Speaker
Feng-Kun Guo
Description
There has been a long-standing puzzle in understanding why the dipion invariant distribution for the $ \Upsilon(3S) \to \Upsilon(1S) \pi \pi$ transition shows a double-bump structure while other analogous transitions have only one. By including the $\pi \pi$ final state interaction and effects from the $Z_b$ states, we show that this phenomenon can be understood. However, this requires the partial widths of the $Z_b$ states to be much larger than those naively calculated from the measured branching fractions. The analysis is further extended to the dipion transition from the $\Upsilon(4S)$ to the $\Upsilon(1S)$, and we predict a nontrivial structure at around 1 GeV in the dipion invariant spectrum for this process.
