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 $e^+ e^- \to \Upsilon(1S)\; \pi^+\pi^-, \Upsilon(2S)\; \pi^+\pi^-$ near the peak of the $\Upsilon(5S)$ resonance, which are found to be anomalously large in rates compared to similar dipion transitions between the lower $\Upsilon$ 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 $Y_b \to \Upsilon(1S)\; \pi^+\pi^-, \Upsilon(2S)\; \pi^+\pi^-$ 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.
Categories: hep-ph hep-ex