Heavy Quarkonium \pi^+\pi^- Transitions and a Possible b\bar{b}q\bar{q}   State
  $\pi^+\pi^-$ transitions of heavy quarkonia, especially the $ \Upsilon(3S)\to\Upsilon(1S)\pi^+\pi^-$ decay process, are revisited. In the framework of the Chiral Unitary Theory (ChUT), the $S$ wave $\pi\pi$ final state interaction (FSI) is included. It is found that when an additional intermediate state with $J^P=1^+$ and I=1 is introduced, not only the $ \pi\pi$ invariant mass spectrum and the $cos\theta_\pi^*$ distribution in the $\Upsilon(3S)\to\Upsilon(1S)\pi^+\pi^-$ process can simultaneously be well-explained, but also a consistent description for other bottomonia $ \pi^+\pi^-$ transitions can be obtained. As a consequence, the mass and the width of the intermediate state are predicted. From the quark content analysis, this state should be a $b\bar bq\bar q$ state. 
F.-K. Guo, P.-N. Shen, H.-C. Chiang, R.-G. Ping