The Compact X and Z and their Invisible Molecular Partners

TL;DR

This paper presents a model explaining isospin violations in X(3872) decays by proposing a compact tetraquark structure and mixing with molecular states, clarifying the nature of Z resonances.

Contribution

It introduces a novel model combining compact tetraquark and molecular states to explain isospin violations and the nature of Z resonances.

Findings

The X(3872) involves mixing between isosinglet and isotriplet states.

Large isospin violations are explained by mixing mechanisms.

Z(3900) resonances are consistent with a compact isotriplet structure.

Abstract

We propose a model explaining the strong isospin violations observed in X(3872) decays. The X is assumed to be a compact isosinglet tetraquark and its charged partners to form an isotriplet, together with an additional neutral state mixing with the isosinglet. The isotriplet results from the loose binding of open charm mesons and, in isolation, has strongly suppressed production rates. However, its neutral component can still mix with the isosinglet and induce the large isospin violations observed. The compact-isosinglet/molecular-isotriplet pattern appears reversed when the Z(3900) resonances are considered. The Z particles observed correspond to a compact isotriplet and there is no evidence of a neutral isosinglet Z. We show that no isospin violation in the Z decays is expected.