Heavy quark spin structure in Z_b resonances

TL;DR

This paper analyzes the heavy quark spin structure of the Z_b(10610) and Z_b(10650) resonances, proposing they are molecular states with mixed spin configurations, consistent with recent experimental data.

Contribution

It introduces a model where the Z_b resonances are molecular states with a specific heavy quark spin mixture, explaining observed amplitude relations.

Findings

The Z_b states have a mixed spin-triplet and spin-singlet structure.

The model's predictions agree with Belle experimental data.

Possible isotopic and parity partners are discussed.

Abstract

We discuss the heavy quark spin structure of the recently observed `twin' resonances $Z_b(10610)$ and $Z_b(10650)$ assuming that these are mostly of a `molecular' type, i.e. that their internal dynamics is dominated by the coupling to meson pairs $B^* \bar B - B {\bar B}^*$ and $B^* {\bar B}^*$. We find that the state of the $b \bar b$ pair within the $Z_b(10610)$ and $Z_b(10650)$ resonances is a mixture of a spin-triplet and a spin-singlet of equal amplitude and with the phase orthogonal between the two resonances. Such a structure gives rise to specific relations between observable amplitudes that are in agreement with the data obtained recently by Belle. We also briefly discuss possible properties of the isotopically singlet counterparts of the newly found resonances, and also of their $C$ ($G$) parity opposites that likely exist in the same mass range near the open $B$ flavor threshold.