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

The observation of associated production of an $Υ$(1S) 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 √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 $σ$(pp → Z $+$$Υ$(1S))$B$(Z $→μ⁺μ⁻)B($Υ$(1S) →μ^+μ^-$ ) to $σ$(pp $→$ Z)$B$(Z $→$ 4$μ$) is measured to be $RZ+Υ(1S)$ = (21.1 $±$ 55 (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 $σ_eff$ = 13.0$+7.7-3.4$. In addition, for the first time, $σ_eff$ is measured in bins of the transverse momentum of the $Υ$(1S) meson or of the Z boson.