An explanation of Belle states Z_b(10610) and Z_b(10650)

TL;DR

This paper explains the observed peaks in Belle's data on Z_b states as resulting from threshold cusps and phase space effects, rather than genuine molecular states.

Contribution

It provides a novel interpretation of Z_b(10610) and Z_b(10650) peaks as threshold cusps influenced by phase space, challenging the molecular state hypothesis.

Findings

Peaks are explained by threshold cusps and phase space effects.

Masses align with Bbar-B^* and B^*bar-B^* thresholds.

The model reproduces observed peak positions.

Abstract

Belle report data on Upsilon (5S) -> Upsilon(1S,2S,3S)-piplus-piminus and chi_b(1P,2P)-piplus-piminus; they observe peaks in Upsilon-pi and chi_b-pi consistent with J^P=1^+. They interpret the peaks as molecular states Z_b(10608) and Z_b(10653). Their masses are just above Bbar-B^* and B^*bar-B^* thresholds at 10604.6 and 10650.2 MeV. An explanation in terms of cusps at these thresholds is presented here. The product of the rising phase space for Bbar-B^* and B^*bar B^* with the cusps creates peaks a few MeV higher in Bbar-B^* and B^*bar-B^*, and these peaks can de-excite to piplus-piminus-Upsilon(1S,2S,3S) and piplus-piminus-chi_b(1P,2P).