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Tetraquark interpretation of the BELLE data on the anomalous $Υ(1S) π^+π^-$ and $Υ(2S) π^+π^-$ production near the $Υ(5S)$ resonance

We analyze the Belle data [K. F. Chen {\it et al.} (Belle Collaboration), Phys.\ Rev.\ Lett.\ {\bf 100}, 112001 (2008); I. Adachi {\it et al.} (Belle Collaboration), arXiv:0808.2445] on the processes $e^+ e^- \to Υ(1S)\; π^+π^-, Υ(2S)\; π^+π^-$ near the peak of the $Υ(5S)$ resonance, which are found to be anomalously large in rates compared to similar dipion transitions between the lower $Υ$ resonances. Assuming these final states arise from the production and decays of the $J^{PC}=1^{--}$ state $Y_b(10890)$, which we interpret as a bound (diquark-antidiquark) tetraquark state $[bq][\bar{b}\bar{q}]$, a dynamical model for the decays $Y_b \to Υ(1S)\; π^+π^-, Υ(2S)\; π^+π^-$ is presented. Depending on the phase space, these decays receive significant contributions from the scalar $0^{++}$ states, $f_0(600)$ and $f_0(980)$, and from the $2^{++}$ $q\bar{q}$-meson $f_2(1270)$. Our model provides excellent fits for the decay distributions, supporting $Y_b$ as a tetraquark state.