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.