We present the observation of doubly-produced J/ψ mesons with the D0 detector at Fermilab in ppˉ collisions at s=1.96 TeV. The production cross section for both singly and doubly-produced J/ψ mesons is measured using a sample with an integrated luminosity of 8.1fb−1. For the first time, the double J/ψ production cross section is separated into contributions due to single and double parton scatterings. Using these measurements, we determine the effective cross section σeff, a parameter characterizing an effective spatial area of the parton-parton interactions and related to the parton spatial density inside the nucleon.
@article{arxiv.1406.2380,
title = {Observation and studies of double J/psi production at the Tevatron},
author = {D0 Collaboration},
journal= {arXiv preprint arXiv:1406.2380},
year = {2016}
}