Publication
The first observation of the concurrent production of two J/psi mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead (pPb) data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC and corresponding to an integrated luminosity of 174.6 nb(-1). The two J/psi mesons are reconstructed in their mu(+)mu(-) decay channels with transverse momenta p(T) > 6.5 GeV and rapidity vertical bar y vertical bar < 2.4. Events where one of the J/psi mesons is reconstructed in the dielectron channel are also considered in the search. The pPb -> J/psi J/psi +X process is observed with a significance of 5.3 standard deviations. The measured inclusive fiducial cross section, using the four-muon channel alone, is sigma(pPb (-> J/psi J/psi + X) = 22.0 +/- 8.9(stat) +/- 1.5(syst) nb. A fit of the data to the expected rapidity separation for pairs of J/psi mesons produced in single (SPS) and double (DPS) parton scatterings yields sigma(pPb)(SPS) (-> J/psi J/psi + X) = 16.5 +/- 10.8(stat) +/- 0.1(syst) nb and sigma(pPb)(DPS) (-> J/psi J/psi + X) = 5.4 +/- 6.2(stat) +/- 0.4(syst) nb, respectively. This latter result can he transformed into a lower hound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of sigma(eff) > 1.0 mb at 95% confidence level.