Approaching nuclear interactions with lattice QCD
- Autores: Marc Illa Subiña
- Directores de la Tesis: Assumpta Parreño García (dir. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2021
- Idioma: inglés
- Tribunal Calificador de la Tesis: Johann Haidenbauer (presid.), Federico Mescia (secret.), M. Elvira Gámiz Sánchez (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Física
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Nuclei make up the majority of the visible matter in the Universe; obtaining a first principles description of the nuclear properties and interactions between nuclei directly from the underlying theory of the strong interaction, Quantum Chromodynamics (QCD), is one of the main goals of the nuclear physics community. Although the theory was established nearly fifty years ago, the complexities of QCD at low energies precludes analytical solutions of the simplest hadronic systems, let alone the features of the nuclear forces.
Until the beginning of the century, the only way to overcome this handicap in the low-energy regime was to use phenomenological descriptions of nuclei or effective field theories (EFTs). While they have been very successful, these approaches rely heavily on experimental data. In contrast to what happens in the study of nucleon-nucleon interactions, where the amount of experimental data is overwhelming, the study of hadronic systems beyond the up-down quarks sector becomes more limited. This is because hyperons (baryons containing the next lightest quark, the strange quark), are unstable against weak interaction processes, making the experimental study of the interaction between hyperons and nucleons, and among hyperons, very difficult.
In this thesis we follow the lattice QCD (LQCD) approach, according to which QCD is solved non-perturvatibely in a discretized space-time via large-scale numerical calculations. Specifically, the interactions between two octet baryons are studied at low energies with larger-than-physical quark masses corresponding to a pion mass of 450 MeV and a kaon mass of 596 MeV. The two-baryon systems that are analyzed have strangeness ranging from 0 to -4 and include the spin-singlet and triplet N-N, Sigma-N (I=3/2), and Xi-Xi states, the spin-singlet Sigma-Sigma (I=2) and Xi-Sigma (I=3/2) states, and the spin-triplet Xi-N (I=0) state.
Due to the inherent large noise in multi-baryon calculations (mitigated by the use of unphysical quark masses), the finite-volume energies are extracted using a robust fitting methodology, where in order to reliably estimate the systematic uncertainties, both the fitting form and the fitting range are varied. Then, the corresponding S-wave scattering phase shifts, low-energy scattering parameters, and binding energies when applicable, are extracted using Lüscher's formalism. While the results are consistent with most of the systems being bound at this pion mass, the interactions in the spin-triplet Sigma-N and Xi-Xi channels are found to be repulsive and do not support bound states. Using results from previous studies of these systems at a larger pion mass, an extrapolation of the binding energies to the physical point is performed and is compared with available experimental values and phenomenological predictions.
The low-energy coefficients in pionless EFT relevant for two-baryon interactions, including those responsible for SU(3) flavor-symmetry breaking, are constrained. The SU(3) flavor symmetry is observed to hold approximately at the chosen values of the quark masses, as well as the SU(6) spin-flavor symmetry, predicted at large Nc. A remnant of an accidental SU(16) symmetry found previously at a larger pion mass is further observed. The SU(6)-symmetric EFT constrained by these LQCD calculations is used to make predictions for two-baryon systems for which the low-energy scattering parameters could not be determined within the present LQCD study, and to constrain the coefficients of all leading SU(3) flavor-symmetric interactions, demonstrating the predictive power of two-baryon EFTs matched to LQCD.
