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 $$ sigma^{J/psi J/psi} = 5.1pm1.0pm1.1 nb,$$ where the first uncertainty is statistical and the second systematic.