The production of $J/\psi$ pairs in protonproton collisions at a centreofmass energy of 7 TeV has been observed using an integrated luminosity of $37.5 pb^{1}$ collected with the LHCb detector. The production crosssection for pairs with both \jpsi in the rapidity range $2<y^{J/\psi}<4.5$ and transverse momentum $p_{T}^{J/\psi}<10 GeV/c$ is $$ \sigma^{J/\psi J/\psi} = 5.1\pm1.0\pm1.1 nb,$$ where the first uncertainty is statistical and the second systematic.
The fitted yields of $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \rightarrow \left(\mu ^+\mu ^ \right)_1$ in bins of $(\mu ^+\mu ^ )_2$ invariant mass: (a) the raw signal yield observed in the data; (b) the efficiency corrected yield (Sect. 6). The result of a fit with a doublesided Crystal Ball function for the signal and an exponential background is superimposed. 
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Differential production crosssection for $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ pairs as a function of the invariant mass of the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}{ J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ system. The points correspond to the data. Only statistical uncertainties are included in the error bars. The shaded area corresponds to prediction by the model described in Ref. [20]. 
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Animated gif made out of all figures. 
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Relative systematic uncertainties on the crosssection measurement. The total uncertainty is calculated as the quadratic sum of the individual components. 
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Created on 05 March 2021.