Observation of double $J/\psi$ production in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV

The production of $J/\psi J/\psi$~pairs in proton-proton collisions at a centre-of-mass energy of $\sqrt{\mathrm{s}}=7~\mathrm{TeV}$ has been observed with the LHCb detector with an integrated luminosity of $35.2~\mathrm{pb}^{-1}$. The production cross section for $J/\psi J/\psi$~pairs with both $J/\psi$ in the rapidity range $2<y^{J/\psi}<4.5$ and with transverse momentum $p_{\mathrm{T}}^{J/\psi}<10~\mathrm{GeV}/c$ has been measured to be \begin{equation*} \sigma^{J/\psi J/psi} = 5.6\pm1.1\pm1.2~\mathrm{nb}, \end{equation*} where the first uncertainty is statistical, and the second is systematic.