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 ) → η ϒ(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.
Highlights
The Belle experiment [1] at the KEKB e+e− asymmetric collider [2] collected the world largest samples of Υ(1S ), Υ(2S ) and Υ(5S )
Once the large combinatorial background is fitted and subtracted, the remaining distribution (Figure 1) shows different peaks, due to different dipion transitions: the hb(1P) and hb(2P) signals are clearly visible with a significance of 6.2 σ and 12.4 σ respectively, together with the Υ(5S ) → π+π−Υ(nS ) signals
The calculation of B [Υ(5S ) → π+π−hb(1P)] includes amplitudes related to a spin flip of the b quark, whose presence is predicted by non-relativistic QCD (NRQCD) to suppress this transition with respect to a non spin-flipping process such as Υ(5S )
Summary
B mesons are commonly expected not to have large transitions to the lower states. In addition to that, the calculation of B [Υ(5S ) → π+π−hb(1P)] includes amplitudes related to a spin flip of the b quark, whose presence is predicted by non-relativistic QCD (NRQCD) to suppress this transition with respect to a non spin-flipping process such as Υ(5S ).
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