Observation of J/ψ-pair production in pp collisions at s=7 TeV

The production of J/ψ 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 J/ψ in the rapidity range 2