmonic nanostructured optical fibers.
    (2021-04-29) Ghimire, Indra Mani, 1985-; Lee, Ho Wai (Howard); Zhang, Zhenrong.
    Conventional optical fibers are well-known for their efficient light guiding mechanism. However, the dielectric properties of core and cladding materials (e.g.- doped silica and silica glasses) limit the functionality of the optical fibers. Therefore, the optical properties of the fibers, such as phase, polarization state, amplitude, mode profile are fixed and cannot be altered once after the fiber drawing fabrication. The advent of new fabrication technologies for optical nanostructures such as plasmonic and metasurfaces allows us to overcome these limitations by tailoring those optical properties for advanced light manipulation and the development of novel optical fiber devices. In this dissertation, I have reported four main projects on integrating plasmonic and metasurface nanostructures into an optical fiber for novel optical functions. In the first project, I demonstrated in-fiber polarization-dependent color filters by nanopatterning asymmetric metallic metasurface array on the end-facet of polarization-dependent photonic-crystal fibers. The asymmetric cross-typed nanoslit metasurface arrays are fabricated on the core of the optical fiber using a focused ion beam milling technique. Highly polarization- and wavelength-dependent transmission with transmission efficiency ~ 70 % in the telecommunication wavelength observed by launching light into two orthogonal linear polarization states of the fiber. In the second project, I have extended the use of the focused ion beam milling technique to fabricate Berry phase metasurfaces on conventional single-mode optical fiber. A focusing effect has been observed in these metasurface-optical fibers, leading to the development of in-fiber metalens. The third project involves the integration of plasmonic nano-circuits on the facet of polarization-maintaining photonic crystal fiber (PM-PCF) and panda-shaped PM-optical fibers. A Yagi-Uda antenna-coupled low loss plasmonic slot waveguide is directly integrated on the optical fiber by direct milling technique. We demonstrated efficient coupling of light from the fiber core to the plasmonic slot waveguide. The light is then propagated in the plasmonic slot waveguide via the propagation of surface plasmon polaritons and emitted to the far-field in the output antenna located in the cladding. We further extend the design of the complex circuit by integrating multi-channels plasmonic waveguide with different waveguide lengths, polarization splitters, and optical directional coupler (ODC) onto the optical fiber. This project first proof-of-concept demonstration on developing an ultra-compact plasmonic network on the tip of optical fiber for advanced optical communications applications. The final project consists of the study of optical modulation by incorporating vanadium dioxide (VO2) nanocrystals into the air holes of anti-resonant hollow-core photonic crystal fibers (ARHCF). Efficient optical modulation is observed by inducing the insulator-to-metal phase transition of VO2 at different temperatures.
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    Testing theories of gravity by gravitational wave observations.
    (2021-04-01) Zhang, Chao, 1993-; Wang, Anzhong.
    In this dissertation, Einstein-æther theory is studied. As a candidate of modified theories to general relativity, Einstein-æther theory shows some different features compared to Einstein’s theory. The studies of these features can serve for the test and development of gravitational theories. In this dissertation, the study of Einstein-æther theory is closely related to the gravitational wave observations. In fact, those observations can potentially provide a variety of ways to test general relativity and put severe constraints on Einstein-æther theory. This thesis will mainly focus on the gravitational waves emitted by compact celestial bodies in the universe, and calculate the gravitational waveforms, innermost stable circular orbits, universal horizons, quasi-normal modes, etc. in the framework of Einstein-æther theory. With the continuous improvement of the accuracy of detectors around the world, as well as the accumulation of gravitational wave events, these investigations will become more and more important for understanding the nature of gravity and the dynamics of the universe.
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    Precision theory for LHC/FCC : new results for the five point function and interface between KKMC-hh and MG5_aMC@NLO.
    (2021-01-11) Liu, Yang, 1984-; Ward, B. F. L.
    The development of large colliders provides us with the opportunity to discover the fundamental particles in nature and explore the interactions among them. The Standard Model (SM) of particle physics reflects our best knowledge of elementary particles and their interactions at present, which is formulated by a gauge quantum field theory with gauge symmetry SU(3)C ⊗ SU(2)L ⊗ U(1)Y. With the discovery of the Higgs boson, the era of the sub-1% precision on processes such as Z and W production is approaching us. In order to achieve the 1% theoretical precision tag, we have to take radiative corrections into account and develop more precise Monte Carlo generators. In this dissertation, we first developed the computer realization of the magic spinor product method in loop integrals proposed by B. F. L. Ward to evaluate the general five-point function numerically. The result from magic spinor product method agrees with that from LoopTools overall. Additionally, we also developed an approach to achieve the next-to-the-leading order QCD and the electroweak (EW) exact O(αs ⊗ α2L) corrections, interfacing MG5_aMC@NLO with KKMC-hh by merging their LHE files. By comparing the results of the Drell-Yan process obtained by KKMC-hh, MG5_aMC@NLO and KKMC-hh interfaced with MG5_aMC@NLO , at √s = 13 TeV with the ATLAS cuts on the Z/γ∗ production and decay to lepton pairs, respectively, we find that the results derived from KKMC-hh interfaced with MG5_aMC@NLO would generate enhancements from those derived from MG5_aMC@NLO, which is due to the EW corrections provided by KKMC-hh.
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    Investigations of the plasma conditions in the sheath using dust grain as probes.
    (2020-11-03) Ashrafi, Khandaker Sharmin, 1986-; Matthews, Lorin Swint.
    Dust particles can be levitated against gravity in the sheath of the plasma, and different types of ordered structures can be formed by adjusting the pressure and power of the system. To explain the formation and stability of the ordered structures, it is important to know the plasma parameters such as temperature and number densities of the plasma species, the charges on dust particles, electric fields that accelerate the particles, the ion wake field formed downstream of the dust grains, as well as the ion flow velocities. The main focus of this dissertation is to determine these quantities that are difficult to measure experimentally. In a dusty plasma experiment, the dust grains themselves can be used as probes to measure plasma parameters as the perturbations produce by the grains are minimal. In this study, a molecular dynamics simulation of ion and dust dynamics is used to probe the plasma characteristics. The simulation allows calculation of the charges collected by the dust particles, including the effects of ion-neutral collisions. The model is used to investigate the variations in the electron and ion density within the sheath region. The results obtained from this study agree with the previous numerical and experimental studies, and also allow the ion flow velocities to be determined. The potential structure around the dust grains that is responsible for the attraction between the same polarity charged dust grains is also resolved using this model.
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    Search for supersymmetric top quarks in the CMS Run 2 data set.
    (2020-10-21) Smith, Caleb James, 1993-; Dittmann, Jay R.
    Elementary particle physics is described very accurately by the Standard Model. With the discovery of the Higgs boson at CERN by ATLAS and CMS in 2012, the full set of fundamental particles in the Standard Model has been confirmed to exist by experimentation. The LHC and the CMS detector continue to probe physics at higher energies to determine if additional fundamental particles exist that are not present in the Standard Model. An analysis of the CMS Run 2 data set collected during the years 2016–2018 at center-of-mass energy 13 TeV corresponding to an integrated luminosity of 137.0 fb−1 is presented. This analysis searches for supersymmetric top quarks in the all-hadronic final state. The search targets multiple simplified SUSY models. Custom algorithms are used to identify top quarks and W bosons. The leading Standard Model background processes are t¯t, W(→ lν)+jets, Z(→ ν¯ν)+jets, QCD, and t¯tZ. A complete description of the Z(→ ν¯ν)+jets data-driven background prediction is given. The results are interpreted for several simplified SUSY models, and limits are placed on the masses of the supersymmetric top quark (up to 1.3 TeV), the gluino (up to 2.3 TeV), and the lightest supersymmetric particle (up to 1.4 TeV).
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    Multi-pulse nonlinear optical spectroscopy and light-matter interactions in layered materials.
    (2020-10-28) Ko, Brian Alexander, 1992-; Scully, Marlan O. (Marlan Orvil), 1939-
    The interaction between light and matter is one that underscores many applications from solar energy conversion to optical sensing of biological materials. The use of multiple pulses to study these interactions are especially useful in understanding the nonlinear optical properties of materials. Transition metal dichalcogenides such as molybdenum disulfide (MoS2) are attractive materials due to the existence of a direct excitonic resonance that can be used to enhance nonlinear optical phenomena, such as Raman spectroscopy. Here, we have investigated four-wave mixing (FWM) processes in bulk MoS2 using a multiplex coherent anti-Stokes Raman spectroscopy setup. The observed FWM signal has a resonance at approximately 680 nm, corresponding to the energy of the A excitonic transition of MoS2. This resonance can be attributed to the increased third-order nonlinear susceptibility near the excitonic transition. This phenomenon shows the potential of MoS2 as a substrate for enhancing FWM processes. Understanding how particles and light interact in a liquid environment is vital for optical and biological applications. The interaction between two femtosecond pulses and MoS2 nanoparticles suspended in liquid is studied. The laser pulses induce bubble formation on the surface of a nanoparticle and a nanoparticle aggregate then forms on the surface of the trapped bubble. Two-dimensional organometallic lead halide perovskites are generating great interest due to their optoelectronic characteristics, such as a direct band gap in the visible regime. However, the presence of defect states within the crystal structure can affect these properties, resulting in changes to their emission and the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the hybrid perovskite (BA)2(MA)2Pb3Br10. When two pulses are incident on a perovskite flake, FWM occurs, with peaks corresponding to the defect energy levels present within the crystal. The longer lifetime of the defect state, in comparison to that of virtual transitions, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energies. This technique has the potential to detect defect energy levels in bulk crystals using FWM.
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    Deflation methods in lattice QCD.
    (2020-07-20) Whyte, Travis, 1992-; Wilcox, Walter, 1954-
    The inversion of the Dirac operator is a necessary feature of calculating physical observables within lattice QCD. The calculation of fermionic forces within hybrid Monte Carlo and the formation of quark propagators are two examples where such an inversion is needed. The many discretizations of the Dirac operator pose an algorithmic and computational challenge due to their size and their eigenspectra. As the quark mass approaches its physical value, the low lying eigenspectra of the Dirac operator approaches zero. From this arises the phenomena of critical slowing down, where the number of iterations to obtain an approximate solution for an iterative solver increases as a power law. Deflation and multigrid are two techniques that combat the effects of critical slowing down. We present a deflated multigrid preconditioner of FGMRES for the Wilson-Dirac operator in the lattice Schwinger model. Our method of deflation within the preconditioner demonstrates a remarkable reduction in cost for the inversion of the Wilson-Dirac operator, and also displays very mild scaling with respect to lattice size. The calculation of physical quantities arising from disconnected quark loops is one of the largest challenges in lattice QCD. A direct approach is to calculate the propagator for all lattice sites to all lattice sites. For large lattices, this approach is intractable so stochastic methods are used. The physical signal must be extracted from the noise created by these methods, and thus noise subtraction techniques are mandatory. We present deflation based noise subtraction techniques for the scalar, local vector and non-local vector operators in the quenched approximation at zero quark mass and with the inclusion of dynamical sea quarks at larger than physical pion mass. In both cases, the deflation based methods show dramatic reduction in the variance of these noisy calculations.
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    Nanoscale chemical imaging with novel fiber tip-enhanced Raman spectroscopy and microscale study of surface reactions on monolayer MoS2.
    (2020-07-30) Birmingham, Blake, 1991-; Zhang, Zhenrong.
    Study of molecule-surface interaction dynamics requires nanoscale chemical and topological information. Full dynamic understanding cannot be provided by microscale optical spectroscopy and has been aided by nanoscale imaging techniques, such as scanning probe microscopy (SPM). Recently, Tip-Enhanced Raman spectroscopy (TERS) has demonstrated single molecule chemical imaging with <1 nm resolution. However, nanoscale experiments require restrictive sample conditions and TERS microscope configurations prohibit easy operation in real-world environments, such as liquid. In this work, I helped develop and demonstrate a novel fiber optic-based technique, fiber-based TERS (FTERS), which overcomes the limitations of traditional SPMs, microspectroscopies, and current TERS techniques. The FTERS probe utilizes plasmonic coupling between an optical fiber and metal coating to excite and collect nanoscale chemical scattering and emission. FTERS probes were fabricated and plasmonic nanofocusing of FTERS was demonstrated experimentally. Delivery of optical excitation to a diffraction limited volume was confirmed by polarization dependent optical imaging of FTERS probe emission. The back-collection plasmonic coupling of FTERS was demonstrated by Photoluminescence (PL) emission of quantum dots (QDs) placed on the tip. Fiber-in excitation fiber-out collection of PL from QDs on a Au surface was demonstrated in Scanning Tunneling Microscopy (STM) configuration. Single-nanometer dependence of the tip-sample distance confirms the plasmonic excitation/collection mechanism. Finally, FTERS probes were demonstrated to operate in liquid for Raman collection and STM surface imaging. Two studies of the industrial material monolayer (ML) MoS2 were conducted with scanning Raman microspectroscopy in environment-controlled cells. ML MoS2 has a direct bandgap that is dependent on its local surface structure and is modulated by a small number of adsorbed molecules. The first study imaged localized photoreactions with ambient atmospheric components O2, N2, and H2O at flake edges. Laser irradiation in ambient conditions causes increasing PL at edge sites of ML MoS2, but has little effect on the basal plane PL. A second study showed a dramatic PL modulation by reactant phase: gaseous pyridine and thiophene nearly completely quenched PL but liquid exposure reduced emission by only half. These studies demonstrate molecule interaction dynamics are strongly dependent on localized surface features and full understanding requires accessible nanoscale chemical imaging techniques.