Bound state nature of the exotic Z_b states

TL;DR

This paper investigates whether the exotic Z_b states are molecular bound states of bottom mesons by comparing theoretical predictions with experimental invariant mass distributions, emphasizing the importance of non-Breit-Wigner analysis.

Contribution

It provides a nonrelativistic effective field theory framework up to two loops to support the molecular interpretation of Z_b states and challenges the use of Breit-Wigner parametrization.

Findings

Z_b states are consistent with being Bar B^*+c.c. and B^*ar B^* bound states.

Data supports molecular nature over alternative explanations.

Nonrelativistic effective field theory accurately describes the invariant mass distributions.

Abstract

The assumption that the newly observed charged bottomonia states Z_b(10610) and Z_b(10650) are of molecular nature is confronted with the measured invariant mass distributions for the transitions of the Upsilon(5S) to the final states h_b pi^+ pi^- and h_b(2P) pi^+ pi^-. It is shown that the assumption that the Z_b(10610) and Z_b(10650) are B\bar B^*+{\rm c.c.} and B^*\bar B^* bound states, respectively, with very small binding energies is consistent with the data. The calculation is based on a power counting for bottom meson loops, which is explicitly given up to two-loop in the framework of a nonrelativistic effective field theory. We also show that if the Z_b states are of molecular nature, then the data should not be analyzed by using a Breit-Wigner parametrization.