Observation of $ϒ$(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 $ϒ$⁡(1⁢S) 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 $ϒ$⁡(1⁢S) meson and the 𝑍 boson are identified via decays into a pair of opposite-sign muons. The analysis is based on proton-proton (𝑝⁢𝑝) 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⁡[𝑝⁢𝑝→𝑍+$ϒ$⁡(1⁢S)]⁢ℬ⁡(𝑍→𝜇$^+$⁢𝜇$^−$)⁢ℬ⁡[ϒ⁡(1⁢S)→𝜇$^+$⁢𝜇$^−$] to 𝜎⁡(𝑝⁢𝑝→𝑍)⁢ℬ⁡(𝑍→4⁢𝜇) is measured to be ℛ$_{𝑍+$ϒ$⁡(1⁢S)}$=[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 𝜎$_{eff}$=13.0$^{+7.7}_{−3.4}$  mb. In addition, for the first time, 𝜎$_{eff}$ is measured in bins of the transverse momentum of the $ϒ$⁡(1⁢S) meson or of the 𝑍 boson.