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

Abstract

The observation of associated production of an $\Upsilon$(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 $\Upsilon$(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 $\sqrt{s}$ = 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 $\sigma$(pp $\to$ Z $+$ $\Upsilon$(1S))$\mathcal{B}$(Z $\to$ $\mu^+\mu^-$)$\mathcal{B}$($\Upsilon$(1S) $\to$ $\mu^+\mu^-$ ) to $\sigma$(pp $\to$ Z)$\mathcal{B}$(Z $\to$ 4$\mu$) is measured to be $\mathcal{R}_{\mathrm{Z+\Upsilon}\mathrm{(1S)}}$ = (21.1 $\pm$ 55 (stat) $\pm$ 0.6 (syst) $\times$ 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 $\sigma_\text{eff}$ = 13.0$^{+7.7}_{-3.4}$. In addition, for the first time, $\sigma_\text{eff}$ is measured in bins of the transverse momentum of the $\Upsilon$(1S) meson or of the Z boson.