New results from a number operator interpretation of the compositeness of bound and resonant states

TL;DR

This paper introduces a new theoretical framework based on number operator expectation values to analyze the compositeness of bound and resonant states, applicable to effective field theories and relativistic states, providing universal criteria for elementariness.

Contribution

It develops a novel formalism for calculating the compositeness of states using number operators, applicable to energy-dependent potentials and relativistic cases, with criteria for elementariness and elementary resonance qualification.

Findings

X=1 for finite-range energy-independent potentials

X is independent of cutoff regulators in certain energy-dependent potentials

A universal criterion for elementariness of bound states is established

Abstract

A novel theoretical approach to the problem of the compositeness ($X$) of a resonance or bound state is developed on the basis of the expectation values of the number operators of the free particles in the continuum. This formalism is specially suitable for effective field theories in which the bare elementary states are integrated out but that give rise to resonance and bound states when implemented in nonperturbative calculations. We demonstrate that $X=1$ for finite-range energy-independent potentials, either regular or singular. A non-trivial example for an energy-dependent potential is discussed where it is shown that $X$ is independent of any type of cutoff regulator employed. The generalization of these techniques to relativistic states is developed. We also explain how to obtain a meaningful compositeness with respect to the open channels for resonances, even if it is complex in a first turn, by making use of suitable phase-factor transformations. Defining elementariness as $X=0$, we derive a new universal criterion for the elementariness of a bound state. Along the same lines, a necessary condition for a resonance to be qualified as elementary is given. The application of the formalism here developed might be of considerable practical interest.