Observation of $J/\psi$ pair production in pp collisions at $\sqrt{s}=7   TeV$
  The production of $J/\psi$ pairs in proton-proton collisions at a centre-of-mass energy of 7 TeV has been observed using an integrated luminosity of $37.5 pb^{-1}$ collected with the LHCb detector. The production cross-section for pairs with both \jpsi in the rapidity range $2<y^{J/\psi}<4.5$ and transverse momentum $p_{T}^{J/\psi}<10 GeV/c$ is $$ \sigma^{J/\psi J/\psi} = 5.1\pm1.0\pm1.1 nb,$$ where the first uncertainty is statistical and the second systematic. 
LHCb Collaboration: R. Aaij, B. Adeva, M. Adinolfi, C. Adrover, A.   Affolder, Z. Ajaltouni, J. Albrecht, F. Alessio, M. Ale...
hep-ex