Observation of $\Upsilon(\mathrm{1S})$ pair production in proton-proton collisions at $\sqrt{s} = $ 8 TeV

Pair production of $\Upsilon(\mathrm{1S})$ mesons is observed at the LHC in proton-proton collisions at $ \sqrt{s} = $ 8 TeV by the CMS experiment in a data sample corresponding to an integrated luminosity of 20.7 fb$^{-1}$. Both $\Upsilon(\mathrm{1S})$ candidates are fully reconstructed via their decays to $\mu^{+}\mu^{- } $. The fiducial acceptance region is defined by an absolute $\Upsilon(\mathrm{1S})$ rapidity smaller than 2.0. The fiducial cross section for the production of $\Upsilon(\mathrm{1S})$ pairs, assuming that both mesons decay isotropically, is measured to be 68.8 $\pm$ 12.7 (stat) $\pm$ 7.4 (syst) $\pm$ 2.8 ($\mathcal{B}$) pb, where the third uncertainty comes from the uncertainty in the branching fraction of $\Upsilon(\mathrm{1S})$ decays to $\mu^{+}\mu^{-}$. Assuming instead that the $\Upsilon(\mathrm{1S})$ mesons are produced with different polarizations leads to variations in the measured cross section in the range from $-38\%$ to $+36\%$.