# Browsing by Subject "Particle physics"

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Item Open Access A High Precision Measurement of the Proton Charge Radius at JLab(2020) Xiong, WeizhiThe elastic electron-proton ($e-p$) scattering and the spectroscopy of hydrogen atoms are the two traditional methods to determine the proton charge radius ($r_{p}$). In 2010, a new method using the muonic hydrogen ($\mu$H)\footnote{A muonic hydrogen has its orbiting electron replaced by a muon.} spectroscopy reported a $r_{p}$ result that was nearly ten times more precise but significantly smaller than the values from the compilation of all previous $r_{p}$ measurements, creating the ``proton charge radius puzzle".

In order to investigate the puzzle,

the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}) was first proposed in 2011 and performed in 2016 in Hall B at the Thomas Jefferson National Accelerator Facility, with both 1.1 and 2.2 GeV electron beams. The experiment measured the $e-p$ elastic scattering cross sections in an unprecedented low values of momentum transfer squared region ($Q^2 = 2.1\times10^{-4} - 0.06~\rm{(GeV/c)}^2$), with a sub-percent precision.

The PRad experiment utilized a calorimetric method that was magnetic-spectrometer-free. Its detector setup included a large acceptance and high resolution calorimeter (HyCal), and two large-area, high-spatial-resolution Gas Electron Multiplier (GEM) detectors. To have a better control over the systematic uncertainties, the absolute $e-p$ elastic scattering cross section was normalized to that of the well-known M$\o$ller scattering process, which was measured simultaneously during the experiment. For each beam energy, all data with different $Q^{2}$ were collected simultaneously with the same detector setup, therefore sharing a common normalization parameter. The windowless H$_2$ gas-flow target utilized in the experiment largely removed a typical background source, the target cell windows. The proton charge radius was determined as $r_{p} = 0.831 \pm 0.007_{\rm{stat.}} \pm 0.012_{\rm{syst.}}$~fm, which is smaller than the average $r_{p}$ from previous $e-p$ elastic scattering experiments, but in agreement with the $\mu$H spectroscopic results within the experimental uncertainties.

Item Open Access A Measurement of the Eta Meson Radiative Decay Width via the Primakoff Effect(2024) Smith, DrewThe $\eta$ meson is an interesting tool to study fundamental symmetries in Quantum Chromodynamics (QCD). In particular, its radiative decay width, $\Gamma\left(\eta\rightarrow\gamma\gamma\right)$, is an important quantity that can be predicted in the framework of Chiral Perturbation Theory. A precision measurement of this quantity would provide critical inputs to understanding the mixing of the $\eta$ and $\eta'$ mesons and extracting constants with wide-ranging applications in low-energy QCD. This decay width has been measured in the past using two different experimental techniques. The more popular technique utilized $e^{+}e^{-}$ collisions to produce $\eta$ mesons through electromagnetic interactions. Today, the Particle Data Group (PDG) averages the results of five such experiments to obtain their currently-accepted value of the decay width as: 0.515$\pm$0.018~keV. However the first measurement of this quantity was obtained from a fixed-target experiment that measured the cross section for photoproduction of $\eta$ mesons on a nuclear target via the Primakoff effect. Their result of 0.324$\pm$0.046~keV shows strong tension with the average of the collider measurements, motivating a new, high precision measurement using the Primakoff method.

For this purpose, the PrimEx-\textit{eta} experiment was conducted in Hall D of the Thomas Jefferson National Accelerator Facility (Jefferson Lab or JLab). The data is currently being analyzed to measure the differential cross section for the photoproduction of $\eta$ mesons on a liquid, $^{4}$He target. Preliminary results obtained from the analysis of the first phase of the PrimEx-\textit{eta} experiment show reasonable agreement with the currently-accepted PDG value of the radiative decay width. However, as will be discussed, there are many challenges to this precision measurement which must be studied before any results can be finalized and compared with previous measurements.

In parallel to the $\eta$ decay width measurement, the PrimEx-\textit{eta} experiment measured the total cross section for the fundamental, Quantum Electrodynamics (QED) process of Compton scattering from the atomic electrons inside the target. The results obtained from this measurement are in strong agreement with the next-to-leading order QED calculations, and the total combined uncertainties are below 3\% for incident photon energies between 7-10~GeV. In addition to providing the first precision measurement of the total Compton scattering cross section within this beam energy range, this measurement verifies the capability of the PrimEx-\textit{eta} experimental setup to perform absolute cross section measurements at forward angles, and serves as a reference process for the calibration of systematic uncertainties.

Item Open Access A Measurement of the Production Cross Section of a Single Top Quark in Association with a $W$-boson at $\sqrt{s} = 13$ TeV with the ATLAS Detector at the Large Hadron Collider(2020) Davis, DouglasA measurement of the Standard Model cross section for the production of a single top quark in association with a $W$-boson from proton-proton collisions is presented. Collision data at a center-of-mass energy of 13 TeV totaling 139 inverse femtobarns collected by the ATLAS detector on the Large Hadron Collider is used to study the process. Binary classifiers trained using kinematic properties of observable particles are used to separate the signal process from a large top pair production background. A binned maximum likelihood fit is performed to extract the Standard Model cross section for the signal process, along with a secondary top pair cross section measurement and uncertainties related to object reconstruction with the detector and theoretical models. The measured cross section is the most precise measurement performed using the ATLAS detector and is in good agreement with the Standard Model prediction.

Item Open Access A Search for Supersymmetry in Multi-b Jet Events with the ATLAS Detector(2019) Epland, Matthew BergA search for supersymmetry in pair-produced gluinos decaying via top squarks to the lightest neutralino is presented. Events with multiple hadronic jets, of which at least three must be identified as originating from b-quarks, and large amounts of missing transverse energy in the final state, are selected for study. The dataset utilized encompasses proton-proton collisions with a center-of-mass energy of sqrt(s) = 13 TeV and integrated luminosity of 79.9 fb-1 collected by the ATLAS experiment at the LHC from 2015 to 2017. The search employs a parameterized boosted decision tree (BDT) to separate supersymmetric signal events from standard model backgrounds. New methods for optimal BDT parameter point selection and signal region creation, as well as new soft kinematic variables, are exploited to increase the search's expected exclusion limit beyond prior analyses of the same dataset by 100-200 GeV in the gluino and neutralino mass plane. No excess is observed in data above the predicted background, extending the previous exclusion limit at the 95% confidence level by 250 GeV to approximately 1.4 TeV in neutralino mass. The analytical and machine learning techniques developed here will benefit future analysis of additional Run 2 data from 2018.

Item Open Access Alternative Tests of Quarkonium Production Theory Using Jets(2017) Makris, YiannisIn this thesis I discuss an alternative approach for investigating quarkonium production in hadron colliders. I present a complete framework for developing observables for studies of charmonium states produced within a jet. My work is based on the use of effective field theories of quantum chromodynamics that allow for the approximate factorization of jet cross sections in perturbative calculable terms and universal non-perturbative functions that are extracted from data. Particularly in this thesis I explore the factorization approach of non-relativistic quantum chromodynamics and soft-collinear effective theory. The fragmenting jet functions play central role in factorization theorems for cross sections for identified hadrons within jets. This cross sections can depend on the hadron-jet energy ratio and possibly on other jet observables. I expand this concept to jet-shape observables known as angularities and introduce the transverse momentum dependent fragmenting jet functions. Applications of these advanced methods to J/ψ production from gluon fragmentation in electron-positron annihilation are presented and I develop the tools for expanding this work in hadron colliders. Additionally, I compare predictions for J/ψ production in jets, based on the framework of fragmenting jet functions, against recent experimental data from the LHCb collaboration.

Item Open Access An Improved W Boson Mass Measurement using the Collider Detector at Fermilab(2012) Zeng, YuThe mass of the $W$ boson is one of the most important parameters in the

Standard Model. A precise measurement of the $W$ boson mass, together

with a precise measurement of the top quark mass, can constrain the

mass of the undiscovered Higgs boson within the Standard Model

framework or give a hint for physics beyond the Standard Model.

This dissertation describes a measurement of the $W$ boson mass

through its decay into a muon and a neutrino using

$\approx$ 2.2 fb$^{-1}$ of $\sqrt{s} = 1.96$ TeV $p\bar{p}$ data taken

with the CDF II detector at Fermilab. We measure the $W$ boson mass

to be ($80.374 \pm 0.015_{\rm stat.} \pm 0.016_{\rm syst.}$)

GeV/c$^2$. This result, when combined with the $W$ mass

measurement in the electron channel, leads to the single most

precise $m_W$ value and greatly constrains the possible mass

range of the undiscovered Higgs boson.

Item Open Access An Inclusive Analysis of Top Quark Pair, W Boson Pair, and Drell-Yan Tau Lepton Pair Production in the Dilepton Final State from Proton-Proton Collisions at Center-of-Mass Energy 7 TeV with the ATLAS Detector(2013) Finelli, KevinA simultaneous measurement of three Standard Model cross-sections using 4.7 inverse femtobarns of proton-proton collision data at a center-of-mass energy of 7 TeV is presented. Collision data were collected using the ATLAS detector at the Large Hadron Collider. The signal production cross-sections studied are for top quark pair production, charged weak boson pair production, and Drell-Yan production of tau lepton pairs with invariant mass greater than 40 GeV. A data sample is defined from events with isolated high-energy electron-muon pairs arranged in a phase space defined by missing transverse momentum and jet multiplicity. A binned maximum likelihood fit is employed to determine signal yields in this phase space. Signal event yields are in turn used to measure full cross-section values and cross-section values within a fiducial region of the detector, and unlike conventional measurements the signal measurements are performed simultaneously. This is the first such simultaneous measurement of these cross-sections using the ATLAS detector. Measured cross-sections are found in good agreement with the most precise published theoretical predictions.

Item Open Access Application of Effective Field Theory in Nuclear Physics(2019) Yao, XiaojunThe production of heavy quarkonium in heavy ion collisions has been used as an important probe of the quark-gluon plasma (QGP). Due to the plasma screening effect, the color attraction between the heavy quark antiquark pair inside a quarkonium is significantly suppressed at high temperature and thus no bound states can exist, i.e., they ``melt". In addition, a bound heavy quark antiquark pair can dissociate if enough energy is transferred to it in a dynamical process inside the plasma. So one would expect the production of quarkonium to be considerably suppressed in heavy ion collisions. However, experimental measurements have shown that a large amount of quarkonia survive the evolution inside the high temperature plasma. It is realized that the in-medium recombination of unbound heavy quark pairs into quarkonium is as crucial as the melting and dissociation. Thus, phenomenological studies have to account for static screening, dissociation and recombination in a consistent way. But recombination is less understood theoretically than the melting and dissociation. Many studies using semi-classical transport equations model the recombination effect from the consideration of detailed balance at thermal equilibrium. However, these studies cannot explain how the system of quarkonium reaches equilibrium and estimate the time scale of the thermalization. Recently, another approach based on the open quantum system formalism started being used. In this framework, one solves a quantum evolution for in-medium quarkonium. Dissociation and recombination are accounted for consistently. However, the connection between the semi-classical transport equation and the quantum evolution is not clear.

In this dissertation, I will try to address the issues raised above. As a warm-up project, I will first study a similar problem: $\alpha$-$\alpha$ scattering at the $^8$Be resonance inside an $e^-e^+\gamma$ plasma. By applying pionless effective field theory and thermal field theory, I will show how the plasma screening effect modifies the $^8$Be resonance energy and width. I will discuss the need to use the open quantum system formalism when studying the time evolution of a system embedded inside a plasma. Then I will use effective field theory of QCD and the open quantum system formalism to derive a Lindblad equation for bound and unbound heavy quark antiquark pairs inside a weakly-coupled QGP. Under the Markovian approximation and the assumption of weak coupling between the system and the environment, the Lindblad equation will be shown to turn to a Boltzmann transport equation if a Wigner transform is applied to the open system density matrix. These assumptions will be justified by using the separation of scales, which is assumed in the construction of effective field theory. I will show the scattering amplitudes that contribute to the collision terms in the Boltzmann equation are gauge invariant and infrared safe. By coupling the transport equation of quarkonium with those of open heavy flavors and solving them using Monte Carlo simulations, I will demonstrate how the system of bound and unbound heavy quark antiquark pairs reaches detailed balance and equilibrium inside the QGP. Phenomenologically, my calculations can describe the experimental data on bottomonium production. Finally I will extend the framework to study the in-medium evolution of heavy diquarks and estimate the production rate of the doubly charmed baryon $\Xi_{cc}^{++}$ in heavy ion collisions.

Item Open Access Applications of Gauge/Gravity Duality in Heavy Ion Collisions(2014) Yang, Di-LunIn order to analyze the strongly interacting quark gluon plasma in heavy ion collisions, we study different probes by applying the gauge/gravity duality to facilitate our qualitative understandings on such a non-perturbative system. In this dissertation, we utilize a variety of holographic models to tackle many problems in heavy ion physics including the rapid thermalization, jet quenching, photon production, and anomalous effects led by external electromagnetic fields. We employ the AdS-Vaidya metric to study the gravitational collapse corresponding to the thermalization of a strongly coupled gauge theory, where we compute the approximated thermalization time and stopping distances of light probes in such a non-equilibrium medium. We further generalize the study to the case with a nonzero chemical potential. We find that the non-equilibrium effect is more influential for the probes with smaller energy. In the presence of a finite chemical potential, the decrease of thermalization times for both the medium and the light probes is observed.

On the other hand, we also investigate the anisotropic effect on the stopping distance related to jet quenching of light probes and thermal-photon production. The stopping distance and photoemission rate in the anisotropic background depend on the moving directions of probes.

The influence from a magnetic field on photoemission is as well investigated in the framework of the D3/D7 system, where the contributions from massive quarks are involved. The enhancement of photon production for photons generated perpendicular to the magnetic field is found. Given that the mass of massive quarks is close to the critical embedding, the meson-photon transition will yield a resonance in the spectrum. We thus evaluate flow coefficient $v_2$ of thermal photons in a 2+1 flavor strongly interacting plasma. The magnetic-field induced photoemission results in large $v_2$ and the resonance from massive quarks gives rise to a mild peak in the spectrum. Moreover, we utilize Sakai-Sugimoto model to analyze the chiral electric separation effect, where an axial current is generated parallel to the applied electric field in the presence of both the vector and axial chemical potentials. Interestingly, the axial conductivity is approximately proportional to the product of the vector chemical potential and the axial chemical potential for arbitrary magnitudes of the chemical potentials.

Item Open Access Coherent Elastic Neutrino-Nucleus Scattering in Large-Scale Scintillators(2024) Major, AdryannaThe growth in the neutrino sector over the last several decades has offered interesting answers to questions about the neutrino's fundamental nature and the essential role it plays in astrophysical processes. The field's success allows a trend towards bolder and more precise observations of the oft-eluding particle, and concrete cross section measurements are possible like never before. Coherent elastic neutrino-nucleus scattering (CEvNS) is a neutral-current process in which a neutrino scatters off a nucleus as a cohesive unit, depositing a tiny recoil energy (few-to-tens-of-keV). Observed for the first time by the COHERENT experiment in 2017, the clean theoretical cross section prediction allows CEvNS to function as not only a probe for non-standard interactions and nuclear form factors, but also as a predictable flavor-blind signature from all manner of sources. The process is important in core-collapse supernovae and also presents an opportunity for detection of a burst of core-collapse neutrinos in low-threshold detectors designed for solar neutrino and dark matter detection. Often partnered with neutrino beam facilities, a second trend in the field has been leveraging new technologies and techniques to scale up to the ton-scale and beyond.

The work presented here will cover the ability of ton-scale scintillators to measure CEvNS interactions with neutrinos from two sources. The first covers the prospects for flavor-blind supernova neutrino burst detection via CEvNS (E$\nu$=10s of MeV) in existing and future large scintillating detectors. This study will present an analytic method for obtaining the expected photon spectra and provide predictions on the CEvNS observation power during the exceedingly neutrino-luminous burst. The second undertaking details the deployment of COHERENT's new multi-ton NaI[Tl] subsystem, a scintillating detector designed to observe CEvNS from pulsed, stopped-pion neutrinos at the Spallation Neutron Source (also 10s of MeV). Analysis of the in-situ backgrounds of the first half-ton module is conducted to lay the foundation for a long-term CEvNS measurement on sodium.

Item Open Access Constraining Non-Standard Neutrino Interactions and Estimating Future Neutrino-Magnetic-Moment Sensitivity With COHERENT(2020) Sinev, GlebNeutrinos represent a rich field of physics that contains many theoretical problems that are yet to be solved and experimental results hinting at physics beyond the standard model of particle physics (BSM). An experiment studying neutrino physics and that is the source of the data used in the studies presented here is COHERENT. Its primary goals are to measure and characterize coherent elastic neutrino-nucleus scattering (CEvNS). Studying CEvNS, a standard-model process, provides a direct way to constrain BSM theories. The area of Neutrino Physics that is primarily studied in this work is non-standard neutrino interactions (NSI). I use the data taken by the CsI and CENNS-10 detectors of the COHERENT experiment to improve the constraint on the vector electron-electron

NSI couplings with the up and down quarks. In addition to combining the data of those detectors, I use the Feldman-Cousins technique to improve the NSI limit, obtaining a result that is stronger than prior constraints. Multiple future improvements are discussed.

Another topic investigated here is non-zero neutrino magnetic moments, that, if measured, would point to BSM physics. I estimate the sensitivity of the future COHERENT program to the muon neutrino magnetic moment by minimizing the likelihood function of observing nuclear recoils due to that neutrino magnetic moment in the COHERENT Ge detector. The obtained predicted sensitivity is not as strong as indirect limits, but is similar to existing direct constraints.

Item Open Access Evidence of Higgs Boson Production through Vector Boson Fusion(2015) Cerio, Benjamin CThe discovery of the Higgs boson in 2012 provided confirmation of the

proposed mechanism for preserving the electroweak $SU(2) \times U(1)$

gauge symmetry of the Standard Model of particle physics. It also

heralded in a new era of precision Higgs physics. This thesis presents a

measurement of the rate at which the Higgs boson is produced by vector

boson fusion in the $WW^{(\ast)}\rightarrow\,\ell\nu\ell\nu$ decay channel. With gauge boson couplings

in both the production and decay vertices, a VBF measurement in this

channel is a powerful probe of the $VVH$ vertex strength. Using

$4.5$~fb$^{-1}$ and $20.3$~fb$^{-1}$ of $pp$ collision data collected

at respective center-of-mass energies of 7 and $8 \tev$ in the ATLAS

detector, measurements of the statistical significance and the signal

strength are carried out in the Higgs mass range $100 \leq m_H \leq

200 \gev$. These measurements are enhanced with a boosted decision

tree that exploits the correlations between eight kinematic inputs in

order to separate signal and background processes. At the benchmark

Higgs mass of $125.36 \gev$, the significance of the data assuming the

background-only hypothesis to be true has been observed to be

$3.2\sigma$ ($2.7\sigma$ expected), constituting evidence of VBF Higgs boson

production. The measured signal strength (ratio of observed cross section times

branching ratio to that predicted by the SM) is

$1.27^{+0.53}_{-0.45}$. The inclusive cross section times

branching ratio is found to be $0.51^{+0.22}_{-0.17}$~pb at $\sqrts =

8 \tev$, consistent with the SM prediction of $0.34$~pb. No

significant deviations from the SM predictions for VBF Higgs boson

production are observed.

Item Open Access Exclusive Photodisintegration of 3He(2019) Friesen, Forrest Quinn ListerKinematically complete measurements of three-body photodisintegration of $^3$He were performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) with nearly monoenergetic 15 MeV photons. The experiment relied on two-nucleon coincidence measurements in which the nucleons are emitted on opposite sides of the incident $\gamma$-ray beam axis. The setup consisted of seven 10 cm long cylindrical gas targets pressurized near 4 atm with thin windows to allow low-energy charged particles to exit with acceptable energy loss. Charged particles were detected in silicon strip detectors with angular acceptance constrained by a collimator system. Neutrons were detected in arrays of liquid organic scintillator cells. Data for neutron-proton (np) coincidences were acquired in configurations which selectively include or exclude the np final state interaction. Measurements of proton-proton (pp) coincidences along the same kinematic locus containing the np final state interaction (FSI) were also taken in-situ. Products from the two-body reaction were used as a luminosity monitor. Theory predictions were propagated through a GEANT4 simulation of the experimental setup. There was good agreement between predictions and measurements in the vicinity of the collinear point in which a proton remains at rest as measured by np coincidences. The measured np FSI peak included additional low-energy neutrons not anticipated by the simulation, which are likely associated with intermediate neutron scattering. The np FSI peak was found to be underpredicted by about 20$\%$. The pp coincidence data were consistently about 39$\%$ above predictions.

Item Open Access Experiments of Search for Neutron Electric Dipole Moment and Spin-Dependent Short-Range Force(2012) Zheng, WangzhiIt is of great importance to identify new sources of discrete symmetry violations because it can explain the baryon number asymmetry of our universe and also test the validity of various models beyond the standard model. Neutron Electric Dipole Moment (nEDM) and short-range force are such candidates for the new sources of P&T violations. A new generation nEDM experiment was proposed in USA in 2002, aiming at improving the current nEDM upperlimit by two orders of magnitude. Polarized 3He is crucial in this experiment and Duke is responsible for the 3He injection, measurements of 3He nuclear magnetic resonance (NMR) signal and some physics properties related to polarized 3He.

A Monte-Carlo simulation is used to simulate the entire 3He injection process in order to study whether polarized 3He can be successfully delivered to the measurement cell. Our simulation result shows that it is achievable to maintain more than 95% polarization after 3He atoms travel through very complicated paths in the presence of non-uniform magnetic fiels.

We also built an apparatus to demonstrate that the 3He precession signal can be measured under the nEDM experimental conditions using the Superconducting Quantum Interference Device (SQUID). Based on the measurement result in our lab, we project that the signal-to-noise ratio in the nEDM experiment will be at least 10.

During this SQUID test, two interesting phenomena were discovered. One is the pressure dependence of the T

_{1}of the polarized 3He which has never been reported before. The other is the discrepancy between the theoretically predicted T_{2}and the experimentally measured T_{2}of the 3He precession signal. To investigate these two interesting phenomena, two dedicated experiments were built, and two papers have been published in Physical Review A.In addition to the nEDM experiment, polarized 3He is also used in the search for the exotic short-range force. The high pressure 3He cell used in this experiment has a very thin window (~250 μm) to maximize the effect from the force. We demonstrate that our new method could improve the current best experimental limit by two orders of magnitude. A rapid communication demonstrating the technique and the result was published in Physical Review D.

Item Open Access Exploring Quantum Field Theories with Qubit Lattice Models(2020) Singh, HershThe framework of quantum field theory (QFT) underlies our modern understanding of both particle physics and condensed matter physics. Despite its importance, precise quantitative calculations in strongly-coupled theories in QFTs have generally only been possible through non-perturbative lattice Monte Carlo (MC) methods. Traditionally, such lattice MC methods proceed by starting from a lattice regularization of the continuum QFT of interest, which has the same (possibly infinite dimensional) local Hilbert space at each lattice site as the continuum QFT. In this thesis, we explore an alternative regularization where the local Hilbert space is also replaced by a smaller finite dimensional Hilbert space. Motivated by the appeal of such models for near-term quantum computers, we dub this approach qubit regularization. Using this approach, in this thesis, we present three main results. First, we develop a qubit-regularization for the O(N) nonlinear sigma model (NLSM) in D $\geq$ 3 spacetime dimensions. We show using numerical lattice calculations that the O(N ) qubit model lies in the correct universality class for N = 2, 4, 6, 8, and reproduces the universal physics of the O(N) Wilson-Fisher (WF) fixed point in D = 3 spacetime dimensions by computing some well-known critical exponents. Next, we explore sectors of large global charges of the O(N) WF conformal field theory (CFT) using the O(N) qubit model. This allows us to test the predictions of a recently proposed large-charge effective field theory (EFT) and extract the two leading low-energy constants (LECs) in the EFT. Performing computations for N = 2, 4, 6, 8, we are also able to quantitatively test predictions of a recent large-N analysis in the large-charge sectors. Finally, we show that our qubit approach can also be used to study the few-body physics of non-relativistic particles. In particular, we consider a system of two species of mass-imbalanced fermions in $1 + 1$ dimensions. We compute the ground state energies for a range of mass-imbalances and interaction strengths, and uncover some problems with recent results obtained from the Complex Langevin (CL) method for the same system.

Item Open Access Fermion Bag Approach for Hamiltonian Lattice Field Theories(2018) Huffman, EmilieUnderstanding the critical behavior near quantum critical points for strongly correlated quantum many-body systems remains intractable for the vast majority of scenarios. Challenges involve determining if a quantum phase transition is first- or second-order, and finding the critical exponents for second-order phase transitions. Learning about where second-order phase transitions occur and determining their critical exponents is particularly interesting, because each new second-order phase transition defines a new quantum field theory.

Quantum Monte Carlo (QMC) methods are one class of techniques that, when applicable, offer reliable ways to extract the nonperturbative physics near strongly coupled quantum critical points. However, there are two formidable bottlenecks to the applicability of QMC: (1) the sign problem and (2) algorithmic update inefficiencies. In this thesis, I overcome both these difficulties for a class of problems by extending the fermion bag approach recently developed by Shailesh Chandrasekharan to the Hamiltonian formalism and by demonstrating progress using the example of a specific quantum system known as the $t$-$V$ model, which exhibits a transition from a semimetal to an insulator phase for a single flavor of four-component Dirac fermions.

I adapt the fermion bag approach, which was originally developed in the context of Lagrangian lattice field theories, to be applicable within the Hamiltonian formalism, and demonstrate its success in two ways: first, through solutions to new sign problems, and second, through the development of new efficient QMC algorithms. In addressing the first point, I present a solution to the sign problem for the $t$-$V$ model. While the $t$-$V$ model is the simplest Gross-Neveu model of the chiral Ising universality class, the specter of the sign problem previously prevented its simulation with QMC for 30 years, and my solution initiated the first QMC studies for this model. The solution is then extended to many other Hamiltonian models within a class that involves fermions interacting with quantum spins. Some of these models contain an interesting quantum phase transition between a massless/semimetal phase to a massive/insulator phase in the so called Gross-Neveu universality class. Thus, the new solutions to the sign problem allow for the use of the QMC method to study these universality classes.

The second point is addressed through the construction of a Hamiltonian fermion bag algorithm. The algorithm is then used to compute the critical exponents for the second-order phase transition in the $t$-$V$ model. By pushing the calculations to significantly larger lattice sizes than previous recent computations ($64^2$ sites versus $24^2$ sites), I am able to compute the critical exponents more reliably here compared to earlier work. I show that the inclusion of these larger lattices causes a significant shift in the values of the critical exponents that was not evident for the smaller lattices. This shift puts the critical exponent values in closer agreement with continuum $4-\epsilon$ expansion calculations. The largest lattice sizes of $64^2$ at a comparably low temperature are reachable due to efficiency gains from this Hamiltonian fermion bag algorithm. The two independent critical exponents I find, which completely characterize the phase transition, are $\eta=.51(3)$ and $\nu=.89(1)$, compared to previous work that had lower values for these exponents. The finite size scaling fit is excellent with a $\chi^2/DOF=.90$, showing strong evidence for a second-order critical phase transition, and hence a non-perturbative QFT can be defined at the critical point.

Item Open Access Initial Conditions of Bulk Matter in Ultrarelativistic Nuclear Collisions(2019) Moreland, John ScottDynamical models based on relativistic fluid dynamics provide a powerful tool to extract the properties of the strongly-coupled quark-gluon plasma (QGP) produced in the first ${\sim}10^{-23}$ seconds of an ultrarelativistic nuclear collision. The largest source of uncertainty in these model-to-data extractions is the choice of theoretical initial conditions used to model the distribution of energy or entropy at the hydrodynamic starting time.

Descriptions of the QGP initial conditions are generally improved through iterative cycles of testing and refinement. Individual models are compared to experimental data; the worst models are discarded and best models retained. Consequently, successful traits (assumptions) are passed on to subsequent generations of the theoretical landscape. This so-called bottom-up approach correspondingly describes a form of theoretical trial and error, where each trial proposes a first principles solution to the problem at hand.

A natural complement to this strategy is to employ a top-down or data driven approach which is able to reverse engineer properties of the initial conditions from the constraints imposed by the experimental data. In this dissertation, I motivate and develop a parametric model for initial energy and entropy deposition in ultrarelativistic nuclear collisions which is based on a family of functions known as the generalized means. The ansatz closely mimics the variability of first-principle calculations and hence serves as a reasonable parametric form for exploring QGP energy and entropy deposition assuming imperfect knowledge of the complex physical processes which lead to its creation.

With the parametric model in hand, I explore broad implications of the proposed ansatz using recently adapted Bayesian methods to simultaneously constrain properties of the initial conditions and QGP medium using experimental data from the Large Hadron Collider. These analyses show that the QGP initial conditions are highly constrained by available measurements and provide evidence of a unified hydrodynamic description of small and large nuclear collision systems.

Item Open Access Low Energy Neutrino-Nucleus Interactions at the Spallation Neutron Source(2021) Hedges, Samuel CarterThere are few existing measurements of low energy neutrino-nucleus interactions. The COHERENT collaboration is seeking to measure several of these processes using the intense pulsed neutrinos produced at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL).

The primary process of interest to COHERENT is coherent elastic neutrino-nucleus scattering (CEvNS), a process predicted in 1974 but only first measured by COHERENT in 2017. In a CEvNS interaction, a neutrino elastically scatters off a nucleus, causing its nucleons to recoil in phase, leading to a large increase in the scattering cross section. The large cross section provides several potential applications of CEvNS, however the signature of interaction, a keV-scale nuclear recoil, can be difficult to detect.

This thesis highlights experimental work to develop and measure neutrino-nucleus interactions on a variety of targets, including both CEvNS interactions and inelastic neutrino-nucleus interactions. This includes the development a ton-scale sodium-iodide scintillator array, a 185-kg prototype NaI detector, and analysis of neutrino-induced neutron detectors seeking to study unobserved neutrino-nucleus interactions on lead and iron. In addition, supporting measurements carried out at the Triangle Universities Nuclear Laboratory (TUNL) to measure quenching factors in NaI[Tl] are discussed.

Item Open Access Measurement of Atmospheric Flux-Weighted Charged-Current $\nu_{e} - {}^{16}\text{O}$ Cross Section with the Super-Kamiokande Experiment(2023) Bodur, BaranA first measurement of $\nu_{e} + {}^{16}\text{O} \to e^{-} + {}^{16}\text{F}^{*}$ cross section from 45 MeV to 125 MeV was performed using the atmospheric neutrinos incident to the Super-Kamiokande detector over 24 years. This corresponds to an exposure of 485 kTon$\cdot$years, and 125 (after cuts) expected $\nu_{e} - {}^{16}\text{O}$ events between 45 and 125 MeV. An event generator to simulate $\nu_{e} - {}^{16}\text{O}$ interactions, a multivariate method to separate events with de-excitation gammas, and an unbinned likelihood fitter to extract the flux-weighted cross section are developed. Using these tools, a scaling factor of $1.87^{+0.35\text{(stat)}+0.62\text{(syst)}}_{-0.36\text{(stat)}-0.34\text{(syst)}}$ to the Haxton prediction was measured, corresponding to $233^{+89}_{-62}$ observed events and an atmospheric $\nu_{e}$ flux-weighted cross section of $10.8^{+4.1}_{-3.0} \times 10^{-40} \text{cm}^{2}$. This is $1.7\sigma$ larger than the Haxton prediction of $5.8 \times 10^{-40} \text{cm}^{2}$, and $3.6\sigma$ away from the null hypothesis. The measured ratio can be used to rescale the $\nu_{e} - {}^{16}\text{O}$ cross section in supernova burst studies and diffuse supernova neutrino background searches. Finally, atmospheric neutrinos at this energy range will be a background for the future WIMP dark matter searches via coherent elastic neutrino-nucleus scattering. Combined with an independent $\nu_{e} - {}^{16}\text{O}$ cross section measurement, this measurement can be used to constrain the uncertainties in the low energy atmospheric neutrino flux, which is relevant for the estimation of the WIMP neutrino floor.

Item Open Access Measurement of Muon Neutrino Disappearance with the T2K Experiment(2014) Wongjirad, TaritreeWe describe the measurement of muon neutrino disappearance due to

neutrino oscillation using the Tokai-2-Kamiokande (T2K) experiment's Run 1-4 (6.57×1020 POT)

data set. We analyze the data using the conventional

Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing

matrix for the three Standard Model neutrinos. The output of the

analysis is a measurement of the parameters sin2θ

_{23}, Δm2_{32}for the normal hierarchy and sin2θ_{23}, Δm2_{13}forthe inverted hierarchy. The best-fit oscillation

parameters for the normal hierarchy are found to be

(sin2θ

_{23}, Δm2_{32}) = ( 0.514, 2.51×10-3 eV2/c4}). The 90% 1D confidence interval -- determined for both parametersusing the Feldman-Cousins procedure -- is for the normal hierarchy

0.428 < sin2θ

_{23}< 0.598 and2.34×10-3 eV2/c4 < Δm2

_{32}< 2.68\times10^{-3} eV2/c4.For the inverted hierarchy, the best-fit oscillation parameters are

(sin2θ

_{23}, Δm2_{13}) = (0.511, 2.48×10-3 eV2/c4. The 90\% 1D Feldman-Cousins confidence intervals for the inverted hierarchy are 2.31×10-3 eV2/c4 < \Delta m^2_{13} < 2.64×10-3 eV2/c4.