1
From a sample of ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ events without converted photons.
References:
LEES
2014M
PR D90 112010
Bottomonium Spectroscopy and Radiative Transitions Involving the ${{\mathit \chi}_{{bJ}}{(1P,2P)}}$ States at BABAR