Friday·November 4, 2016

[Submitted on 3 Nov 2016] (cross-list from hep-ph)

Effects of $Z_b$ states and bottom meson loops on ${\rm Y}(4S)\to {\rm Y}(1S,2S){\pi }^{+}{\pi }^{-}$ transitions

Yun-Hua Chen🇩🇪 · Martin Cleven🇪🇸 · Johanna T. Daub🇩🇪 · Feng-Kun Guo🇨🇳 · Christoph Hanhart🇩🇪 · Bastian Kubis🇩🇪 · Ulf-G. Meißner🇩🇪 · Bing-Song Zou🇨🇳

We study the dipion transitions ${\rm Y}(4S)\to {\rm Y}(nS){\pi }^{+}{\pi }^{-}$ $(n=1,2)$. In particular, we consider the effects of the two intermediate bottomoniumlike exotic states $Z_b(10610)$ and $Z_b(10650)$ as well as bottom meson loops. The strong pion-pion final-state interactions, especially including channel coupling to $K\bar{K}$ in the $S$-wave, are taken into account model-independently by using dispersion theory. Based on a nonrelativistic effective field theory we find that the contribution from the bottom meson loops is comparable to those from the chiral contact terms and the $Z_b$-exchange terms. For the ${\rm Y}(4S)\to {\rm Y}(2S){\pi }^{+}{\pi }^{-}$ decay, the result shows that including the effects of the $Z_b$-exchange and the bottom meson loops can naturally reproduce the two-hump behavior of the $\pi \pi$ mass spectra. Future angular distribution data are decisive for the identification of different production mechanisms. For the ${\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ decay, we show that there is a narrow dip around 1 GeV in the $\pi \pi$ invariant mass distribution, caused by the final-state interactions. The distribution is clearly different from that in similar transitions from lower ${\rm Y}$ states, and needs to be verified by future data with high statistics. Also we predict the decay width and the dikaon mass distribution of the ${\rm Y}(4S)\to {\rm Y}(1S)K^{+}{K}^{-}$ process.