Bound state nature of the exotic Z_b states

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.