Measurement of the $J/\psi$ pair production cross-section in $pp$   collisions at $\sqrt{s} = 13 \,{\mathrm{TeV}}$
  The production cross-section of $J/\psi$ pairs is measured using a data sample of $pp$ collisions collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s} = 13 \,{\mathrm{TeV}}$, corresponding to an integrated luminosity of $279 \pm 11 \,{\mathrm{pb^{-1}}}$. The measurement is performed for $J/\psi$ mesons with a transverse momentum of less than $10 \,{\mathrm{GeV}}/c$ in the rapidity range $2.0<y<4.5$. The production cross-section is measured to be $15.2 \pm 1.0 \pm 0.9 \,{\mathrm{nb}}$. The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the $J/\psi$ pair are measured and compared to theoretical predictions. 
LHCb collaboration: R. Aaij, B. Adeva, M. Adinolfi, Z. Ajaltouni, S.   Akar, J. Albrecht, F. Alessio, M. Alexander, S. Ali, ...
hep-ex