Absence of Ground States in the Renormalized Massless Translation-Invariant Nelson Model Thomas Norman Dam Benjamin Hinrichs Aarhus Universitet∗ Friedrich Schiller Universität Jena Nordre Ringgade 1 Ernst-Abbe-Platz 2 8000 Aarhus C 07743 Jena Denmark Germany
[email protected] July 28, 2021 Abstract We consider the model of a massive uncharged non-relativistic particle interacting with a mass- less bosonic field, widely referred to as the Nelson model. It is well known that an ultraviolet renormalized Hamilton operator exists in this case. Further, due to translation-invariance, it decomposes into fiber operators. In this paper, we treat the renormalized fiber operators. We give a description of the operator and form domains and prove that the fiber operators do not have a ground state. Our results especially hold for any non-zero coupling constant and arbitrary total momentum. Our proof for the absence of ground states is a new generalization of methods recently applied to related models. A major enhancement we provide is that the method can be applied to models with degenerate ground state eigenspaces. 1 Introduction arXiv:1909.07661v2 [math-ph] 11 Jan 2021 In this paper, we prove absence of ground states for the fiber operators in the translation invariant renormalized massless Nelson model. The method used in the proof follows along the lines of [Dam20], but crucially avoids non-degeneracy of ground states as an ingredient, even though it would be available (see [Miy18]). Instead, we rely on a momentum estimate for hypothetical ground states derived via rotation invariance. The purpose of avoiding non-degeneracy of ground states as an ingredient is threefold: First of all, the method found in this paper can in principle be applied to translation invariant models which cannot be expected to have non-degenerate ground states, such as models with spin.