# Browsing by Subject "Effective field theories"

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Item Embargo 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 Probing Quarkonium Production in Jets Using Effective Field Theories(2017) Bain, Reginald AlexanderWhile bound states of heavy quarks $Q\bar{Q}$ called quarkonia have been studied for nearly a half-century, their production is still not well understood. We examine how techniques from Effective Field Theories (EFTs) of the strong force can be used to probe the production of the $J/\psi(c\bar{c})$. The focus will be to study how quarkonia are produced in jets, highly-collimated sprays of hadrons ubiquitous at particle colliders. We review the study of quarkonium production using Non-relativistic QCD (NRQCD) and the study of jet substructure observables using the Soft-Collinear Effective Theory (SCET). The concept of Fragmenting Jet Functions (FJF), which describe a hadron's energy distribution inside a jet of measured substructure, is extended to jets where the angularity is measured or where the transverse momenta of a hadron relative to the jet axis is measured. Predictions of the energy distribution of $J/\psi$ in jets at the LHC using FJFs are compared with the latest LHCb data using various extractions of the non-perturbative NRQCD long-distance-matrix-elements (LDMEs) in the literature. These distributions are also calculated using a modification of the Pythia Monte Carlo, which is shown to have an unphysical model of quarkonium production and gives results consistent with our FJF calculations. Our predictions of the energy of $J/\psi$ produced in jets fit the data much better than default Monte Carlo results. We also demonstrate that LDMEs extracted from high transverse momentum data do a better job at predicting the LHCb measurements.