1. Belle collaboration, K.F. Chen et al., Observation of anomalous Υ(1S)π + π − and Υ(2S)π + π − production near the Υ(5S) resonance, Phys. Rev. Lett. 100 (2008) 112001 [ arXiv:0710.2577 ] [ SPIRES ].
2. Belle collaboration, I. Adachi et al., Observation of an enhancement in e + e − → Υ(1S)π + π − , Υ(2S)π + π − , and Υ(3S)π + π − production at Belle, arXiv:0808.2445 [ SPIRES ].
3. S.L. Olsen, Hadronic Spectrum-Multiquark States, Nucl. Phys. A 827 (2009) 53c–60c [ arXiv:0901.2371 ] [ SPIRES ].
4. Belle collaboration, A. Zupanc, Hadron Spectroscopy Results from Belle, arXiv:0910.3404 [ SPIRES ].
5. W.-S. Hou, Searching for the bottom counterparts of X (3872) and Y (4260) via π + π −Υ, Phys. Rev. D 74 (2006) 017504 [ hep-ph/0606016 ] [ SPIRES ].