Observation and studies of double $J/\psi$ production at the Tevatron

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

Figure 1

The $c\tau$ distribution of background subtracted single $J/\psi$ events after all selection criteria. The distributions for the signal and background templates are shown normalized to their respective fitted fractions. The uncertainty band corresponds to the total systematic uncertainty on the sum of signal and background events.

Figure 2

Dimuon invariant mass distribution in data for two muon pairs $M_{\mu \mu }^{(1)}$, $M_{\mu \mu }^{(2)}$ after the DJ selection criteria.

Figure 3

Comparison of the signal and background contributions to data projected on the axis of one muon pair $M_{\mu \mu }$ while events along the second pair are integrated over the mass range 2.85–3.35 GeV.

Figure 4

The $|\mathrm{\Delta }\eta (J/\psi ,J/\psi )|$ distribution of background subtracted double $J/\psi$ events after all selection criteria. The distributions for the SP and DP templates are shown normalized to their respective fitted fractions. The uncertainty band corresponds to the total systematic uncertainty on the sum of SP and DP events.