New bottomonium(-like) resonances’ spectroscopy and decays at Belle

Abstract

We report the recent progresses in the study of bottomonium spectroscopy and decays obtained with the ϒ(1S ), ϒ(2S ), and ϒ(5S ) samples collected by the Belle experiment. Among these results, particular interest is generated by the observation, with high significance, of the missing singlet states hb(1P) and hb(2P) in ϒ(5S ) hadronic decays. The transition ϒ(5S ) → π+π−hb(1, 2P) in particular is found to be mediated by two new, bottomonium-like, charged state Zb and Zb′, whose nature is still argument of discussion. The large samples of hb(1P) and hb(2P) provided by these transitions allow the word most precise measurement of the hyperfine splitting in the 1S system and the first measurement of this quantity in the 2S system, together with a new high significance observation of the ηb(1S ) and a new, precise measurements of its parameters. The study of the radiative transition hb(2P) → ηb(2S ) resulted in the first evidennce of the 2S singlet state. We report also new studies of the ϒ(nS ) → ηϒ(mS ) transitions, the results of an intensive search for bottomonium decays to charmonium and the first observation of ϒ(1S ) and ϒ(2S ) decaying in low multiplicity, mesonic final states.