New insights into many-particle bound states in quantum field theory

by Fabio van Dissel (IFAE)

Thursday 3 Jul 2025, 11:00 → 14:00 Europe/Madrid

IFAE Seminar Room (In-person)

Supervisors: Oriol Pujolas Boix and Evangelos Sfakianakis.

Committee: Francesc Ferrer, Andrzej Wereszczyński and Clara Murgui.

Abstract: In this thesis we study various aspects of many-particle bound states in quantum field theories. In bosonic theories many fundamental quanta can occupy the same state, resulting in macroscopic solutions that behave semiclassically on large timescales. If additionally attractive interactions exist between the individual quanta the particles can settle into bound states: localized solutions that are (meta)stable and have well defined properties. We expand and enhance the understanding of these configurations, focusing on real scalar field theories. The thesis is centered around five research works. In the first part we give a broad introduction to the topic of many-particle bound states in quantum field theory, focusing on the aspects most relevant to the papers on which this thesis is based. In paper 1 we construct multi-field oscillons and highlight how these objects form in the early Universe. In paper 2 we show that multi-field wave dark matter is a viable extension of the popular ULDM paradigm, with better agreement to experimental data in certain areas. In paper 3 we elucidate the resonant structure of oscillons in the Sine-Gordon model, showing that a tower of long lived excited states exists. In paper 4 we find a new type of oscillon solution dubbed soft oscillons, that contrary to common lore can have arbitrary longevity in gapless theories. Finally, in paper 5, we assess the accuracy of various approximation schemes used to extend the validity of the semiclassical limit of many-particle bound states.