Observation and studies of double<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>J</mml:mi><mml:mo stretchy="false">/</mml:mo><mml:mi>ψ</mml:mi></mml:math>production at the Tevatron

We present the observation of doubly produced $J/\ensuremath{\psi}$ mesons with the D0 detector at Fermilab in $p\overline{p}$ collisions at $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$. The production cross section for both singly and doubly produced $J/\ensuremath{\psi}$ mesons is measured using a sample with an integrated luminosity of $8.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. For the first time, the double $J/\ensuremath{\psi}$ production cross section is separated into contributions due to single and double parton scatterings. Using these measurements, we determine the effective cross section ${\ensuremath{\sigma}}_{\text{eff}}$, a parameter characterizing an effective spatial area of the parton-parton interactions and related to the parton spatial density inside the nucleon.