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Item Probing Scalars, Vectors, Vector-Like Quarks, Supersymmetry, and Jet Quenching at the Large Hadron Collider(Vanderbilt University. Dept. of Physics and Astronomy, 2025-04-14) Umar Sohail QureshiA model based on a U(1)T 3 R extension of the Standard Model can address the mass hierarchy between generations of fermions, explain thermal dark matter abundance, and the muon g − 2, R(D), and R(D∗) anomalies. The model contains a light scalar boson φ and a heavy vector-like quark χu that can be probed at CERN’s large hadron collider (LHC). We perform a phenomenology study on the production of φ and χu particles from proton–proton (pp) collisions at the LHC at √s = 13.6 TeV, primarily through g−g and t−χu fusion. We work under a simplified model approach and directly take the χu and φ masses as free parameters. We perform a phenomenological analysis considering χu final states to b-quarks, muons, and neutrinos, and φ decays to μ+μ−. A machine learning algorithm is used to maximize the signal sensitivity, considering an integrated luminosity of 3000 fb−1. The proposed methodology can be a key mode for discovery over a large mass range, including low masses, traditionally considered difficult due to experimental constraints.Item First-Principles Calculations of Electronic and Vibrational Properties in Semiconductors(Vanderbilt University. Dept. of Physics and Astronomy, 2025-04-23) Demos Negash; Professor Sokrates Pantelides, Ph.D.Item Fragmentation in Coulomb Explosion of Hydrocarbons(Vanderbilt University. Dept. of Physics and Astronomy, 2024-12-02) Samuel S. Taylor; Professor Kalman VargaFragmentation dynamics in the Coulomb explosion of hydrocarbons—specifically methane, ethane, propane, and butane—are investigated using time-dependent density functional theory (TDDFT) simulations. The goal of this work is to elucidate the distribution of fragments generated under laser-driven Coulomb explosion conditions. A detailed analysis reveals the types of fragments formed, their respective charge states, and the optimal laser intensities required to achieve various fragmentation pathways. The results indicate distinct fragmentation patterns for each hydrocarbon, which correlate with differences in molecular structure and ionization potential. Additionally, the laser parameters that maximize fragmentation efficiency are identified, offering valuable insights for guiding experimental setups. This research advances our understanding of Coulomb explosion mechanisms and provides a foundation for further studies on controlled molecular fragmentation.Item The Origins of the Evolution of Thermal Conductivity of Perovskite Superlattices(Vanderbilt University. Dept. of Physics and Astronomy, 2024.12) Matthew Lu; Professor Sokrates Pantelides, Ph.D.The thermal conductivities of (SrTiO3)n/(CaTiO3)n and other such superlattices have been measured for several layer thicknesses, revealing a trend that dips to a minimum and then rises as the interface density approaches the unit-cell scale. We employ density-functional-theory (DFT)-based machine-learning molecular-dynamics simulations to investigate the thermal conductivities in (SrTiO3)n/(CaTiO3)n superlattices, examining how interface phonons influence the thermal and vibrational properties for n = 1, 2, 4, 6, and 8. The simulations reproduce the thermal conductivity trends with n values and reveal the absence of true interfacial phonons in SL1 and SL2. This study sheds light on the potential for tailoring thermal transport in nanoscale materials by understanding and manipulating the role of interface-specific phonons in superlattices.Item MC Study of the Impact of Jet Quenching on Energy Correlators in Heavy Ion Collisions (A JEWEL-based Analysis on ENCs for N = 2 and 3)(Vanderbilt University. Dept. of Physics and Astronomy, 2025-04) Junxing Sheng; Prof. Raghav Kunnawalkam ElayavalliIn the framework of QCD, energy correlators (ENCs) in jets offer a sensitive probe of the quark–gluon plasma (QGP) formed in relativistic heavy-ion collisions. We employ the JEWEL Monte Carlo generator to study QGP-induced modifications to ENCs measured on jets reconstructed with the anti-kt algorithm. We analyze both N = 2 and N = 3 correlators to characterize the medium effects on the reconstructed jet. For N = 2, our simulations reveal pronounced enhancements at both large and small angles, in agreement with CMS data. The small-angle enhancement is attributed to the flavor dependence of the initiating parton combined with jet pT selection biases induced by energy loss. For N = 3, we observe almost all the features of the N = 2 case and find that the E3C distribution with respect to RS is far more sensitive to medium enhancement compared to RL. It is noteworthy that the E3C(ϕ) distribution remains largely unaltered at LHC energies, suggesting that its ϕ profile—predominantly set by the first parton splitting—is robust against QGP effects; however, further simulations indicate that medium modifications become observable at RHIC energies. Additionally, we introduce a new (X, Y) coordinate system to map recoil-induced modifications, showing that the enhancement is concentrated along the isosceles region of the particle-triplet triangle, in contrast to the more equilateral pattern expected from the wake effect.Item The Data Acquisition and Calibration of Luminosity at the Compact Muon Solenoid Experiment(Vanderbilt University. Dept. of Physics and Astronomy, 2024) Tang, JessicaItem Deformed Explicitly Correlated Gaussians(Vanderbilt University. Dept. of Physics and Astronomy, 2022-12) Beutel, MatthewStrong coupling of cavity electromagnetic modes and molecules has emerged as an area of intense theoretical and experimental interest. Such systems are of particular interest due to their ability to modify the physical and chemical properties of materials. In this work, I use a stochastic variational method (SVM) to construct optimized light-matter coupled wave function. By using SVMs to select the best basis states, we are able to achieve highly accurate energies and wave functions. In this work, I will be solving for the Pauli-Fierz (PF) nonrelativistic QED Hamiltonian. In this work I will introduce a new basis type Deformed Explicitly Correlated Gaussians (DECGs). DECGs are a modified form of explicitly correlated Gaussians (ECGs) where the basis is chosen such that the dipole self-interaction term can be eliminated. These calculations will be compared to those performed with traditional ECGs, demonstrating their superiority in cases where a non-spherical potential exists, such as the dipole self-interaction term.Item Analytic Solutions of Two Electrons in Harmonic Confinement in an Optical Cavity(Vanderbilt University. Dept. of Physics and Astronomy, 2021-12) Huang, ChenhangThe possibility to control quantum systems with photons has stimulated recent interest in the study of quantum optical systems. While simple classical quantum systems admit well-known solutions, analysis of light-coupling quantum regimes remains lacking. In this work, we obtain analytic solutions for a light-coupling electron pair in harmonic confinement in a cavity by separating center-of-mass (CM) and relative motions. The CM part can be calculated in closed form or by exact diagonalization of the Hamiltonian, and the relative part is quasi-exactly solvable. We analyze the 2D results produced by the exact diagonalization method and reach conclusions on the effects of different system parameters. We also present 1D numerical simulations by Stochastic Variational Method (SVM) using Explicitly Correlated Gaussian (ECG) bases, which agree with our analysis in 2D. Our analytic solutions may provide a valuable calibration point for simulations in the quantum optical regime.Item Axionlike Dark Energy and Particle Decay in theFuture of the Accelerating Universe(Vanderbilt University. Dept. of Physics and Astronomy, 2021-04-30) Norton, CameronThe 1998 discovery that the universe was accelerating in its expansion has yet to be explained theoretically, meriting the continual theoretical and observational study of this phenomena. In this thesis, we undergo a phenomenological study of the cosmological implications of this “dark energy” in two different ways. In the first part of this thesis, we examine the cosmological evolution of ultralight axionlike (ULA) scalar fields with potentials of the form V (φ) = m2f 2 [1 − cos (φ/f)]2 , with particular emphasis on the deviation in their behavior from the corresponding small-φ powerlaw approximations to these potentials: V (φ) ∝ φ 2n . We show that in the slow-roll regime, when φ˙2/2 V (φ), the full ULA potentials yield a more interesting range of possibilities for quintessence than do the corresponding power law approximations. For rapidly oscillating scalar fields, we derive the equation of state parameter and oscillation frequency for the ULA potentials and show how they deviate from the corresponding power-law values. We derive an analytic expression for the equation of state parameter that better approximates the ULA value than does the pure power-law approximation. In the second part, we study particle decay in the future of the accelerating universe. We generalize the result that in a cosmological constant dominated universe, the decay of matter into relativistic particles can never cause radiation to once again dominate over matter. We study both models of dark energy comprised of quintessence and cosmologies ending in a “big rip” in this context.Item A web-based application of the Cellular Force Inference Toolkit (CellFIT)(Vanderbilt University. Dept. of Physics and Astronomy, 2019-04-18) Xu, XiaojiaGiven an image of an epithelial cell sheet, CellFIT can infer cellular forces by segmenting the image into individual cells, constructing equilibrium equations for the points where cells meet at triple junctions, and finding a least-squares solution for the tensions at cell-cell interfaces. Similarly, cellular pressures can be estimated by constructing Laplace equations that relate the edge tensions, curvatures and cellular pressure differences. Despite these capabilities, the accessibility of CellFIT to scientists of all backgrounds is not yet optimized. We will present an updated web-based application of CellFIT that allows users to access the software from a browser. The updated version includes improved error handling and the implementation of additional functionality for reading and processing image stacks. Application of the web-based CellFIT to time-resolved image stacks of wound healing in Drosophila epithelia demonstrates spatial and temporal variations in cellular forces as the wounds close.Item LieART 2.0--An Improved Way to Compute Branching Rules(Vanderbilt University. Dept. of Physics and Astronomy, 2019-04-18) Saskowski, Robert; Kephart, ThomasIn this thesis, we present LieART 2.0 which contains substantial extensions to the Mathematica application LieART (Lie Algebras and Representation Theory) for computations frequently encountered in Lie algebras and representation theory, such as tensor product decomposition and subalgebra branching of irreducible representations. LieART 2.0 can now handle all classical and exceptional Lie algebras up through rank 15. The basic procedure is unchanged–it computes root systems of Lie algebras, weight systems and several other properties of irreducible representations, but new features and procedures have been included to allow the extensions to be seamless. The new version of LieART continues to be user friendly. Some extended tables of branching rules of irreducible representations are included in the supplementary material for use without Mathematica software.Item Probing Heavy Spin-2 Bosons with γγ final states from Vector Boson Fusion Processes at the LHC(Vanderbilt University. Dept. of Physics and Astronomy, 2019) Guo, YuhanNew massive spin-2 particles are predicted in theoretical extensions to the Standard Model (SM) attempting to solve the hierarchy problem. Such theories postulate that gravity is diluted compared to the other fundamental forces because it can propagate in extra spatial dimensions. While such theoretical models are of high experimental interest because they predict massive spin-2 particles (Y2) potentially detectable by collider experiments, searches at the Large Hadron Collider (LHC) have thus far produced no significant evidence for their existence. This work considers a hypothetical physics scenario where low coupling strengths between the Y2 and quarks/gluons is the underlying reason behind the null Y2 search results at the LHC, which have mainly relied on Drell-Yan and gluon-gluon fusion production mechanisms. The focus of this paper is a feasibility study to search for Y2 particles using vector boson fusion (VBF) processes at the LHC. In the context of an effective field theory approach with varying couplings κV between Y2 and the weak bosons of the SM, we consider the Y2 → γγ decay mode to show that the requirement of a diphoton pair combined with two high pT forward jets with large dijet mass and with large separation in pseudorapidity can significantly √ reduce the SM backgrounds. Assuming proton-proton collisions at present the total VBF production cross sections, Y2 decay widths, and Y2 → γγ branching ratios as a function of m(Y2), considering universal and non-universal couplings to the SM particles. The unitarity-violating phase space is described. The proposed VBF Y2 → γγ search is expected to achieve a discovery reach with signal significance greater than 5σ for Y2 masses up to 4.4 TeV and κV couplings down to 0.5.Item Nanoscale dolmen structure exhibiting a tunable Fano resonance(Vanderbilt University. Dept. of Physics and Astronomy, 2016-05-02) Beier, Nicholas; Haglund, Richard F., Jr., 1942-A Fano resonance is an asymmetric, resonant scattering phenomenon which occurs in a multitude of fields, such as atomic physics, nuclear physics, nonlinear optics, and nanophotonics. The Fano resonance is a many-particle excitation arising from a single-particle excitation, and occurs due to the interference of a narrow discrete resonance overlapping with a spectrally broad resonance. Multi-element nanoparticles are explored as a means to realize this resonance type, which has a characteristic, steep dispersion useful in sensors, among other devices. A gold dolmen nanostructure consisting of a bright, radiative, dipolar mode coupled with a dark, quadrupolar mode is investigated, which produces the plasmonic analogue of electromagnetically induced transparency through the interference of the bright and dark modes. Additionally, the plasmonic resonance of a metallic nanostructure is extremely sensitive to its local dielectric environment. Vanadium dioxide exhibits a large change in its dielectric function during its metal to insulator phase transition. The combination of the gold dolmen nanostructure with vanadium dioxide produces a tunable Fano resonance. The dimensions of the structure are optimized such that the resonance is near symmetric in shape and located in the near infrared to allow for spectroscopic measurement. The author is unable to obtain experimental results of this hybrid structure, but previous work, combined with simulation data, suggest the proposed structure will exhibit the expected result.Item Emittance and entropy of electron beams(Vanderbilt University. Department of Physics and Astronomy, 2014-04) Erickson, Collin; Brau, Charles A., 1938-Creating high-brightness electron beams, which have many practical applications, is done with cathodes in regions with large electric field by field emission. The brightness is high when the current is high and the volume of the beam in phase space is small. An estimate of the phase space volume is the rms emittance. The simulations described in this report show that the beam emitted by a gated diamond field emitter of the type fabricated at Vanderbilt has an exquisitely small emittance, on the order of a few nm. This is probably too small to measure. The rms emittance is generally thought to be constant for a beam or increasing in the presence of aberrations in the beam optics, but simulations show that this is not true. The rms emittance rises and falls according to the geometry that surrounds the beam. The entropy of an electron beam can be calculated, and should be constant if done in six dimensional phase space, and approximately constant in four dimensions. Calculations show that the entropy is not constant in two nor four dimensions. Rather, the entropy changes over time with a shape similar to how the emittance changes, which includes a local maxima at the entrance to an aperture and minima on either side. It is not clear if the fault is with the calculations or theory. To our knowledge, this is the first time that the entropy of a particle beam has been computed and used in a quantitative fashion. Future research should explore the applications and limitations of this concept.Item The quadratic approximation for quintessence with arbitrary initial conditions(Vanderbilt University. Department of Physics and Astronomy, 2014-04) Swaney, Jeffrey; Scherrer, Robert J. (Robert Joseph), 1959-We examine models of quintessence in which a minimally-coupled scalar field phi evolves near a local extremum of its potential V ( phi) at phi_*. Assuming that (1/V )(dV/dphi) is small and w ~ -1, we Taylor expand the potential about phi_* and derive a general expression for w(a). The dynamics of this field are determined by the initial and final equation of state parameters w_i and w_0, the quantity V''( phi_*)/V(phi_*), and the direction of \dot\phi_i in relation to \dot\phi _0. This approximation is then tested for six values of V''( phi_*)/V(phi_*) and shown to lie within 2% of the exact solution for five of these cases. However, the model becomes less precise near certain values of V''( phi_*)/V(phi_*) where \dot\phi becomes very large.Item Simulation and optimization of pulsed Chemical Exchange Saturation Transfer for clinical application at 3T(Vanderbilt University. Deptartment of Physics and Astronomy, 2013-04-22) Dewey, Blake; Smith, Seth A.Chemical Exchange Saturation Transfer (CEST) is often overlooked as a method for the investigation of metabolites in vivo due to the time required to obtain a full spectrum. We investigated the feasibility and optimization of a pulsed CEST technique that interleaves an echo planer imaging (EPI) readout with saturation in order to reduce time. In addition, we incorporated a multi-shot EPI sequence that reduces distortions. To achieve this, computer simulations based on the Bloch equations were used to optimize scan parameters while keeping scan time in the clinical timeframe. To analyze the data, a number of Lorentizian fitting algorithms were investigated to evaluate their ability to isolate CEST contrast. By using a 30 ms pulse at 2 μT, we were able to achieve CEST contrast on the order of 2% and could provide APT maps based on an adapted Lorentzian fitting method. In the process of this fitting, it was also discovered that MTR contrast could also be recovered from the CEST data, allowing for MT and CEST to be acquired at the same time.Item An anomalous measurement of delta m31 squared from neutrino oscillations at the Daya Bay Reactor Neutrino Experiment(Vanderbilt University. Department of Physics and Astronomy, 2013-04-22) Burroughs, Hunter; Ernst, DavidIn 2012, the collaboration overseeing the Daya Bay Reactor Neutrino Experiment announced results which determined the magnitude of the mixing angle \theta_{13} with unprecedented precision. However, no attempt was made in the collaboration’s publications to predict the value of the most relevant mass-squared difference to the observed oscillation, \delta m^2_{31}. This paper presents the results of an analysis which suggests that the Daya Bay data prefers a value of \delta m^2_{31} which is far greater than its presently recognized value. Specifically, it is found that Daya Bay predicts \delta m^2_{31} = 3.53_(-1.07)^(+.74) × 10^(-3) eV^2, where the cited uncertainties correspond to the 99% confidence bounds. This measurement excludes the most precise current measurement of \delta m^2_{31}, the MINOS result, at a 99% confidence level and is in turn excluded by the MINOS data at a 10 \sigma level. The possibility that sterile neutrino effects are the cause of this anomalous result is considered and used to suggest further work.Item Simulations of Nano-Structures in Time Dependent External Fields(Vanderbilt University. Department of Physics and Astronomy, 2013-04-22) Atkinson, Mackenzie; Varga, Kálmán, 1963-Time Dependent Density Functional Theory is used to probe the structure of matter. Coulomb explosion of small hydrocarbons driven by strong laser pulses and electron holography of molecules are studied in a theoretical framework. The spectra of the ejected protons obtained computationally is in good agreement with experimental data of Coulomb explosion. TDDFT allowed us to obtain time-dependent data, giving us a deeper understanding of the process. Our computational approach to electron holography provides 3-d reconstructions of simple molecules. Further investigation is needed in order to reconstruct larger molecules.Item Dark radiation from particle decays during big bang nucleosynthesis(Vanderbilt University. Dept. of Physics and Astronomy, 2012-04) Menestrina, Justin; Scherrer, Robert J. (Robert Joseph), 1959-Cosmic microwave background (CMB) observations suggest the possibility of an extra dark radiation component, while the current evidence from big bang nucleosynthesis (BBN) is more ambiguous. Dark radiation from a decaying particle can affect these two processes differently. Early decays add an additional radiation component to both the CMB and BBN, while late decays can alter the radiation content seen in the CMB while having a negligible effect on BBN. Here we quantify this difference and explore the intermediate regime by examining particles decaying during BBN, i.e., particle lifetimes τ_X satisfying 0.1 sec < τ_X < 1000 sec. We calculate the change in the effective number of neutrino species, Neff, as measured by the CMB, ΔN_CMB, and the change in the effective number of neutrino species as measured by BBN, ΔN_BBN, as a function of the decaying particle initial energy density and lifetime, where DNBBN is defined in terms of the number of additional two-component neutrinos needed to produce the same change in the primordial 4He abundance as our decaying particle. As expected, for short lifetimes (τ_X < 0.1 sec), the particles decay before the onset of BBN, and DNCMB = DNBBN, while for long lifetimes (τ_X >1000 sec), ΔN_BBN is dominated by the energy density of the nonrelativistic particles before they decay, so that ΔN_BBN remains nonzero and becomes independent of the particle lifetime. By varying both the particle energy density and lifetime, one can obtain any desired combination of N_BBN and ΔN_CMB, subject to the constraint that DNCMB N_BBN. We present limits on the decaying particle parameters derived from observational constraints on ΔN_CMB, and N_BBN.Item Real-time real-space density functional theory calculations of electron scattering in materials(Vanderbilt University. Dept. of Physics and Astronomy, 2012-05-04) Wyatt, Benjamin; Varga, Kálmán, 1963-Density functional theory is utilized in real-time, real-space simulations of LEED measurements and attosecond electron scattering. For LEED measurements, we find that our simulation results agree well with experimental data and other theoretical approaches. For attosecond electron scattering, we find that the wavefunction of the scattered electron is not signficantly changed by the scattering process, and the measured electron density seems to be related to the initial form of the wave-packet. However, further investigation is needed to confirm these results for different choices of initial form.