Observation of $J\mathrm{/}\psi$ pair production in pp collisions at $\sqrt{s}=7TeV$

The production of $J\mathrm{/}\psi$ pairs in proton-proton collisions at a centre-of-mass energy of 7 TeV has been observed using an integrated luminosity of $37.5p{b}^{-1}$ collected with the LHCb detector. The production cross-section for pairs with both \jpsi in the rapidity range $2<y^{J\mathrm{/}\psi }<4.5$ and transverse momentum $p_T^{J\mathrm{/}\psi }<10GeV\mathrm{/}c$ is $${\sigma }^{J\mathrm{/}\psi J\mathrm{/}\psi }=5.1\pm 1.0\pm 1.1nb,$$ where the first uncertainty is statistical and the second systematic.