Measurement of associated J/ψ-ψ(2S) production cross-section in pp collisions at √s = 13 TeV

A bstract The cross-section of associated J/ψ - ψ (2 S ) production in proton-proton collisions at a centre-of-mass energy of √s s = 13 TeV is measured using a data sample corresponding to an integrated luminosity of 4 . 2 fb − 1 , collected by the LHCb experiment. The measurement is performed for both J/ψ and ψ (2 S ) mesons having transverse momentum p T < 14 GeV/ c and rapidity 2 . 0 < y < 4 . 5, assuming negligible polarisation of the J/ψ and ψ (2 S ) mesons. The production cross-section is measured to be 4 . 5 ± 0 . 7 ± 0 . 3 nb, where the first uncertainty is statistical and the second systematic. The differential cross-sections are measured as functions of several kinematic variables of the J/ψ - ψ (2 S ) candidates. The results are combined with a measurement of J/ψ - J/ψ production, giving a cross-section ratio between J/ψ - ψ (2 S ) and J/ψ - J/ψ production of 0 . 274 ± 0 . 044 ± 0 . 008, where the first uncertainty is statistical and the second systematic.