A New Look at the X Compositeness from its Lineshape

TL;DR

This paper compares different theoretical models of the X(3872) particle's lineshape, showing that a compact four-quark model fits data well, while a molecular model requires a different effective theory approach.

Contribution

It introduces an alternative effective theory framework for describing the X(3872) lineshape, contrasting it with the traditional Flatte' distribution approach.

Findings

The compact four-quark model aligns with existing data.

A molecular model needs an auxiliary field approach for accurate description.

The auxiliary field method provides a consistent description of scattering data.

Abstract

The existing analyses on the X(3872) lineshape are consistent with the hypothesis that it is a compact particle composed of four quarks. This conclusion follows from fitting available data with a Flatte' distribution derived from an effective Lagrangian in which X, D, and D* are all elementary fields. A molecular X, i.e., a deuteron-like bound state of D and Dbar*, must instead be described by an effective theory where the X field is initially absent or introduced as an auxiliary, unphysical, field, yielding a lineshape parametrization that differs significantly from the Flatte' one. We demonstrate, in the analogous theory of nucleons with an auxiliary deuteron field, that this parametrization gives a fully consistent description of np--> deuteron --> np scattering data, and motivate applying the same method to the X lineshape.