Observation of Y(1S) + Z associated production and measurement of the effective double-parton scattering cross section in proton-proton collisions at sqrt(s) = 13 TeV

The observation of the associated production of an ϒð1SÞ meson with a Z boson and a measurement of the ratio of its fiducial cross section to the fiducial cross section of the Z boson are presented. Both the ϒ(1S) meson and the Z boson are identified via decays into a pair of opposite-sign muons. The analysis is based on proton-proton (pp) collision data at radical(s) = 13 TeV collected with the CMS detector in 2016–2018 and corresponding to an integrated luminosity of 138 fb−1. Using the production of the Z boson decaying into four muons as a normalization channel, the ratio of the fiducial cross sections σ½pp → Z þ ϒ(1S)B(Z → μ+μ−)B(ϒ(1S) → μ+μ−) to σ(pp → Z)B(Z → 4μ) is measured to be R(Z+ϒ(S) =21.1+- 5.5(stat)+- 0.6(syst) × 10−3, where stat and syst denote the statistical and systematic uncertainties, respectively. The result is used to extract the effective double-parton scattering cross section σeff = 13.0 +7.7 −3.4 mb. In addition, for the first time, σeff is measured in bins of the transverse momentum of the ϒ(1S) meson or of the Z boson.