The bottomonium spectrum is the perfect testing ground for the confining potential and unitarisation effects. The bottom quark is about three times heavier than the charm quark, so that $b\bar{ b}$ systems probe primarily the short-range part of that potential. Also, the much smaller colour-hyperfine interaction in the $B$ mesons make the $B\bar{ B}$ threshold lie significantly higher than the $D\bar{ D}$ threshold in charmonium, on a relative scale of course. A further complicating circumstance is that none of the experimentally observed vector $b\bar{ b}$ mesons has been positively identified as a $^{ 3\!}D_1$ state, contrary to the situation in charmonium. This makes definite conclusions about level splittings very problematic. Finally, there are compelling indications that the $\Upsilon(10580)$ is not the $\Upsilon(4S)$ state, as is generally assumed. Here we review an analysis of experimental bottomonium data which show indications of the two lowest and so far unlisted $^{ 3\!}D_1$ states below the $B\bar{ B}$ threshold. Next an empirical modelling of vector $b\bar{ b}$ resonances above the open-bottom threshold is revisited, based on the Resonance-Spectrum-Expansion production formalism applied to other experimental data. A recent effective-Lagrangian study supporting our non-resonant assignment of the $\Upsilon(10580)$ is briefly discussed as well.
Categories: hep-ph hep-ex