# Browsing by Author "Springer, Roxanne"

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Item Open Access Effective Field Theory Studies of Few-nucleon Systems: Fundamental Symmetry Violation, Electromagnetic Interactions, and Direct Detection of Dark Matter(2023) Nguyen, Thai SonEffective field theory (EFT) has evolved as a powerful model-independent theoretical framework for illuminating complicated interactions across a wide range of physics areas and subfields. It takes advantage of the scale separation exsiting in physical systems to invoke a systematic expansion to capture the physics at a certain energy scale. The symmetries of the high-energy/short-distance theory constrain these interactions, limiting the number of unknown low-energy coefficients (LECs) that must be extracted from the experiment or calculated directly from the underlying theory.

The utilization of EFTs in nuclear physics has facilitated our understanding of atomic nuclei and bridged the gap between quantum chromodynamics (QCD), the theory of strong interactions, and nuclear structure and interactions. In particular, EFTs have been proven successful in describing the structure and dynamics of few-body nuclei. In this dissertation, we present several studies on applying the EFT technique to research problems in nuclear physics. We first apply pionless EFT (\eftnopi) to study parity violation in two-nucleon systems and the dark matter scattering off light nuclei. The operators contributing to these elusive processes are accompanied by unknown LECs. We show that the large-\Nc expansion can systematically separate these LECs into those that occur at leading order in $N_c$ and those that occur at next-to-leading order in \Nc. The large-\Nc ordering could provide a powerful reduction in the number of experiments needed to understand these processes at every order in this combined expansion, as well as help prioritize future experiments and lattice QCD calculations.

In the second part, we consider the low-energy proton–deuteron and deuteron-Helium-4 systems at low energies in cluster EFT. Below the deuteron breakup threshold, the deuteron and Helium-4 can be treated as structureless degrees of freedom. In particular, we focus on the deuteron + Helium-4 cluster configuration of the Lithium-6 nucleus. We illustrate how to directly extract the asymptotic normalization coefficient, $\mathcal{C}_0$, and the asymptotic $D/S$ ratio, $\eta_{sd}$, from the three electromagnetic form factors of Lithium-6. The fitting to these form factor data yields $C_0\approx 2.20$ fm$^{-1/2}$ and $\eta_{sd}\approx -0.0224$.

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 Transverse Asymmetry in Nucleon Deuteron Scattering in Pionless Effective Field Theory(2017) Margaryan, ArmanIn this dissertation we applied the pionless effective field theory (EFTπ{ ) to low energy neu- tron deuteron elastic scattering process. We calculated some of the polarization observables in neutron deuteron scattering to next-to-next-to-next-to-leading-order, in particular the transverse asymmetry Ay. All of the previous calculations have the same characteristic fea- ture of under-predicting this observable, this is known as the Ay-puzzle. At this order of the EFTπ{ new two-body P-wave interaction terms enter into the Lagrangian. This interaction terms give crucial contributions to the Ay observable. By varying the interaction coefficients within the allowed error estimates of the EFTπ{ we find results that at this order are consis- tent with the experimental data. Our conclusion is that the Ay-puzzle is likely to be solved within the next few orders of the EFTπ{ . Other observables in neutron-deuteron scattering process are also calculated and are with a good agreement with the experimental data.