Observation of double J/psi meson production in pPb collisions at sqrt(s_NN) = 8.16 TeV

he first observation of the concurrent production of two J/ψ mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead (p⁢Pb) 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/ψ mesons are reconstructed in their μ+⁢μ− decay channels with transverse momenta pT >6.5 GeV and rapidity |y| <2.4. Events where one of the J/ψ mesons is reconstructed in the dielectron channel are also considered in the search. The p⁢Pb →J/ψ⁢J/ψ+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 σ⁡(p⁢Pb →J/ψ⁢J/ψ +X) = 22.0 ±8.9⁢(stat) ±1.5⁢(syst) nb. A fit of the data to the expected rapidity separation for pairs of J/ψ mesons produced in single (SPS) and double (DPS) parton scatterings yields σp⁢Pb→J/ψ⁢J/ψ+X SPS=16.5±10.8⁢(stat)±0.1⁢(syst) nb and σp⁢Pb→J/ψ⁢J/ψ+X DPS =5.4 ±6.2⁢(stat) ±0.4⁢(syst) nb, respectively. This latter result can be transformed into a lower bound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of σeff >1.0 mb at 95% confidence level.